Universe

Universe

Overview
The Universe is commonly defined as the totality of everything that exists
Existence
In common usage, existence is the world we are aware of through our senses, and that persists independently without them. In academic philosophy the word has a more specialized meaning, being contrasted with essence, which specifies different forms of existence as well as different identity...

, including all matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

 and energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

, the planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s, star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s, galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos
Cosmos
In the general sense, a cosmos is an orderly or harmonious system. It originates from the Greek term κόσμος , meaning "order" or "ornament" and is antithetical to the concept of chaos. Today, the word is generally used as a synonym of the word Universe . The word cosmos originates from the same root...

, the world and nature
Nature
Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...

. Scientific observation of earlier stages in the development of the Universe, which can be seen at great distances, suggests that the Universe has been governed by the same physical laws and constants throughout most of its extent and history.
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Quotations

Considering the size of the universe, we might conclude that God likes to exaggerate!

Mehmet ildan, Quotations
Encyclopedia
The Universe is commonly defined as the totality of everything that exists
Existence
In common usage, existence is the world we are aware of through our senses, and that persists independently without them. In academic philosophy the word has a more specialized meaning, being contrasted with essence, which specifies different forms of existence as well as different identity...

, including all matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

 and energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

, the planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s, star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s, galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos
Cosmos
In the general sense, a cosmos is an orderly or harmonious system. It originates from the Greek term κόσμος , meaning "order" or "ornament" and is antithetical to the concept of chaos. Today, the word is generally used as a synonym of the word Universe . The word cosmos originates from the same root...

, the world and nature
Nature
Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...

. Scientific observation of earlier stages in the development of the Universe, which can be seen at great distances, suggests that the Universe has been governed by the same physical laws and constants throughout most of its extent and history. There are various multiverse
Multiverse
The multiverse is the hypothetical set of multiple possible universes that together comprise all of reality.Multiverse may also refer to:-In fiction:* Multiverse , the fictional multiverse used by DC Comics...

 theories, in which physicists have suggested that our universe is one among many universes that likewise exist.

History


Throughout recorded history, several cosmologies
Cosmology
Cosmology is the discipline that deals with the nature of the Universe as a whole. Cosmologists seek to understand the origin, evolution, structure, and ultimate fate of the Universe at large, as well as the natural laws that keep it in order...

 and cosmogonies
Cosmogony
Cosmogony, or cosmogeny, is any scientific theory concerning the coming into existence or origin of the universe, or about how reality came to be. The word comes from the Greek κοσμογονία , from κόσμος "cosmos, the world", and the root of γίνομαι / γέγονα "to be born, come about"...

 have been proposed to account for observations of the universe. The earliest quantitative geocentric models were developed by the ancient Greek philosophers
Ancient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...

. Over the centuries, more precise observations and improved theories of gravity led to Copernicus's
Nicolaus Copernicus
Nicolaus Copernicus was a Renaissance astronomer and the first person to formulate a comprehensive heliocentric cosmology which displaced the Earth from the center of the universe....

 heliocentric model
Heliocentrism
Heliocentrism, or heliocentricism, is the astronomical model in which the Earth and planets revolve around a stationary Sun at the center of the universe. The word comes from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center...

 and the Newtonian
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...

 model of the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, respectively. Further improvements in astronomy led to the realization that the Solar System is embedded in a galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 composed of billions of stars, the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

, and that other galaxies exist outside it, as far as astronomical instruments can reach. Careful studies of the distribution of these galaxies and their spectral line
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...

s have led to much of modern cosmology
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...

. Discovery of the red shift
Red shift
-Science:* Redshift, the increase of wavelength of detected electromagnetic radiation with respect to the original wavelength of the emission* Red shift, an informal term for a bathochromic shift...

 and cosmic microwave background radiation revealed that the universe is expanding and apparently had a beginning.
According to the prevailing scientific model of the universe, known as the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

, the universe expanded from an extremely hot, dense phase called the Planck epoch
Planck epoch
In physical cosmology, the Planck epoch , named after Max Planck, is the earliest period of time in the history of the universe, from zero to approximately 10−43 seconds , during which, it is believed, quantum effects of gravity were significant...

, in which all the matter and energy of the observable universe
Observable universe
In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light from those objects has had time to reach us since the beginning of the cosmological expansion...

 was concentrated. Since the Planck epoch, the universe has been expanding to its present form, possibly with a brief period (less than 10−32
Scientific notation
Scientific notation is a way of writing numbers that are too large or too small to be conveniently written in standard decimal notation. Scientific notation has a number of useful properties and is commonly used in calculators and by scientists, mathematicians, doctors, and engineers.In scientific...

 seconds) of cosmic inflation
Cosmic inflation
In physical cosmology, cosmic inflation, cosmological inflation or just inflation is the theorized extremely rapid exponential expansion of the early universe by a factor of at least 1078 in volume, driven by a negative-pressure vacuum energy density. The inflationary epoch comprises the first part...

. Several independent experimental measurements support this theoretical expansion
Metric expansion of space
The metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...

 and, more generally, the Big Bang theory. Recent observations indicate that this expansion is accelerating because of dark energy
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...

, and that most of the matter in the universe may be in a form which cannot be detected by present instruments, and so is not accounted for in the present models of the universe; this has been named dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

. The imprecision of current observations has hindered predictions of the ultimate fate of the universe
Ultimate fate of the universe
The ultimate fate of the universe is a topic in physical cosmology. Many possible fates are predicted by rival scientific theories, including futures of both finite and infinite duration....

.

Current interpretations of astronomical observations
Observable universe
In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light from those objects has had time to reach us since the beginning of the cosmological expansion...

 indicate that the age of the universe
Age of the universe
The age of the universe is the time elapsed since the Big Bang posited by the most widely accepted scientific model of cosmology. The best current estimate of the age of the universe is 13.75 ± 0.13 billion years within the Lambda-CDM concordance model...

 is 13.75 ± 0.17 billion
1000000000 (number)
1,000,000,000 is the natural number following 999,999,999 and preceding 1,000,000,001.In scientific notation, it is written as 109....

 years, and that the diameter of the observable universe
Observable universe
In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light from those objects has had time to reach us since the beginning of the cosmological expansion...

 is at least 93 billion light years or metres. According to general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

, space can expand faster than the speed of light, although we can view only a small portion of the universe due to the limitation imposed by light speed. Since we cannot observe space beyond the limitations of light (or any electromagnetic radiation), it is uncertain whether the size of the universe is finite or infinite.

Etymology, synonyms and definitions



The word universe derives from the Old French
Old French
Old French was the Romance dialect continuum spoken in territories that span roughly the northern half of modern France and parts of modern Belgium and Switzerland from the 9th century to the 14th century...

 word Univers, which in turn derives from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 word universum. The Latin word was used by Cicero
Cicero
Marcus Tullius Cicero , was a Roman philosopher, statesman, lawyer, political theorist, and Roman constitutionalist. He came from a wealthy municipal family of the equestrian order, and is widely considered one of Rome's greatest orators and prose stylists.He introduced the Romans to the chief...

 and later Latin authors in many of the same senses as the modern English
English language
English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

 word is used. The Latin word derives from the poetic contraction Unvorsum — first used by Lucretius
Lucretius
Titus Lucretius Carus was a Roman poet and philosopher. His only known work is an epic philosophical poem laying out the beliefs of Epicureanism, De rerum natura, translated into English as On the Nature of Things or "On the Nature of the Universe".Virtually no details have come down concerning...

 in Book IV (line 262) of his De rerum natura
On the Nature of Things
De rerum natura is a 1st century BC didactic poem by the Roman poet and philosopher Lucretius with the goal of explaining Epicurean philosophy to a Roman audience. The poem, written in some 7,400 dactylic hexameters, is divided into six untitled books, and explores Epicurean physics through richly...

 (On the Nature of Things) — which connects un, uni (the combining form of unus', or "one") with vorsum, versum (a noun made from the perfect passive participle of vertere, meaning "something rotated, rolled, changed").

An alternative interpretation of unvorsum is "everything rotated as one" or "everything rotated by one". In this sense, it may be considered a translation of an earlier Greek word for the universe, , (periforá, "circumambulation"), originally used to describe a course of a meal, the food being carried around the circle of dinner guests. This Greek word refers to celestial spheres
Celestial spheres
The celestial spheres, or celestial orbs, were the fundamental entities of the cosmological models developed by Plato, Eudoxus, Aristotle, Ptolemy, Copernicus and others...

, an early Greek model of the universe. Regarding Plato's Metaphor of the sun, Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

 suggests that the rotation of the sphere of fixed stars inspired by the prime mover
Primum movens
Primum movens , usually referred to as the Prime mover or first cause in English, is a term used in the philosophy of Aristotle, in the theological cosmological argument for the existence of God, and in cosmogony, the source of the cosmos or "all-being".-Aristotle's ontology:In book 12 of his...

, motivates, in turn, terrestrial change via the Sun. Careful astronomical
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 and physical measurements (such as the Foucault pendulum
Foucault pendulum
The Foucault pendulum , or Foucault's pendulum, named after the French physicist Léon Foucault, is a simple device conceived as an experiment to demonstrate the rotation of the Earth. While it had long been known that the Earth rotated, the introduction of the Foucault pendulum in 1851 was the...

) are required to prove the Earth rotates on its axis.

A term for "universe" in ancient Greece was (tó pán, The All
The All
The All is the Hermetic or panentheistic view of God, which is that everything that is, or at least that can be experienced, collectively makes up The All...

, Pan (mythology)
Pan (mythology)
Pan , in Greek religion and mythology, is the god of the wild, shepherds and flocks, nature, of mountain wilds, hunting and rustic music, as well as the companion of the nymphs. His name originates within the Greek language, from the word paein , meaning "to pasture." He has the hindquarters, legs,...

). Related terms were matter, ' onMouseout='HidePop("94281")' href="http://www.absoluteastronomy.com/topics/Hyle">Hyle
Hyle
In philosophy, hyle refers to matter or stuff. It can also be the material cause underlying a change in Aristotelian philosophy. The Greeks originally had no word for matter in general, as opposed to raw material suitable for some specific purpose or other, so Aristotle adapted the word for...

, lit. wood) and place . Other synonyms for the universe among the ancient Greek philosophers included (cosmos
Cosmos
In the general sense, a cosmos is an orderly or harmonious system. It originates from the Greek term κόσμος , meaning "order" or "ornament" and is antithetical to the concept of chaos. Today, the word is generally used as a synonym of the word Universe . The word cosmos originates from the same root...

) and (meaning Nature
Nature
Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...

, from which we derive the word physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

). The same synonyms are found in Latin authors (totum, mundus, natura) and survive in modern languages, e.g., the German words Das All, Weltall, and Natur for universe. The same synonyms are found in English, such as everything (as in the theory of everything
Theory of everything
A theory of everything is a putative theory of theoretical physics that fully explains and links together all known physical phenomena, and predicts the outcome of any experiment that could be carried out in principle....

), the cosmos (as in cosmology
Cosmology
Cosmology is the discipline that deals with the nature of the Universe as a whole. Cosmologists seek to understand the origin, evolution, structure, and ultimate fate of the Universe at large, as well as the natural laws that keep it in order...

), the world (as in the many-worlds hypothesis), and Nature
Nature
Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...

 (as in natural law
Natural law
Natural law, or the law of nature , is any system of law which is purportedly determined by nature, and thus universal. Classically, natural law refers to the use of reason to analyze human nature and deduce binding rules of moral behavior. Natural law is contrasted with the positive law Natural...

s or natural philosophy
Natural philosophy
Natural philosophy or the philosophy of nature , is a term applied to the study of nature and the physical universe that was dominant before the development of modern science...

).

Broadest definition: reality and probability


The broadest definition of the universe can be found in De divisione naturae
De divisione naturae
De divisione naturae was the magnum opus of ninth century theologian Johannes Scotus Eriugena....

 by the medieval
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

 philosopher and theologian
Theology
Theology is the systematic and rational study of religion and its influences and of the nature of religious truths, or the learned profession acquired by completing specialized training in religious studies, usually at a university or school of divinity or seminary.-Definition:Augustine of Hippo...

 Johannes Scotus Eriugena
Johannes Scotus Eriugena
Johannes Scotus Eriugena was an Irish theologian, Neoplatonist philosopher, and poet. He is known for having translated and made commentaries upon the work of Pseudo-Dionysius.-Name:...

, who defined it as simply everything: everything that is created and everything that is not created. In the path integral formulation
Path integral formulation
The path integral formulation of quantum mechanics is a description of quantum theory which generalizes the action principle of classical mechanics...

 of Richard Feynman
Richard Feynman
Richard Phillips Feynman was an American physicist known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics and the physics of the superfluidity of supercooled liquid helium, as well as in particle physics...

, the probability amplitude
Probability amplitude
In quantum mechanics, a probability amplitude is a complex number whose modulus squared represents a probability or probability density.For example, if the probability amplitude of a quantum state is \alpha, the probability of measuring that state is |\alpha|^2...

s for the various outcomes of an experiment given a perfectly defined initial state of the system are determined by summing over all possible histories (paths) by which the system could progress from the initial to final state.

Definition as reality


More customarily, the universe is defined as everything that exists, (has existed, and will exist) . According to our current understanding, the universe consists of three principles: spacetime
Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...

, forms of energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

, including momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...

 and matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

, and the physical law
Physical law
A physical law or scientific law is "a theoretical principle deduced from particular facts, applicable to a defined group or class of phenomena, and expressible by the statement that a particular phenomenon always occurs if certain conditions be present." Physical laws are typically conclusions...

s that relate them.

Definition as connected space-time


It is possible to conceive of disconnected space-times, each existing but unable to interact with one another. An easily visualized metaphor is a group of separate soap bubble
Soap bubble
A soap bubble is a thin film of soapy water enclosing air, that forms a hollow sphere with an iridescent surface. Soap bubbles usually last for only a few seconds before bursting, either on their own or on contact with another object. They are often used for children's enjoyment, but they are also...

s, in which observers living on one soap bubble cannot interact with those on other soap bubbles, even in principle. According to one common terminology, each "soap bubble" of space-time is denoted as a universe, whereas our particular space-time is denoted as the universe, just as we call our moon the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

. The entire collection of these separate space-times is denoted as the multiverse
Multiverse
The multiverse is the hypothetical set of multiple possible universes that together comprise all of reality.Multiverse may also refer to:-In fiction:* Multiverse , the fictional multiverse used by DC Comics...

. In principle, the other unconnected universes may have different dimension
Dimension
In physics and mathematics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...

alities and topologies
Topology
Topology is a major area of mathematics concerned with properties that are preserved under continuous deformations of objects, such as deformations that involve stretching, but no tearing or gluing...

 of space-time, different forms of matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

 and energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

, and different physical law
Physical law
A physical law or scientific law is "a theoretical principle deduced from particular facts, applicable to a defined group or class of phenomena, and expressible by the statement that a particular phenomenon always occurs if certain conditions be present." Physical laws are typically conclusions...

s and physical constant
Physical constant
A physical constant is a physical quantity that is generally believed to be both universal in nature and constant in time. It can be contrasted with a mathematical constant, which is a fixed numerical value but does not directly involve any physical measurement.There are many physical constants in...

s, although such possibilities are currently speculative.

Definition as observable reality


According to a still-more-restrictive definition, the universe is everything within our connected space-time that could have a chance to interact with us and vice versa. According to the general theory of relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

, some regions of space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

 may never interact with ours even in the lifetime of the universe, due to the finite speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

 and the ongoing expansion of space. For example, radio messages sent from Earth may never reach some regions of space, even if the universe would live forever; space may expand faster than light can traverse it. It is worth emphasizing that those distant regions of space are taken to exist and be part of reality as much as we are; yet we can never interact with them. The spatial region within which we can affect and be affected is denoted as the observable universe
Observable universe
In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light from those objects has had time to reach us since the beginning of the cosmological expansion...

. Strictly speaking, the observable universe depends on the location of the observer. By traveling, an observer can come into contact with a greater region of space-time than an observer who remains still, so that the observable universe for the former is larger than for the latter. Nevertheless, even the most rapid traveler will not be able to interact with all of space. Typically, the observable universe is taken to mean the universe observable from our vantage point in the Milky Way Galaxy.

Size, age, contents, structure, and laws


The universe is immensely large and possibly infinite in volume. The region visible from Earth (the observable universe
Observable universe
In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light from those objects has had time to reach us since the beginning of the cosmological expansion...

) is a sphere with a radius of about 46 billion light years
Light Years
Light Years is the seventh studio album by Australian recording artist Kylie Minogue. It was released on 25 September 2000 by Parlophone and Mushroom Records. The album's style was indicative of her return to "mainstream pop dance tunes"....

, based on where the expansion of space
Metric expansion of space
The metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...

 has taken
Comoving distance
In standard cosmology, comoving distance and proper distance are two closely related distance measures used by cosmologists to define distances between objects...

 the most distant objects observed. For comparison, the diameter of a typical galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 is only 30,000 light-years, and the typical distance between two neighboring galaxies is only 3 million light-years. As an example, our Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 Galaxy is roughly 100,000 light years in diameter, and our nearest sister galaxy, the Andromeda Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...

, is located roughly 2.5 million light years away. There are probably more than 100 billion (1011) galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

 in the observable universe. Typical galaxies range from dwarfs
Dwarf galaxy
A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200-400 billion stars...

 with as few as ten million (107) star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s up to giants with one trillion (1012) stars, all orbiting the galaxy's center of mass. A 2010 study by astronomers estimated that the observable universe contains 300 sextillion (3) stars.
The observable matter is spread homogeneously (uniformly) throughout the universe, when averaged over distances longer than 300 million light-years. However, on smaller length-scales, matter is observed to form "clumps", i.e., to cluster hierarchically; many atoms are condensed into star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s, most stars into galaxies, most galaxies into clusters, superclusters
Galaxy groups and clusters
Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. They form the densest part of the large scale structure of the universe...

 and, finally, the largest-scale structures such as the Great Wall of galaxies
Great Wall (astronomy)
The Great Wall , sometimes specifically referred to as the CfA2 Great Wall, is one of the largest known super-structures in the Universe...

. The observable matter of the universe is also spread isotropically, meaning that no direction of observation seems different from any other; each region of the sky has roughly the same content. The universe is also bathed in a highly isotropic microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

 that corresponds to a thermal equilibrium
Thermal equilibrium
Thermal equilibrium is a theoretical physical concept, used especially in theoretical texts, that means that all temperatures of interest are unchanging in time and uniform in space...

 blackbody spectrum of roughly 2.725-kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

s. The hypothesis that the large-scale universe is homogeneous and isotropic is known as the cosmological principle
Cosmological Principle
In modern physical cosmology, the cosmological principle is the working assumption that observers on Earth do not occupy an unusual or privileged location within the universe as a whole, judged as observers of the physical phenomena produced by uniform and universal laws of physics...

, which is supported by astronomical observations.

The present overall density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 of the universe is very low, roughly 9.9 × 10−30 grams per cubic centimetre. This mass-energy appears to consist of 73% dark energy
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...

, 23% cold dark matter
Cold dark matter
Cold dark matter is the improvement of the big bang theory that contains the additional assumption that most of the matter in the Universe consists of material that cannot be observed by its electromagnetic radiation and whose constituent particles move slowly...

 and 4% ordinary matter. Thus the density of atoms is on the order of a single hydrogen atom for every four cubic meters of volume. The properties of dark energy and dark matter are largely unknown. Dark matter gravitates as ordinary matter, and thus works to slow the expansion of the universe
Metric expansion of space
The metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...

; by contrast, dark energy accelerates its expansion
Accelerating universe
The accelerating universe is the observation that the universe appears to be expanding at an increasing rate, which in formal terms means that the cosmic scale factor a has a positive second derivative, implying that the velocity at which a given galaxy is receding from us should be continually...

.

The most precise estimate
Wilkinson Microwave Anisotropy Probe
The Wilkinson Microwave Anisotropy Probe — also known as the Microwave Anisotropy Probe , and Explorer 80 — is a spacecraft which measures differences in the temperature of the Big Bang's remnant radiant heat — the Cosmic Microwave Background Radiation — across the full sky. Headed by Professor...

 of the universe's age
Age of the universe
The age of the universe is the time elapsed since the Big Bang posited by the most widely accepted scientific model of cosmology. The best current estimate of the age of the universe is 13.75 ± 0.13 billion years within the Lambda-CDM concordance model...

 is 13.73±0.12 billion years old, based on observations of the cosmic microwave background radiation
Cosmic microwave background radiation
In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....

. Independent estimates (based on measurements such as radioactive dating) agree at 13–15 billion years. The universe has not been the same at all times in its history; for example, the relative populations of quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...

s and galaxies have changed and space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

 itself appears to have expanded
Metric expansion of space
The metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...

. This expansion accounts for how Earth-bound scientists can observe the light from a galaxy 30 billion light years away, even if that light has traveled for only 13 billion years; the very space between them has expanded. This expansion is consistent with the observation that the light from distant galaxies has been redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...

ed; the photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s emitted have been stretched to longer wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

s and lower frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 during their journey. The rate of this spatial expansion is accelerating
Accelerating universe
The accelerating universe is the observation that the universe appears to be expanding at an increasing rate, which in formal terms means that the cosmic scale factor a has a positive second derivative, implying that the velocity at which a given galaxy is receding from us should be continually...

, based on studies of Type Ia supernova
Type Ia supernova
A Type Ia supernova is a sub-category of supernovae, which in turn are a sub-category of cataclysmic variable stars, that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion...

e and corroborated by other data.

The relative fractions
Abundance of the chemical elements
The abundance of a chemical element measures how relatively common the element is, or how much of the element is present in a given environment by comparison to all other elements...

 of different chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

s — particularly the lightest atoms such as hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

, deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

 and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 — seem to be identical throughout the universe and throughout its observable history. The universe seems to have much more matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

 than antimatter
Antimatter
In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...

, an asymmetry possibly related to the observations of CP violation
CP violation
In particle physics, CP violation is a violation of the postulated CP-symmetry: the combination of C-symmetry and P-symmetry . CP-symmetry states that the laws of physics should be the same if a particle were interchanged with its antiparticle , and left and right were swapped...

. The universe appears to have no net electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

, and therefore gravity appears to be the dominant interaction on cosmological length scales. The universe also appears to have neither net momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...

 nor angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

. The absence of net charge and momentum would follow from accepted physical laws (Gauss's law
Gauss's law
In physics, Gauss's law, also known as Gauss's flux theorem, is a law relating the distribution of electric charge to the resulting electric field. Gauss's law states that:...

 and the non-divergence of the stress-energy-momentum pseudotensor
Stress-energy-momentum pseudotensor
In the theory of general relativity, a stress–energy–momentum pseudotensor, such as the Landau–Lifshitz pseudotensor, is an extension of the non-gravitational stress–energy tensor which incorporates the energy-momentum of gravity. It allows the energy-momentum of a system of gravitating matter to...

, respectively), if the universe were finite.

The universe appears to have a smooth space-time continuum consisting of three spatial
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

 dimension
Dimension
In physics and mathematics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...

s and one temporal (time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

) dimension. On the average, space is observed to be very nearly flat (close to zero curvature
Curvature
In mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry. Intuitively, curvature is the amount by which a geometric object deviates from being flat, or straight in the case of a line, but this is defined in different ways depending on the context...

), meaning that Euclidean geometry
Euclidean geometry
Euclidean geometry is a mathematical system attributed to the Alexandrian Greek mathematician Euclid, which he described in his textbook on geometry: the Elements. Euclid's method consists in assuming a small set of intuitively appealing axioms, and deducing many other propositions from these...

 is experimentally true with high accuracy throughout most of the Universe. Spacetime also appears to have a simply connected
Simply connected space
In topology, a topological space is called simply connected if it is path-connected and every path between two points can be continuously transformed, staying within the space, into any other path while preserving the two endpoints in question .If a space is not simply connected, it is convenient...

 topology
Topology
Topology is a major area of mathematics concerned with properties that are preserved under continuous deformations of objects, such as deformations that involve stretching, but no tearing or gluing...

, at least on the length-scale of the observable universe. However, present observations cannot exclude the possibilities that the universe has more dimensions and that its spacetime may have a multiply connected global topology, in analogy with the cylindrical or toroid
Toroid
Toroid may refer to*Toroid , a doughnut-like solid whose surface is a torus.*Toroidal inductors and transformers which have wire windings on circular ring shaped magnetic cores.*Vortex ring, a toroidal flow in fluid mechanics....

al topologies of two-dimensional space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

s.

The universe appears to behave in a manner that regularly follows a set of physical law
Physical law
A physical law or scientific law is "a theoretical principle deduced from particular facts, applicable to a defined group or class of phenomena, and expressible by the statement that a particular phenomenon always occurs if certain conditions be present." Physical laws are typically conclusions...

s and physical constant
Physical constant
A physical constant is a physical quantity that is generally believed to be both universal in nature and constant in time. It can be contrasted with a mathematical constant, which is a fixed numerical value but does not directly involve any physical measurement.There are many physical constants in...

s. According to the prevailing Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...

 of physics, all matter is composed of three generations of lepton
Lepton
A lepton is an elementary particle and a fundamental constituent of matter. The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons , and neutral...

s and quark
Quark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...

s, both of which are fermion
Fermion
In particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....

s. These elementary particle
Elementary particle
In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...

s interact via at most three fundamental interaction
Fundamental interaction
In particle physics, fundamental interactions are the ways that elementary particles interact with one another...

s: the electroweak interaction which includes electromagnetism
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...

 and the weak nuclear force; the strong nuclear force described by quantum chromodynamics
Quantum chromodynamics
In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...

; and gravity, which is best described at present by general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

. The first two interactions can be described by renormalized
Renormalization
In quantum field theory, the statistical mechanics of fields, and the theory of self-similar geometric structures, renormalization is any of a collection of techniques used to treat infinities arising in calculated quantities....

 quantum field theory
Quantum field theory
Quantum field theory provides a theoretical framework for constructing quantum mechanical models of systems classically parametrized by an infinite number of dynamical degrees of freedom, that is, fields and many-body systems. It is the natural and quantitative language of particle physics and...

, and are mediated by gauge boson
Gauge boson
In particle physics, gauge bosons are bosonic particles that act as carriers of the fundamental forces of nature. More specifically, elementary particles whose interactions are described by gauge theory exert forces on each other by the exchange of gauge bosons, usually as virtual particles.-...

s that correspond to a particular type of gauge symmetry. A renormalized quantum field theory of general relativity has not yet been achieved, although various forms of string theory
String theory
String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything , a manner of describing the known fundamental forces and matter in a mathematically complete system...

 seem promising. The theory of special relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

 is believed to hold throughout the universe, provided that the spatial and temporal length scales are sufficiently short; otherwise, the more general theory of general relativity must be applied. There is no explanation for the particular values that physical constant
Physical constant
A physical constant is a physical quantity that is generally believed to be both universal in nature and constant in time. It can be contrasted with a mathematical constant, which is a fixed numerical value but does not directly involve any physical measurement.There are many physical constants in...

s appear to have throughout our universe, such as Planck's constant h or the gravitational constant
Gravitational constant
The gravitational constant, denoted G, is an empirical physical constant involved in the calculation of the gravitational attraction between objects with mass. It appears in Newton's law of universal gravitation and in Einstein's theory of general relativity. It is also known as the universal...

 G. Several conservation law
Conservation law
In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves....

s have been identified, such as the conservation of charge, momentum, angular momentum and energy
Conservation of energy
The nineteenth century law of conservation of energy is a law of physics. It states that the total amount of energy in an isolated system remains constant over time. The total energy is said to be conserved over time...

; in many cases, these conservation laws can be related to symmetries
Symmetry
Symmetry generally conveys two primary meanings. The first is an imprecise sense of harmonious or aesthetically pleasing proportionality and balance; such that it reflects beauty or perfection...

 or mathematical identities.

Fine tuning


It appears that many of the properties of the universe have special values in the sense that a universe where these properties only differ slightly would not be able to support intelligent life. Not all scientists agree that this fine-tuning
Fine-tuned universe
The fine-tuned universe is the proposition that the conditions that allow life in the Universe can only occur when certain universal fundamental physical constants lie within a very narrow range, so that if any of several fundamental constants were only slightly different the universe would be...

 exists. In particular, it is not known under what conditions intelligent life could form and what form or shape that would take. A relevant observation in this discussion is that for an observer to exist to observe fine-tuning, the universe must be able to support intelligent life. As such the conditional probability
Conditional probability
In probability theory, the "conditional probability of A given B" is the probability of A if B is known to occur. It is commonly notated P, and sometimes P_B. P can be visualised as the probability of event A when the sample space is restricted to event B...

 of observing a universe that is fine-tuned to support intelligent life is 1. This observation is known as the anthropic principle
Anthropic principle
In astrophysics and cosmology, the anthropic principle is the philosophical argument that observations of the physical Universe must be compatible with the conscious life that observes it. Some proponents of the argument reason that it explains why the Universe has the age and the fundamental...

 and is particularly relevant if the creation of the universe was probabilistic or if multiple universes with a variety of properties exist (see below).

Historical models


Many models of the cosmos (cosmologies) and its origin (cosmogonies) have been proposed, based on the then-available data and conceptions of the universe. Historically, cosmologies and cosmogonies were based on narratives of gods acting in various ways. Theories of an impersonal universe governed by physical laws were first proposed by the Greeks and Indians. Over the centuries, improvements in astronomical observations and theories of motion and gravitation led to ever more accurate descriptions of the universe. The modern era of cosmology began with Albert Einstein's
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...

 1915 general theory of relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

, which made it possible to quantitatively predict the origin, evolution, and conclusion of the universe as a whole. Most modern, accepted theories of cosmology are based on general relativity and, more specifically, the predicted Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

; however, still more careful measurements are required to determine which theory is correct.

Creation


Many cultures have stories describing the origin of the world, which may be roughly grouped into common types. In one type of story, the world is born from a world egg
World egg
A world egg or cosmic egg is a mythological motif found in the creation myths of many cultures and civilizations. Typically, the world egg is a beginning of some sort, and the universe or some primordial being comes into existence by "hatching" from the egg, sometimes lain on the primordial waters...

; such stories include the Finnish epic poem
Epic poetry
An epic is a lengthy narrative poem, ordinarily concerning a serious subject containing details of heroic deeds and events significant to a culture or nation. Oral poetry may qualify as an epic, and Albert Lord and Milman Parry have argued that classical epics were fundamentally an oral poetic form...

 Kalevala
Kalevala
The Kalevala is a 19th century work of epic poetry compiled by Elias Lönnrot from Finnish and Karelian oral folklore and mythology.It is regarded as the national epic of Finland and is one of the most significant works of Finnish literature...

, the Chinese
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

 story of Pangu
Pangu
Pangu was the first living being and the creator of all in Chinese mythology.- The Pangu legend:...

 or the Indian
History of India
The history of India begins with evidence of human activity of Homo sapiens as long as 75,000 years ago, or with earlier hominids including Homo erectus from about 500,000 years ago. The Indus Valley Civilization, which spread and flourished in the northwestern part of the Indian subcontinent from...

 Brahmanda Purana
Brahmanda Purana
The Brahmanda Purana is one of the eighteen Mahapuranas, a genre of eighteen Hindu religious texts and has been assigned the eighteenth place in almost all the lists of the Puranas.Brahma in Sanskrit means "the biggest", anda/andam means globe...

. In related stories, the creation idea is caused by a single entity emanating or producing something by him- or herself, as in the Tibetan Buddhism
Tibetan Buddhism
Tibetan Buddhism is the body of Buddhist religious doctrine and institutions characteristic of Tibet and certain regions of the Himalayas, including northern Nepal, Bhutan, and India . It is the state religion of Bhutan...

 concept of Adi-Buddha
Adi-Buddha
In Vajrayana Buddhism, the Adi-Buddha, or Adibuddha , is the "Primordial Buddha." The term refers to a self-emanating, self-originating Buddha, present before anything else existed. Samantabhadra, Samantabhadri and Vajradhara are the best known names for Adi-Buddha, though there are others...

, the ancient Greek
Ancient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...

 story of Gaia
Gaia (mythology)
Gaia was the primordial Earth-goddess in ancient Greek religion. Gaia was the great mother of all: the heavenly gods and Titans were descended from her union with Uranus , the sea-gods from her union with Pontus , the Giants from her mating with Tartarus and mortal creatures were sprung or born...

 (Mother Earth), the Aztec
Aztec mythology
The aztec civilization recognized a polytheistic mythology, which contained the many deities and supernatural creatures from their religious beliefs. "orlando"- History :...

 goddess Coatlicue
Coatlicue
Coatlicue, also known as Teteoinan , "The Mother of Gods" , is the Aztec goddess who gave birth to the moon, stars, and Huitzilopochtli, the god of the sun and war...

 myth, the ancient Egyptian god
Ennead
The Ennead was a group ofnine deities in Egyptian mythology. The Ennead were worshipped at Heliopolis and consisted of the god Atum, his children Shu and Tefnut, their children Geb and Nut and their children Osiris, Isis, Horus, Set and Nephthys.-Terminology:Egyptian mythology established multiple...

 Atum
Atum
Atum, sometimes rendered as Atem or Tem, is an important deity in Egyptian mythology.- Name :Atum's name is thought to be derived from the word 'tem' which means to complete or finish. Thus he has been interpreted as being the 'complete one' and also the finisher of the world, which he returns to...

 story, or the Genesis creation narrative. In another type of story, the world is created from the union of male and female deities, as in the Maori story
Maori mythology
Māori mythology and Māori traditions are the two major categories into which the legends of the Māori of New Zealand may usefully be divided...

 of Rangi and Papa
Rangi and Papa
In Māori mythology the primal couple Rangi and Papa appear in a creation myth explaining the origin of the world. In some South Island dialects, Rangi is called Raki or Rakinui.-Union and separation:...

. In other stories, the universe is created by crafting it from pre-existing materials, such as the corpse of a dead god — as from Tiamat
Tiamat
In Babylonian mythology, Tiamat is a chaos monster, a primordial goddess of the ocean, mating with Abzû to produce younger gods. It is suggested that there are two parts to the Tiamat mythos, the first in which Tiamat is 'creatrix', through a "Sacred marriage" between salt and fresh water,...

 in the Babylon
Babylon
Babylon was an Akkadian city-state of ancient Mesopotamia, the remains of which are found in present-day Al Hillah, Babil Province, Iraq, about 85 kilometers south of Baghdad...

ian epic Enuma Elish
Enûma Elish
The is the Babylonian creation myth . It was recovered by Austen Henry Layard in 1849 in the ruined Library of Ashurbanipal at Nineveh , and published by George Smith in 1876.The Enûma Eliš has about a thousand lines and is recorded in Old Babylonian on seven clay tablets, each holding...

 or from the giant Ymir
Ymir
In Norse mythology, Ymir, also called Aurgelmir among the giants themselves, was the founder of the race of frost giants and was later killed by the Borrs.-Etymology:...

 in Norse mythology
Norse mythology
Norse mythology, a subset of Germanic mythology, is the overall term for the myths, legends and beliefs about supernatural beings of Norse pagans. It flourished prior to the Christianization of Scandinavia, during the Early Middle Ages, and passed into Nordic folklore, with some aspects surviving...

 – or from chaotic materials, as in Izanagi
Izanagi
is a deity born of the seven divine generations in Japanese mythology and Shinto, and is also referred to in the roughly translated Kojiki as "male-who-invites" or Izanagi-no-mikoto. It is also pronounced Izanaki-no-Okami....

 and Izanami
Izanami
In Japanese mythology, is a goddess of both creation and death, as well as the former wife of the god Izanagi-no-Mikoto. She is also referred to as Izanami-no-kami.-Goddess of Creation:...

 in Japanese mythology
Japanese mythology
Japanese mythology is a system of beliefs that embraces Shinto and Buddhist traditions as well as agriculturally based folk religion. The Shinto pantheon comprises innumerable kami...

. In other stories, the universe emanates from fundamental principles, such as Brahman
Brahman
In Hinduism, Brahman is the one supreme, universal Spirit that is the origin and support of the phenomenal universe. Brahman is sometimes referred to as the Absolute or Godhead which is the Divine Ground of all being...

 and Prakrti
Prakrti
Prakrti or Prakriti or Prakruti means "nature". It is, according to Hindus, the basic nature of intelligence by which the Universe exists and functions. It is described in Bhagavad Gita as the "primal motive force". It is the essential constituent of the universe and is at the basis of all the...

, or the yin and yang
Yin and yang
In Asian philosophy, the concept of yin yang , which is often referred to in the West as "yin and yang", is used to describe how polar opposites or seemingly contrary forces are interconnected and interdependent in the natural world, and how they give rise to each other in turn. Opposites thus only...

 of the Tao
Tao
Dao or Tao is a Chinese word meaning 'way', 'path', 'route', or sometimes more loosely, 'doctrine' or 'principle'...

.

Philosophical models


From the 6th century BCE, the pre-Socratic Greek philosophers
Pre-Socratic philosophy
Pre-Socratic philosophy is Greek philosophy before Socrates . In Classical antiquity, the Presocratic philosophers were called physiologoi...

 developed the earliest known philosophical models of the universe. The earliest Greek philosophers noted that appearances can be deceiving, and sought to understand the underlying reality behind the appearances. In particular, they noted the ability of matter to change forms (e.g., ice to water to steam) and several philosophers proposed that all the apparently different materials of the world are different forms of a single primordial material, or arche
Arche
Arche is a Greek word with primary senses 'beginning', 'origin' or 'first cause' and 'power', 'sovereignty', 'domination' as extended meanings. This list is extended to 'ultimate underlying substance' and 'ultimate undemonstrable principle'...

. The first to do so was Thales
Thales
Thales of Miletus was a pre-Socratic Greek philosopher from Miletus in Asia Minor, and one of the Seven Sages of Greece. Many, most notably Aristotle, regard him as the first philosopher in the Greek tradition...

, who proposed this material is Water
Water (classical element)
Water is one of the elements in ancient Greek philosophy, in the Asian Indian system Panchamahabhuta, and in the Chinese cosmological and physiological system Wu Xing...

. Thales' student, Anaximander
Anaximander
Anaximander was a pre-Socratic Greek philosopher who lived in Miletus, a city of Ionia; Milet in modern Turkey. He belonged to the Milesian school and learned the teachings of his master Thales...

, proposed that everything came from the limitless apeiron
Apeiron (cosmology)
Apeiron is a Greek word meaning unlimited, infinite or indefinite from ἀ- a-, "without" and πεῖραρ peirar, "end, limit", the Ionic Greek form of πέρας peras, "end, limit, boundary".-Apeiron as an origin:...

. Anaximenes
Anaximenes of Miletus
Anaximenes of Miletus was an Archaic Greek Pre-Socratic philosopher active in the latter half of the 6th century BC. One of the three Milesian philosophers, he is identified as a younger friend or student of Anaximander. Anaximenes, like others in his school of thought, practiced material monism...

 proposed Air
Air (classical element)
Air is often seen as a universal power or pure substance. Its supposed fundamental importance to life can be seen in words such as aspire, inspire, perspire and spirit, all derived from the Latin spirare.-Greek and Roman tradition:...

 on account of its perceived attractive and repulsive qualities that cause the arche to condense or dissociate into different forms. Anaxagoras
Anaxagoras
Anaxagoras was a Pre-Socratic Greek philosopher. Born in Clazomenae in Asia Minor, Anaxagoras was the first philosopher to bring philosophy from Ionia to Athens. He attempted to give a scientific account of eclipses, meteors, rainbows, and the sun, which he described as a fiery mass larger than...

, proposed the principle of Nous
Nous
Nous , also called intellect or intelligence, is a philosophical term for the faculty of the human mind which is described in classical philosophy as necessary for understanding what is true or real, very close in meaning to intuition...

 (Mind). Heraclitus
Heraclitus
Heraclitus of Ephesus was a pre-Socratic Greek philosopher, a native of the Greek city Ephesus, Ionia, on the coast of Asia Minor. He was of distinguished parentage. Little is known about his early life and education, but he regarded himself as self-taught and a pioneer of wisdom...

 proposed fire
Fire (classical element)
Fire has been an important part of all cultures and religions from pre-history to modern day and was vital to the development of civilization. It has been regarded in many different contexts throughout history, but especially as a metaphysical constant of the world.-Greek and Roman tradition:Fire...

 (and spoke of logos
Logos
' is an important term in philosophy, psychology, rhetoric and religion. Originally a word meaning "a ground", "a plea", "an opinion", "an expectation", "word," "speech," "account," "reason," it became a technical term in philosophy, beginning with Heraclitus ' is an important term in...

). Empedocles
Empedocles
Empedocles was a Greek pre-Socratic philosopher and a citizen of Agrigentum, a Greek city in Sicily. Empedocles' philosophy is best known for being the originator of the cosmogenic theory of the four Classical elements...

 proposed the elements: earth, wind, air and fire. His four element theory became very popular. Like Pythagoras
Pythagoras
Pythagoras of Samos was an Ionian Greek philosopher, mathematician, and founder of the religious movement called Pythagoreanism. Most of the information about Pythagoras was written down centuries after he lived, so very little reliable information is known about him...

, Plato
Plato
Plato , was a Classical Greek philosopher, mathematician, student of Socrates, writer of philosophical dialogues, and founder of the Academy in Athens, the first institution of higher learning in the Western world. Along with his mentor, Socrates, and his student, Aristotle, Plato helped to lay the...

 believed that all things were composed of number
Number
A number is a mathematical object used to count and measure. In mathematics, the definition of number has been extended over the years to include such numbers as zero, negative numbers, rational numbers, irrational numbers, and complex numbers....

, with the Empedocles' elements taking the form of the Platonic solids. Democritus, and later philosophers—most notably Leucippus
Leucippus
Leucippus or Leukippos was one of the earliest Greeks to develop the theory of atomism — the idea that everything is composed entirely of various imperishable, indivisible elements called atoms — which was elaborated in greater detail by his pupil and successor, Democritus...

—proposed that the universe was composed of indivisible atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s moving through void
Void (astronomy)
In astronomy, voids are the empty spaces between filaments, the largest-scale structures in the Universe, that contain very few, or no, galaxies. They were first discovered in 1978 during a pioneering study by Stephen Gregory and Laird A. Thompson at the Kitt Peak National Observatory...

 (vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

). Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

 did not believe that was feasible because air, like water, offers resistance to motion
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

. Air will immediately rush in to fill a void, and moreover, without resistance, it would do so indefinitely fast.

Although Heraclitus argued for eternal change, his quasi-contemporary Parmenides
Parmenides
Parmenides of Elea was an ancient Greek philosopher born in Elea, a Greek city on the southern coast of Italy. He was the founder of the Eleatic school of philosophy. The single known work of Parmenides is a poem, On Nature, which has survived only in fragmentary form. In this poem, Parmenides...

 made the radical suggestion that all change is an illusion, that the true underlying reality is eternally unchanging and of a single nature. Parmenides denoted this reality as (The One). Parmenides' theory seemed implausible to many Greeks, but his student Zeno of Elea
Zeno of Elea
Zeno of Elea was a pre-Socratic Greek philosopher of southern Italy and a member of the Eleatic School founded by Parmenides. Aristotle called him the inventor of the dialectic. He is best known for his paradoxes, which Bertrand Russell has described as "immeasurably subtle and profound".- Life...

 challenged them with several famous paradoxes
Zeno's paradoxes
Zeno's paradoxes are a set of problems generally thought to have been devised by Greek philosopher Zeno of Elea to support Parmenides's doctrine that "all is one" and that, contrary to the evidence of our senses, the belief in plurality and change is mistaken, and in particular that motion is...

. Aristotle responded to these paradoxes by developing the notion of a potential countable infinity, as well as the infinitely divisible continuum. Unlike the eternal and unchanging cycles of time, he believed the world was bounded by the celestial spheres, and thus magnitude was only finitely multiplicative.

The Indian philosopher
Indian philosophy
India has a rich and diverse philosophical tradition dating back to ancient times. According to Radhakrishnan, the earlier Upanisads constitute "...the earliest philosophical compositions of the world."...

 Kanada
Kanada
It has been claimed that Kashyapa, later known as Kanada was a Hindu sage and philosopher who founded the philosophical school of Vaisheshika. He talked of Dvyanuka and tryanuka...

, founder of the Vaisheshika
Vaisheshika
Vaisheshika or ' is one of the six Hindu schools of philosophy of India. Historically, it has been closely associated with the Hindu school of logic, Nyaya....

 school, developed a theory of atomism
Atomism
Atomism is a natural philosophy that developed in several ancient traditions. The atomists theorized that the natural world consists of two fundamental parts: indivisible atoms and empty void.According to Aristotle, atoms are indestructible and immutable and there are an infinite variety of shapes...

 and proposed that light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 and heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

 were varieties of the same substance. In the 5th century AD, the Buddhist atomist
Buddhist atomism
Buddhist atomism is a school of atomistic Buddhist philosophy that flourished on the Indian subcontinent during two major periods . During the first phase, which began to develop prior to the 4th century BCE, Buddhist atomism had a very qualitative, Aristotelian-style atomic theory. This form of...

 philosopher Dignāga
Dignaga
Dignāga was an Indian scholar and one of the Buddhist founders of Indian logic.He was born into a Brahmin family in Simhavakta near Kanchi Kanchipuram), and very little is known of his early years, except that he took as his spiritual preceptor Nagadatta of the Vatsiputriya school, before being...

 proposed atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s to be point-sized, durationless, and made of energy. They denied the existence of substantial matter and proposed that movement consisted of momentary flashes of a stream of energy.

The theory of temporal finitism
Temporal finitism
Temporal finitism is the idea that time is finite. The context of the idea is the pre-modern era, before mathematicians had understood the concept of infinity and before physical cosmology....

 was inspired by the doctrine of Creation shared by the three Abrahamic religions
Abrahamic religions
Abrahamic religions are the monotheistic faiths emphasizing and tracing their common origin to Abraham or recognizing a spiritual tradition identified with him...

: Judaism
Judaism
Judaism ) is the "religion, philosophy, and way of life" of the Jewish people...

, Christianity
Christianity
Christianity is a monotheistic religion based on the life and teachings of Jesus as presented in canonical gospels and other New Testament writings...

 and Islam
Islam
Islam . The most common are and .   : Arabic pronunciation varies regionally. The first vowel ranges from ~~. The second vowel ranges from ~~~...

. The Christian philosopher
Christian philosophy
Christian philosophy may refer to any development in philosophy that is characterised by coming from a Christian tradition.- Origins of Christian philosophy :...

, John Philoponus
John Philoponus
John Philoponus , also known as John the Grammarian or John of Alexandria, was a Christian and Aristotelian commentator and the author of a considerable number of philosophical treatises and theological works...

, presented the philosophical arguments against the ancient Greek notion of an infinite past and future. Philoponus' arguments against an infinite past were used by the early Muslim philosopher
Early Islamic philosophy
Early Islamic philosophy or classical Islamic philosophy is a period of intense philosophical development beginning in the 2nd century AH of the Islamic calendar and lasting until the 6th century AH...

, Al-Kindi
Al-Kindi
' , known as "the Philosopher of the Arabs", was a Muslim Arab philosopher, mathematician, physician, and musician. Al-Kindi was the first of the Muslim peripatetic philosophers, and is unanimously hailed as the "father of Islamic or Arabic philosophy" for his synthesis, adaptation and promotion...

 (Alkindus); the Jewish philosopher
Jewish philosophy
Jewish philosophy , includes all philosophy carried out by Jews, or, in relation to the religion of Judaism. Jewish philosophy, until modern Enlightenment and Emancipation, was pre-occupied with attempts to reconcile coherent new ideas into the tradition of Rabbinic Judaism; thus organizing...

, Saadia Gaon
Saadia Gaon
Saʻadiah ben Yosef Gaon was a prominent rabbi, Jewish philosopher, and exegete of the Geonic period.The first important rabbinic figure to write extensively in Arabic, he is considered the founder of Judeo-Arabic literature...

 (Saadia ben Joseph); and the Muslim theologian
Kalam
ʿIlm al-Kalām is the Islamic philosophical discipline of seeking theological principles through dialectic. Kalām in Islamic practice relates to the discipline of seeking theological knowledge through debate and argument. A scholar of kalām is referred to as a mutakallim...

, Al-Ghazali
Al-Ghazali
Abu Hāmed Mohammad ibn Mohammad al-Ghazzālī , known as Algazel to the western medieval world, born and died in Tus, in the Khorasan province of Persia was a Persian Muslim theologian, jurist, philosopher, and mystic....

 (Algazel). Borrowing from Aristotle's Physics and Metaphysics, they employed two logical arguments against an infinite past, the first being the "argument from the impossibility of the existence of an actual infinite", which states:
"An actual infinite cannot exist."
"An infinite temporal regress of events is an actual infinite."
" An infinite temporal regress of events cannot exist."


The second argument, the "argument from the impossibility of completing an actual infinite by successive addition", states:
"An actual infinite cannot be completed by successive addition."
"The temporal series of past events has been completed by successive addition."
" The temporal series of past events cannot be an actual infinite."


Both arguments were adopted by Christian philosophers and theologians, and the second argument in particular became more famous after it was adopted by Immanuel Kant
Immanuel Kant
Immanuel Kant was a German philosopher from Königsberg , researching, lecturing and writing on philosophy and anthropology at the end of the 18th Century Enlightenment....

 in his thesis of the first antinomy
Antinomy
Antinomy literally means the mutual incompatibility, real or apparent, of two laws. It is a term used in logic and epistemology....

 concerning time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

.

Astronomical models



Astronomical models of the universe were proposed soon after astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 began with the Babylonian astronomers, who viewed the universe as a flat disk
Flat Earth
The Flat Earth model is a belief that the Earth's shape is a plane or disk. Most ancient cultures have had conceptions of a flat Earth, including Greece until the classical period, the Bronze Age and Iron Age civilizations of the Near East until the Hellenistic period, India until the Gupta period ...

 floating in the ocean, and this forms the premise for early Greek maps like those of Anaximander
Anaximander
Anaximander was a pre-Socratic Greek philosopher who lived in Miletus, a city of Ionia; Milet in modern Turkey. He belonged to the Milesian school and learned the teachings of his master Thales...

 and Hecataeus of Miletus.

Later Greek
Ancient Greece
Ancient Greece is a civilization belonging to a period of Greek history that lasted from the Archaic period of the 8th to 6th centuries BC to the end of antiquity. Immediately following this period was the beginning of the Early Middle Ages and the Byzantine era. Included in Ancient Greece is the...

 philosophers, observing the motions of the heavenly bodies, were concerned with developing models of the universe based more profoundly on empirical evidence. The first coherent model was proposed by Eudoxus of Cnidos. According to Aristotle's physical interpretation of the model, celestial spheres
Celestial spheres
The celestial spheres, or celestial orbs, were the fundamental entities of the cosmological models developed by Plato, Eudoxus, Aristotle, Ptolemy, Copernicus and others...

 eternally rotate with uniform motion
Uniform circular motion
In physics, uniform circular motion describes the motion of a body traversing a circular path at constant speed. The distance of the body from the axis of rotation remains constant at all times. Though the body's speed is constant, its velocity is not constant: velocity, a vector quantity, depends...

 around a stationary Earth. Normal matter, is entirely contained within the terrestrial sphere. This model was also refined by Callippus
Callippus
Callippus or Calippus was a Greek astronomer and mathematician.Callippus was born at Cyzicus, and studied under Eudoxus of Cnidus at the Academy of Plato. He also worked with Aristotle at the Lyceum, which means that he was active in Athens prior to Aristotle's death in 322...

 and after concentric spheres were abandoned, it was brought into nearly perfect agreement with astronomical observations by Ptolemy
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

. The success of such a model is largely due to the mathematical fact that any function (such as the position of a planet) can be decomposed into a set of circular functions (the Fourier modes). Other Greek scientists, such as the Pythagorean philosopher Philolaus
Philolaus
Philolaus was a Greek Pythagorean and Presocratic philosopher. He argued that all matter is composed of limiting and limitless things, and that the universe is determined by numbers. He is credited with originating the theory that the earth was not the center of the universe.-Life:Philolaus is...

 postulated that at the center of the universe was a "central fire" around which the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 and Planets revolved in uniform circular motion.
The Greek astronomer
Greek astronomy
Greek astronomy is astronomy written in the Greek language in classical antiquity. Greek astronomy is understood to include the ancient Greek, Hellenistic, Greco-Roman, and Late Antiquity eras. It is not limited geographically to Greece or to ethnic Greeks, as the Greek language had become the...

 Aristarchus of Samos
Aristarchus of Samos
Aristarchus, or more correctly Aristarchos , was a Greek astronomer and mathematician, born on the island of Samos, in Greece. He presented the first known heliocentric model of the solar system, placing the Sun, not the Earth, at the center of the known universe...

 was the first known individual to propose a heliocentric
Heliocentrism
Heliocentrism, or heliocentricism, is the astronomical model in which the Earth and planets revolve around a stationary Sun at the center of the universe. The word comes from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center...

 model of the universe. Though the original text has been lost, a reference in Archimedes' book The Sand Reckoner describes Aristarchus' heliocentric theory. Archimedes
Archimedes
Archimedes of Syracuse was a Greek mathematician, physicist, engineer, inventor, and astronomer. Although few details of his life are known, he is regarded as one of the leading scientists in classical antiquity. Among his advances in physics are the foundations of hydrostatics, statics and an...

 wrote: (translated into English)


You King Gelon are aware the 'universe' is the name given by most astronomers to the sphere the center of which is the center of the Earth, while its radius is equal to the straight line between the center of the Sun and the center of the Earth. This is the common account as you have heard from astronomers. But Aristarchus has brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the 'universe' just mentioned. His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the circumference of a circle, the Sun lying in the middle of the orbit, and that the sphere of fixed stars, situated about the same center as the Sun, is so great that the circle in which he supposes the Earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface.


Aristarchus thus believed the stars to be very far away, and saw this as the reason why there was no visible parallax, that is, an observed movement of the stars relative to each other as the Earth moved around the Sun. The stars are in fact much farther away than the distance that was generally assumed in ancient times, which is why stellar parallax is only detectable with telescopes. The geocentric model, consistent with planetary parallax, was assumed to be an explanation for the unobservability of the parallel phenomenon, stellar parallax. The rejection of the heliocentric view was apparently quite strong, as the following passage from Plutarch suggests (On the Apparent Face in the Orb of the Moon):


Cleanthes
Cleanthes
Cleanthes , of Assos, was a Greek Stoic philosopher and the successor to Zeno as the second head of the Stoic school in Athens. Originally a boxer, he came to Athens where he took up philosophy, listening to Zeno's lectures. He supported himself by working as water-carrier at night. After the...

 [a contemporary of Aristarchus and head of the Stoics] thought it was the duty of the Greeks to indict Aristarchus of Samos on the charge of impiety for putting in motion the Hearth of the universe [i.e. the earth], . . . supposing the heaven to remain at rest and the earth to revolve in an oblique circle, while it rotates, at the same time, about its own axis. [1]


The only other astronomer from antiquity known by name who supported Aristarchus' heliocentric model was Seleucus of Seleucia
Seleucus of Seleucia
Seleucus of Seleucia was a Hellenistic astronomer and philosopher. Coming from Seleucia on the Tigris, the capital of the Seleucid empire, or, alternatively, Seleukia on the Red Sea, he is best known as a proponent of heliocentrism and for his theory of the origin of tides.- Heliocentric theory...

, a Hellenized
Hellenization
Hellenization is a term used to describe the spread of ancient Greek culture, and, to a lesser extent, language. It is mainly used to describe the spread of Hellenistic civilization during the Hellenistic period following the campaigns of Alexander the Great of Macedon...

 Babylonia
Babylonia
Babylonia was an ancient cultural region in central-southern Mesopotamia , with Babylon as its capital. Babylonia emerged as a major power when Hammurabi Babylonia was an ancient cultural region in central-southern Mesopotamia (present-day Iraq), with Babylon as its capital. Babylonia emerged as...

n astronomer who lived a century after Aristarchus. According to Plutarch
Plutarch
Plutarch then named, on his becoming a Roman citizen, Lucius Mestrius Plutarchus , c. 46 – 120 AD, was a Greek historian, biographer, essayist, and Middle Platonist known primarily for his Parallel Lives and Moralia...

, Seleucus was the first to prove the heliocentric system through reasoning, but it is not known what arguments he used. Seleucus' arguments for a heliocentric theory were probably related to the phenomenon of tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....

s. According to Strabo
Strabo
Strabo, also written Strabon was a Greek historian, geographer and philosopher.-Life:Strabo was born to an affluent family from Amaseia in Pontus , a city which he said was situated the approximate equivalent of 75 km from the Black Sea...

 (1.1.9), Seleucus was the first to state that the tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....

s are due to the attraction of the Moon, and that the height of the tides depends on the Moon's position relative to the Sun. Alternatively, he may have proved the heliocentric theory by determining the constants of a geometric
Geometry
Geometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of pre-modern mathematics, the other being the study of numbers ....

 model for the heliocentric theory and by developing methods to compute planetary positions using this model, like what Nicolaus Copernicus later did in the 16th century. During the Middle Ages
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

, heliocentric models may have also been proposed by the Indian astronomer, Aryabhata
Aryabhata
Aryabhata was the first in the line of great mathematician-astronomers from the classical age of Indian mathematics and Indian astronomy...

, and by the Persian astronomers
Islamic astronomy
Islamic astronomy or Arabic astronomy comprises the astronomical developments made in the Islamic world, particularly during the Islamic Golden Age , and mostly written in the Arabic language. These developments mostly took place in the Middle East, Central Asia, Al-Andalus, and North Africa, and...

, Albumasar
Ja'far ibn Muhammad Abu Ma'shar al-Balkhi
Abū Maʿshar, Jaʿfar ibn Muḥammad al-Balkhī , was a Persian astrologer, astronomer, and Islamic philosopher, thought to be the greatest astrologer of the Abbasid court in Baghdad...

 and Al-Sijzi
Al-Sijzi
Abu Sa'id Ahmed ibn Mohammed ibn Abd Jalil Sijzi was a Persian astronomer and mathematician from Sistan, a region lying in the south-west of Afghanistan and south-east of Iran....

.

The Aristotelian model was accepted in the Western world
Western world
The Western world, also known as the West and the Occident , is a term referring to the countries of Western Europe , the countries of the Americas, as well all countries of Northern and Central Europe, Australia and New Zealand...

 for roughly two millennia, until Copernicus revived Aristarchus' theory that the astronomical data could be explained more plausibly if the earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 rotated on its axis and if the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 were placed at the center of the universe.
As noted by Copernicus himself, the suggestion that the Earth rotates was very old, dating at least to Philolaus
Philolaus
Philolaus was a Greek Pythagorean and Presocratic philosopher. He argued that all matter is composed of limiting and limitless things, and that the universe is determined by numbers. He is credited with originating the theory that the earth was not the center of the universe.-Life:Philolaus is...

 (c. 450 BC), Heraclides Ponticus
Heraclides Ponticus
Heraclides Ponticus , also known as Herakleides and Heraklides of Pontus, was a Greek philosopher and astronomer who lived and died at Heraclea Pontica, now Karadeniz Ereğli, Turkey. He is best remembered for proposing that the earth rotates on its axis, from west to east, once every 24 hours...

 (c. 350 BC) and Ecphantus the Pythagorean
Ecphantus the Pythagorean
Ecphantus or Ecphantos is a shadowy Greek pre-Socratic philosopher. He may not have actually existed. He is identified as a Pythagorean of the 4th century BCE, and as a supporter of the heliocentric theory. Described as from Syracuse, this may or may not be the same figure as the attested...

. Roughly a century before Copernicus, Christian scholar Nicholas of Cusa
Nicholas of Cusa
Nicholas of Kues , also referred to as Nicolaus Cusanus and Nicholas of Cusa, was a cardinal of the Catholic Church from Germany , a philosopher, theologian, jurist, mathematician, and an astronomer. He is widely considered one of the great geniuses and polymaths of the 15th century...

 also proposed that the Earth rotates on its axis in his book, On Learned Ignorance (1440). Aryabhata (476–550), Brahmagupta
Brahmagupta
Brahmagupta was an Indian mathematician and astronomer who wrote many important works on mathematics and astronomy. His best known work is the Brāhmasphuṭasiddhānta , written in 628 in Bhinmal...

 (598–668), Albumasar and Al-Sijzi
Al-Sijzi
Abu Sa'id Ahmed ibn Mohammed ibn Abd Jalil Sijzi was a Persian astronomer and mathematician from Sistan, a region lying in the south-west of Afghanistan and south-east of Iran....

, also proposed that the Earth rotates on its axis. The first empirical evidence
Empirical research
Empirical research is a way of gaining knowledge by means of direct and indirect observation or experience. Empirical evidence can be analyzed quantitatively or qualitatively...

 for the Earth's rotation on its axis, using the phenomenon of comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s, was given by Tusi
Nasir al-Din al-Tusi
Khawaja Muḥammad ibn Muḥammad ibn Ḥasan Ṭūsī , better known as Naṣīr al-Dīn al-Ṭūsī , was a Persian polymath and prolific writer: an astronomer, biologist, chemist, mathematician, philosopher, physician, physicist, scientist, theologian and Marja Taqleed...

 (1201–1274) and Ali Qushji (1403–1474).

This cosmology was accepted by Isaac Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...

, Christiaan Huygens and later scientists. Edmund Halley (1720) and Jean-Philippe de Cheseaux
Jean-Philippe de Cheseaux
Jean-Philippe Loys de Chéseaux was an astronomer from Lausanne in Switzerland. In 1746 he presented a list of nebulae, eight of which were his own new discoveries, to the Académie Française des Sciences. The list was noted privately by Le Gentil in 1759, but only made public in 1892 by Guillaume...

 (1744) noted independently that the assumption of an infinite space filled uniformly with stars would lead to the prediction that the nighttime sky would be as bright as the sun itself; this became known as Olbers' paradox
Olbers' paradox
In astrophysics and physical cosmology, Olbers' paradox is the argument that the darkness of the night sky conflicts with the assumption of an infinite and eternal static universe. It is one of the pieces of evidence for a non-static universe such as the current Big Bang model. The argument is also...

 in the 19th century. Newton believed that an infinite space uniformly filled with matter would cause infinite forces and instabilities causing the matter to be crushed inwards under its own gravity. This instability was clarified in 1902 by the Jeans instability
Jeans instability
In physics, the Jeans instability causes the collapse of interstellar gas clouds and subsequent star formation. It occurs when the internal gas pressure is not strong enough to prevent gravitational collapse of a region filled with matter...

 criterion. One solution to these paradoxes is the Charlier universe
Carl Charlier
Carl Vilhelm Ludwig Charlier was a Swedish astronomer.He received his Ph.D. from Uppsala University in 1887, later worked there and at the Stockholm Observatory and was Professor of Astronomy and Director of the Observatory at Lund University from 1897.He made extensive statistical studies of the...

, in which the matter is arranged hierarchically (systems of orbiting bodies that are themselves orbiting in a larger system, ad infinitum) in a fractal
Fractal
A fractal has been defined as "a rough or fragmented geometric shape that can be split into parts, each of which is a reduced-size copy of the whole," a property called self-similarity...

 way such that the universe has a negligibly small overall density; such a cosmological model had also been proposed earlier in 1761 by Johann Heinrich Lambert
Johann Heinrich Lambert
Johann Heinrich Lambert was a Swiss mathematician, physicist, philosopher and astronomer.Asteroid 187 Lamberta was named in his honour.-Biography:...

. A significant astronomical advance of the 18th century was the realization by Thomas Wright
Thomas Wright (astronomer)
Thomas Wright was an English astronomer, mathematician, instrument maker, architect and garden designer. He was the first to describe the shape of the Milky Way and speculate that faint nebulae were distant galaxies....

, Immanuel Kant
Immanuel Kant
Immanuel Kant was a German philosopher from Königsberg , researching, lecturing and writing on philosophy and anthropology at the end of the 18th Century Enlightenment....

 and others of nebulae.

The modern era of physical cosmology
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...

 began in 1917, when Albert Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...

 first applied his general theory of relativity to model the structure and dynamics of the universe.

Theoretical models


Of the four fundamental interaction
Fundamental interaction
In particle physics, fundamental interactions are the ways that elementary particles interact with one another...

s, gravitation
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...

 is dominant at cosmological length scales; that is, the other three forces are believed to play a negligible role in determining structures at the level of planets, stars, galaxies and larger-scale structures. Since all matter and energy gravitate, gravity's effects are cumulative; by contrast, the effects of positive and negative charges tend to cancel one another, making electromagnetism relatively insignificant on cosmological length scales. The remaining two interactions, the weak and strong nuclear forces, decline very rapidly with distance; their effects are confined mainly to sub-atomic length scales.

General theory of relativity


Given gravitation's predominance in shaping cosmological structures, accurate predictions of the universe's past and future require an accurate theory of gravitation. The best theory available is Albert Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...

's general theory of relativity, which has passed all experimental tests hitherto. However, since rigorous experiments have not been carried out on cosmological length scales, general relativity could conceivably be inaccurate. Nevertheless, its cosmological predictions appear to be consistent with observations, so there is no compelling reason to adopt another theory.

General relativity provides a set of ten nonlinear partial differential equations for the spacetime metric
Metric tensor (general relativity)
In general relativity, the metric tensor is the fundamental object of study. It may loosely be thought of as a generalization of the gravitational field familiar from Newtonian gravitation...

 (Einstein's field equations
Einstein field equations
The Einstein field equations or Einstein's equations are a set of ten equations in Albert Einstein's general theory of relativity which describe the fundamental interaction of gravitation as a result of spacetime being curved by matter and energy...

) that must be solved from the distribution of mass-energy and momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...

 throughout the universe. Since these are unknown in exact detail, cosmological models have been based on the cosmological principle
Cosmological Principle
In modern physical cosmology, the cosmological principle is the working assumption that observers on Earth do not occupy an unusual or privileged location within the universe as a whole, judged as observers of the physical phenomena produced by uniform and universal laws of physics...

, which states that the universe is homogeneous and isotropic. In effect, this principle asserts that the gravitational effects of the various galaxies making up the universe are equivalent to those of a fine dust
Dust solution
In general relativity, a dust solution is an exact solution of the Einstein field equation in which the gravitational field is produced entirely by the mass, momentum, and stress density of a perfect fluid which has positive mass density but vanishing pressure...

 distributed uniformly throughout the universe with the same average density. The assumption of a uniform dust makes it easy to solve Einstein's field equations and predict the past and future of the universe on cosmological time scales.

Einstein's field equations include a cosmological constant
Cosmological constant
In physical cosmology, the cosmological constant was proposed by Albert Einstein as a modification of his original theory of general relativity to achieve a stationary universe...

 (Λ), that corresponds to an energy density of empty space. Depending on its sign, the cosmological constant can either slow (negative Λ) or accelerate (positive Λ) the expansion of the universe
Metric expansion of space
The metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...

. Although many scientists, including Einstein, had speculated that Λ was zero, recent astronomical observations of type Ia supernova
Type Ia supernova
A Type Ia supernova is a sub-category of supernovae, which in turn are a sub-category of cataclysmic variable stars, that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion...

e have detected a large amount of "dark energy
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...

" that is accelerating the universe's expansion. Preliminary studies suggest that this dark energy corresponds to a positive Λ, although alternative theories cannot be ruled out as yet. Russian physicist
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

 Zel'dovich
Yakov Borisovich Zel'dovich
Yakov Borisovich Zel'dovich was a prolific Soviet physicist born in Belarus. He played an important role in the development of Soviet nuclear and thermonuclear weapons, and made important contributions to the fields of adsorption and catalysis, shock waves, nuclear physics, particle physics,...

 suggested that Λ is a measure of the zero-point energy
Zero-point energy
Zero-point energy is the lowest possible energy that a quantum mechanical physical system may have; it is the energy of its ground state. All quantum mechanical systems undergo fluctuations even in their ground state and have an associated zero-point energy, a consequence of their wave-like nature...

 associated with virtual particle
Virtual particle
In physics, a virtual particle is a particle that exists for a limited time and space. The energy and momentum of a virtual particle are uncertain according to the uncertainty principle...

s of quantum field theory
Quantum field theory
Quantum field theory provides a theoretical framework for constructing quantum mechanical models of systems classically parametrized by an infinite number of dynamical degrees of freedom, that is, fields and many-body systems. It is the natural and quantitative language of particle physics and...

, a pervasive vacuum energy
Vacuum energy
Vacuum energy is an underlying background energy that exists in space even when the space is devoid of matter . The concept of vacuum energy has been deduced from the concept of virtual particles, which is itself derived from the energy-time uncertainty principle...

 that exists everywhere, even in empty space. Evidence for such zero-point energy is observed in the Casimir effect
Casimir effect
In quantum field theory, the Casimir effect and the Casimir–Polder force are physical forces arising from a quantized field. The typical example is of two uncharged metallic plates in a vacuum, like capacitors placed a few micrometers apart, without any external electromagnetic field...

.

Special relativity and space-time



The universe has at least three spatial
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

 and one temporal (time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

) dimension. It was long thought that the spatial and temporal dimensions were different in nature and independent of one another. However, according to the special theory of relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

, spatial and temporal separations are interconvertible (within limits) by changing one's motion.

To understand this interconversion, it is helpful to consider the analogous interconversion of spatial separations along the three spatial dimensions. Consider the two endpoints of a rod of length L. The length can be determined from the differences in the three coordinates Δx, Δy and Δz of the two endpoints in a given reference frame


using the Pythagorean theorem
Pythagorean theorem
In mathematics, the Pythagorean theorem or Pythagoras' theorem is a relation in Euclidean geometry among the three sides of a right triangle...

. In a rotated reference frame, the coordinate differences differ, but they give the same length


Thus, the coordinates differences (Δx, Δy, Δz) and (Δξ, Δη, Δζ) are not intrinsic to the rod, but merely reflect the reference frame used to describe it; by contrast, the length L is an intrinsic property of the rod. The coordinate differences can be changed without affecting the rod, by rotating one's reference frame.

The analogy in spacetime
Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...

 is called the interval between two events; an event is defined as a point in spacetime, a specific position in space and a specific moment in time. The spacetime interval between two events is given by


where c is the speed of light. According to special relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

, one can change a spatial and time separation (L1, Δt1) into another (L2, Δt2) by changing one's reference frame, as long as the change maintains the spacetime interval s. Such a change in reference frame corresponds to changing one's motion; in a moving frame, lengths and times are different from their counterparts in a stationary reference frame. The precise manner in which the coordinate and time differences change with motion is described by the Lorentz transformation
Lorentz transformation
In physics, the Lorentz transformation or Lorentz-Fitzgerald transformation describes how, according to the theory of special relativity, two observers' varying measurements of space and time can be converted into each other's frames of reference. It is named after the Dutch physicist Hendrik...

.

Solving Einstein's field equations


The distances between the spinning galaxies increase with time, but the distances between the stars within each galaxy stay roughly the same, due to their gravitational interactions. This animation illustrates a closed Friedmann universe with zero cosmological constant
Cosmological constant
In physical cosmology, the cosmological constant was proposed by Albert Einstein as a modification of his original theory of general relativity to achieve a stationary universe...

 Λ; such a universe oscillates between a Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

 and a Big Crunch
Big Crunch
In physical cosmology, the Big Crunch is one possible scenario for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately ending as a black hole singularity.- Overview :...

.

In non-Cartesian (non-square) or curved coordinate systems, the Pythagorean theorem holds only on infinitesimal length scales and must be augmented with a more general metric tensor
Metric tensor
In the mathematical field of differential geometry, a metric tensor is a type of function defined on a manifold which takes as input a pair of tangent vectors v and w and produces a real number g in a way that generalizes many of the familiar properties of the dot product of vectors in Euclidean...

 gμν, which can vary from place to place and which describes the local geometry in the particular coordinate system. However, assuming the cosmological principle
Cosmological Principle
In modern physical cosmology, the cosmological principle is the working assumption that observers on Earth do not occupy an unusual or privileged location within the universe as a whole, judged as observers of the physical phenomena produced by uniform and universal laws of physics...

 that the universe is homogeneous and isotropic everywhere, every point in space is like every other point; hence, the metric tensor must be the same everywhere. That leads to a single form for the metric tensor, called the Friedmann–Lemaître–Robertson–Walker metric


where (r, θ, φ) correspond to a spherical coordinate system
Spherical coordinate system
In mathematics, a spherical coordinate system is a coordinate system for three-dimensional space where the position of a point is specified by three numbers: the radial distance of that point from a fixed origin, its inclination angle measured from a fixed zenith direction, and the azimuth angle of...

. This metric
Metric (mathematics)
In mathematics, a metric or distance function is a function which defines a distance between elements of a set. A set with a metric is called a metric space. A metric induces a topology on a set but not all topologies can be generated by a metric...

 has only two undetermined parameters: an overall length scale R that can vary with time, and a curvature index k that can be only 0, 1 or −1, corresponding to flat Euclidean geometry
Euclidean geometry
Euclidean geometry is a mathematical system attributed to the Alexandrian Greek mathematician Euclid, which he described in his textbook on geometry: the Elements. Euclid's method consists in assuming a small set of intuitively appealing axioms, and deducing many other propositions from these...

, or spaces of positive or negative curvature
Curvature
In mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry. Intuitively, curvature is the amount by which a geometric object deviates from being flat, or straight in the case of a line, but this is defined in different ways depending on the context...

. In cosmology, solving for the history of the universe is done by calculating R as a function of time, given k and the value of the cosmological constant
Cosmological constant
In physical cosmology, the cosmological constant was proposed by Albert Einstein as a modification of his original theory of general relativity to achieve a stationary universe...

 Λ, which is a (small) parameter in Einstein's field equations. The equation describing how R varies with time is known as the Friedmann equation, after its inventor, Alexander Friedmann.

The solutions for R(t) depend on k and Λ, but some qualitative features of such solutions are general. First and most importantly, the length scale R of the universe can remain constant only if the universe is perfectly isotropic with positive curvature (k=1) and has one precise value of density everywhere, as first noted by Albert Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...

. However, this equilibrium is unstable and since the universe is known to be inhomogeneous on smaller scales, R must change, according to general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

. When R changes, all the spatial distances in the universe change in tandem; there is an overall expansion or contraction of space itself. This accounts for the observation that galaxies appear to be flying apart; the space between them is stretching. The stretching of space also accounts for the apparent paradox that two galaxies can be 40 billion light years apart, although they started from the same point 13.7 billion years ago and never moved faster than the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

.

Second, all solutions suggest that there was a gravitational singularity
Gravitational singularity
A gravitational singularity or spacetime singularity is a location where the quantities that are used to measure the gravitational field become infinite in a way that does not depend on the coordinate system...

 in the past, when R goes to zero and matter and energy became infinitely dense. It may seem that this conclusion is uncertain since it is based on the questionable assumptions of perfect homogeneity and isotropy (the cosmological principle) and that only the gravitational interaction is significant. However, the Penrose–Hawking singularity theorems show that a singularity should exist for very general conditions. Hence, according to Einstein's field equations, R grew rapidly from an unimaginably hot, dense state that existed immediately following this singularity (when R had a small, finite value); this is the essence of the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

 model of the universe. A common misconception is that the Big Bang model predicts that matter and energy exploded from a single point in space and time; that is false. Rather, space itself was created in the Big Bang and imbued with a fixed amount of energy and matter distributed uniformly throughout; as space expands (i.e., as R(t) increases), the density of that matter and energy decreases.

Space has no boundary – that is empirically more certain than any external observation. However, that does not imply that space is infinite...(translated, original German)
Bernhard Riemann
Bernhard Riemann
Georg Friedrich Bernhard Riemann was an influential German mathematician who made lasting contributions to analysis and differential geometry, some of them enabling the later development of general relativity....

 (Habilitationsvortrag, 1854)


Third, the curvature index k determines the sign of the mean spatial curvature of spacetime
Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...

 averaged over length scales greater than a billion light years. If k=1, the curvature is positive and the universe has a finite volume. Such universes are often visualized as a three-dimensional sphere S3 embedded in a four-dimensional space
3-sphere
In mathematics, a 3-sphere is a higher-dimensional analogue of a sphere. It consists of the set of points equidistant from a fixed central point in 4-dimensional Euclidean space...

. Conversely, if k is zero or negative, the universe may have infinite volume, depending on its overall topology
Topology
Topology is a major area of mathematics concerned with properties that are preserved under continuous deformations of objects, such as deformations that involve stretching, but no tearing or gluing...

. It may seem counter-intuitive that an infinite and yet infinitely dense universe could be created in a single instant at the Big Bang when R=0, but exactly that is predicted mathematically when k does not equal 1. For comparison, an infinite plane has zero curvature but infinite area, whereas an infinite cylinder is finite in one direction and a torus
Torus
In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle...

 is finite in both. A toroidal universe could behave like a normal universe with periodic boundary conditions
Periodic boundary conditions
In mathematical models and computer simulations, periodic boundary conditions are a set of boundary conditions that are often used to simulate a large system by modelling a small part that is far from its edge...

, as seen in "wrap-around" video games
Wraparound (Video Games)
Wraparound, in video games, is a gameplay variation on the single-screen in which space is finite but unbounded; objects leaving one side of the screen immediately reappear on the opposite side, maintaining speed and trajectory...

 such as Asteroids; a traveler crossing an outer "boundary" of space going outwards would reappear instantly at another point on the boundary moving inwards.
The ultimate fate of the universe
Ultimate fate of the universe
The ultimate fate of the universe is a topic in physical cosmology. Many possible fates are predicted by rival scientific theories, including futures of both finite and infinite duration....

 is still unknown, because it depends critically on the curvature index k and the cosmological constant Λ. If the universe is sufficiently dense, k equals +1, meaning that its average curvature throughout is positive and the universe will eventually recollapse in a Big Crunch
Big Crunch
In physical cosmology, the Big Crunch is one possible scenario for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately ending as a black hole singularity.- Overview :...

, possibly starting a new universe in a Big Bounce
Big Bounce
The Big Bounce is a theoretical scientific model of the formation of the known universe. It is implied by the cyclic model or oscillatory universe interpretation of the Big Bang where the first cosmological event was the result of the collapse of a previous universe.- Expansion and contraction...

. Conversely, if the universe is insufficiently dense, k equals 0 or −1 and the universe will expand forever, cooling off and eventually becoming inhospitable for all life, as the stars die and all matter coalesces into black holes (the Big Freeze
Future of an expanding universe
Observations suggest that the expansion of the universe will continue forever. If so, the universe will cool as it expands, eventually becoming too cold to sustain life. For this reason, this future scenario is popularly called the Big Freeze....

 and the heat death of the universe
Heat death of the universe
The heat death of the universe is a suggested ultimate fate of the universe, in which the universe has diminished to a state of no thermodynamic free energy and therefore can no longer sustain motion or life. Heat death does not imply any particular absolute temperature; it only requires that...

). As noted above, recent data suggests that the expansion speed of the universe is not decreasing as originally expected, but increasing; if this continues indefinitely, the universe will eventually rip itself to shreds (the Big Rip
Big Rip
The Big Rip is a cosmological hypothesis first published in 2003, about the ultimate fate of the universe, in which the matter of the universe, from stars and galaxies to atoms and subatomic particles, is progressively torn apart by the expansion of the universe at a certain time in the future...

). Experimentally, the universe has an overall density that is very close to the critical value between recollapse and eternal expansion; more careful astronomical observations are needed to resolve the question.

Big Bang model


The prevailing Big Bang model accounts for many of the experimental observations described above, such as the correlation of distance and redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...

 of galaxies, the universal ratio of hydrogen:helium atoms, and the ubiquitous, isotropic microwave radiation background. As noted above, the redshift arises from the metric expansion of space
Metric expansion of space
The metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...

; as the space itself expands, the wavelength of a photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

 traveling through space likewise increases, decreasing its energy. The longer a photon has been traveling, the more expansion it has undergone; hence, older photons from more distant galaxies are the most red-shifted. Determining the correlation between distance and redshift is an important problem in experimental physical cosmology
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...

.

Other experimental observations can be explained by combining the overall expansion of space with nuclear
Nuclear physics
Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...

 and atomic physics
Atomic physics
Atomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. It is primarily concerned with the arrangement of electrons around the nucleus and...

. As the universe expands, the energy density of the electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

 decreases more quickly than does that of matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

, since the energy of a photon decreases with its wavelength. Thus, although the energy density of the universe is now dominated by matter, it was once dominated by radiation; poetically speaking, all was light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

. As the universe expanded, its energy density decreased and it became cooler; as it did so, the elementary particle
Elementary particle
In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...

s of matter could associate stably into ever larger combinations. Thus, in the early part of the matter-dominated era, stable proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s and neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s formed, which then associated into atomic nuclei. At this stage, the matter in the universe was mainly a hot, dense plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

 of negative electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s, neutral neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...

s and positive nuclei. Nuclear reaction
Nuclear reaction
In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce products different from the initial particles...

s among the nuclei led to the present abundances of the lighter nuclei, particularly hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

, deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

, and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

. Eventually, the electrons and nuclei combined to form stable atoms, which are transparent to most wavelengths of radiation; at this point, the radiation decoupled from the matter, forming the ubiquitous, isotropic background of microwave radiation observed today.

Other observations are not answered definitively by known physics. According to the prevailing theory, a slight imbalance of matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

 over antimatter
Antimatter
In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles...

 was present in the universe's creation, or developed very shortly thereafter, possibly due to the CP violation
CP violation
In particle physics, CP violation is a violation of the postulated CP-symmetry: the combination of C-symmetry and P-symmetry . CP-symmetry states that the laws of physics should be the same if a particle were interchanged with its antiparticle , and left and right were swapped...

 that has been observed by particle physicists
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...

. Although the matter and antimatter mostly annihilated one another, producing photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s, a small residue of matter survived, giving the present matter-dominated universe. Several lines of evidence also suggest that a rapid cosmic inflation
Cosmic inflation
In physical cosmology, cosmic inflation, cosmological inflation or just inflation is the theorized extremely rapid exponential expansion of the early universe by a factor of at least 1078 in volume, driven by a negative-pressure vacuum energy density. The inflationary epoch comprises the first part...

 of the universe occurred very early in its history (roughly 10−35 seconds after its creation). Recent observations also suggest that the cosmological constant
Cosmological constant
In physical cosmology, the cosmological constant was proposed by Albert Einstein as a modification of his original theory of general relativity to achieve a stationary universe...

 (Λ) is not zero and that the net mass-energy content of the universe is dominated by a dark energy
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...

 and dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

 that have not been characterized scientifically. They differ in their gravitational effects. Dark matter gravitates as ordinary matter does, and thus slows the expansion of the universe; by contrast, dark energy serves to accelerate the universe's expansion.

Multiverse theory



Some speculative theories have proposed that this universe is but one of a set of disconnected universes, collectively denoted as the multiverse
Multiverse
The multiverse is the hypothetical set of multiple possible universes that together comprise all of reality.Multiverse may also refer to:-In fiction:* Multiverse , the fictional multiverse used by DC Comics...

, challenging or enhancing more limited definitions of the universe. Scientific multiverse theories are distinct from concepts such as alternate planes of consciousness
Plane (esotericism)
In esoteric cosmology, a plane, other than the physical plane is conceived as a subtle state of consciousness that transcends the known physical universe....

 and simulated reality
Simulated reality
Simulated reality is the proposition that reality could be simulated—perhaps by computer simulation—to a degree indistinguishable from "true" reality. It could contain conscious minds which may or may not be fully aware that they are living inside a simulation....

, although the idea of a larger universe is not new; for example, Bishop Étienne Tempier
Étienne Tempier
Étienne Tempier was a French bishop of Paris during the 13th century...

 of Paris ruled in 1277 that God could create as many universes as he saw fit, a question that was being hotly debated by the French theologians.

Max Tegmark
Max Tegmark
Max Tegmark is a Swedish-American cosmologist. Tegmark is a professor at the Massachusetts Institute of Technology and belongs to the scientific directorate of the Foundational Questions Institute.-Early life:...

 developed a four part classification scheme for the different types of multiverses that scientists have suggested in various problem domains. An example of such a theory is the chaotic inflation model of the early universe. Another is the many-worlds interpretation
Many-worlds interpretation
The many-worlds interpretation is an interpretation of quantum mechanics that asserts the objective reality of the universal wavefunction, but denies the actuality of wavefunction collapse. Many-worlds implies that all possible alternative histories and futures are real, each representing an...

 of quantum mechanics. Parallel worlds are generated in a manner similar to quantum superposition
Quantum superposition
Quantum superposition is a fundamental principle of quantum mechanics. It holds that a physical system exists in all its particular, theoretically possible states simultaneously; but, when measured, it gives a result corresponding to only one of the possible configurations.Mathematically, it...

 and decoherence, with all states of the wave function being realized in separate worlds. Effectively, the multiverse evolves as a universal wavefunction
Universal wavefunction
The Universal Wavefunction or Universal Wave Function is a term introduced by Hugh Everett in his Princeton PhD thesis The Theory of the Universal Wave Function, and forms a core concept in the relative state interpretation or many-worlds interpretation of quantum mechanics...

. If the big bang that created our mutliverse created an ensemble of multiverses, the wave function of the ensemble would be entangled in this sense.

The least controversial category of multiverse in Tegmark's scheme is Level I, which describes distant space-time events "in our own universe". If space is infinite, or sufficiently large and uniform, identical instances of the history of Earth's entire Hubble volume
Hubble volume
In cosmology, the Hubble volume, or Hubble sphere, is the region of the Universe surrounding an observer beyond which objects recede from the observer at a rate greater than the speed of light, due to the expansion of the Universe....

 occur every so often, simply by chance. Tegmark calculated our nearest so-called doppelgänger
Doppelgänger
In fiction and folklore, a doppelgänger is a paranormal double of a living person, typically representing evil or misfortune...

, is 1010115 meters away from us (a double exponential function larger than a googolplex). In principle, it would be impossible to scientifically verify an identical Hubble volume. However, it does follow as a fairly straightforward consequence from otherwise unrelated scientific observations and theories. Tegmark suggests that statistical analysis exploiting the anthropic principle
Anthropic principle
In astrophysics and cosmology, the anthropic principle is the philosophical argument that observations of the physical Universe must be compatible with the conscious life that observes it. Some proponents of the argument reason that it explains why the Universe has the age and the fundamental...

 provides an opportunity to test multiverse theories in some cases. Generally, science would consider a multiverse theory that posits neither a common point of causation, nor the possibility of interaction between universes, to be an ideal speculation.

Shape of the universe


The shape or geometry
Geometry
Geometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of pre-modern mathematics, the other being the study of numbers ....

 of the universe includes both local geometry in the observable universe
Observable universe
In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light from those objects has had time to reach us since the beginning of the cosmological expansion...

 and global geometry, which we may or may not be able to measure. Shape can refer to curvature and topology
Topology
Topology is a major area of mathematics concerned with properties that are preserved under continuous deformations of objects, such as deformations that involve stretching, but no tearing or gluing...

. More formally, the subject in practice investigates which 3-manifold
3-manifold
In mathematics, a 3-manifold is a 3-dimensional manifold. The topological, piecewise-linear, and smooth categories are all equivalent in three dimensions, so little distinction is made in whether we are dealing with say, topological 3-manifolds, or smooth 3-manifolds.Phenomena in three dimensions...

 corresponds to the spatial section in comoving coordinates of the four-dimensional space-time
Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...

 of the universe. Cosmologists normally work with a given space-like slice of spacetime called the comoving coordinates
Comoving distance
In standard cosmology, comoving distance and proper distance are two closely related distance measures used by cosmologists to define distances between objects...

. In terms of observation, the section of spacetime that can be observed is the backward light cone
Light cone
A light cone is the path that a flash of light, emanating from a single event and traveling in all directions, would take through spacetime...

 (points within the cosmic light horizon, given time to reach a given observer). If the observable universe is smaller than the entire universe (in some models it is many orders of magnitude smaller), one cannot determine the global structure by observation: one is limited to a small patch.

Among the Friedmann–Lemaître–Robertson–Walker (FLRW) models, the presently most popular shape of the Universe found to fit observational data according to cosmologists is the infinite flat model, while other FLRW models include the Poincaré dodecahedral space and the Picard horn
Picard horn
A Picard horn, also called the Picard topology or Picard model, is a theoretical model for theshape of the Universe. It is a horn topology, meaning it has hyperbolic geometry ....

. The data fit by these FLRW models of space especially include the Wilkinson Microwave Anisotropy Probe
Wilkinson Microwave Anisotropy Probe
The Wilkinson Microwave Anisotropy Probe — also known as the Microwave Anisotropy Probe , and Explorer 80 — is a spacecraft which measures differences in the temperature of the Big Bang's remnant radiant heat — the Cosmic Microwave Background Radiation — across the full sky. Headed by Professor...

 (WMAP) maps of cosmic background radiation. NASA released the first WMAP cosmic background radiation data in February 2003. In 2009 the Planck observatory was launched to observe the microwave background at higher resolution than WMAP, possibly providing more information on the shape of the Universe. The data should be released in late 2012.

See also


  • Cosmic latte
    Cosmic latte
    Cosmic latte is a name assigned to the average color of the universe, given by a team of astronomers from Johns Hopkins University.-Discovery of the color:...

  • Cosmology
    Cosmology
    Cosmology is the discipline that deals with the nature of the Universe as a whole. Cosmologists seek to understand the origin, evolution, structure, and ultimate fate of the Universe at large, as well as the natural laws that keep it in order...

  • Dyson's eternal intelligence
    Dyson's eternal intelligence
    Dyson's eternal intelligence is a concept that states an intelligent being would be able to think an infinite number of thoughts in an open universe....

  • Esoteric cosmology
    Esoteric cosmology
    Esoteric cosmology is cosmology that is an intrinsic part of an esoteric or occult system of thought. It almost always deals with at least some of the following themes: emanation, involution, spiritual evolution, epigenesis, planes of existence or higher worlds , hierarchies of spiritual beings,...

  • False vacuum
    False vacuum
    In quantum field theory, a false vacuum is a metastable sector of space that appears to be a perturbative vacuum, but is unstable due to instanton effects that may tunnel to a lower energy state. This tunneling can be caused by quantum fluctuations or the creation of high-energy particles...

  • Final anthropic principle
  • Fine-tuned Universe
    Fine-tuned universe
    The fine-tuned universe is the proposition that the conditions that allow life in the Universe can only occur when certain universal fundamental physical constants lie within a very narrow range, so that if any of several fundamental constants were only slightly different the universe would be...

  • Hindu cycle of the universe
    Hindu Cycle Of The Universe
    See also Hindu units of measurement.According to Hindu Philosophy, the universe never came to be at some particular point, but always has been, always will be, but is perpetually in flux. Space and time are of cyclical nature. This universe is simply the current one, which is in flux and...

  • Jain cosmology
    Jain cosmology
    Jain cosmology is the description of the shape and functioning of the physical and metaphysical Universe and its constituents according to Jainism, which includes the canonical Jain texts, commentaries and the writings of the Jain philosopher-monks...

  • Kardashev scale
    Kardashev scale
    The Kardashev scale is a method of measuring an advanced civilization's level of technological advancement. The scale is only theoretical and in terms of an actual civilization highly speculative; however, it puts energy consumption of an entire civilization in a cosmic perspective. It was first...

  • The Mysterious Universe
    The Mysterious Universe
    The Mysterious Universe, by the British astrophysicist Sir James Jeans, is a science book for lay persons, first published in 1930 by the Cambridge University Press....

     (book)
  • Nucleocosmochronology
    Nucleocosmochronology
    Nucleocosmochronology, also known as cosmochronology, is a relatively new technique used to determine timescales for astrophysical objects and events...

  • Non-standard cosmology
    Non-standard cosmology
    A non-standard cosmology is any physical cosmological model of the universe that has been, or still is, proposed as an alternative to the big bang model of standard physical cosmology...

  • Observable universe
    Observable universe
    In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light from those objects has had time to reach us since the beginning of the cosmological expansion...

  • Omega Point
    Omega point
    Omega Point is a term coined by the French Jesuit Pierre Teilhard de Chardin to describe a maximum level of complexity and consciousness towards which he believed the universe was evolving....

  • Omniverse
    Omniverse
    The Omniverse is the conceptual ensemble of all possible universes, with all possible laws of physics.In this physical cosmology context, the limitation of the definition of "universe" that it has only one set of "physical laws and constants that govern them," is expanded to include multiple sets...

  • Rare Earth hypothesis
    Rare Earth hypothesis
    In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life on Earth required an improbable combination of astrophysical and geological events and circumstances...

  • Vacuum genesis
    Vacuum genesis
    Vacuum genesis is a hypothesis about the Big Bang that states that the universe began as a single particle arising from an absolute vacuum, similar to how virtual particles come into existence and then fall back into non-existence....

  • World view
    World view
    A comprehensive world view is the fundamental cognitive orientation of an individual or society encompassing the entirety of the individual or society's knowledge and point-of-view, including natural philosophy; fundamental, existential, and normative postulates; or themes, values, emotions, and...

  • Zero-energy universe
    Zero-energy Universe
    The zero-energy universe hypothesis states that the total amount of energy in the universe is exactly zero. When the energy of the universe is considered from a pseudo-tensor point of view, zero values are obtained in the resulting calculations...



External links




Videos

  • The Known Universe created by the American Museum of Natural History
    American Museum of Natural History
    The American Museum of Natural History , located on the Upper West Side of Manhattan in New York City, United States, is one of the largest and most celebrated museums in the world...

  • Understand The Size Of The Universe – by Powers of Ten
    Powers of Ten
    Powers of Ten is a 1968 American documentary short film written and directed by Charles and Ray Eames. The film depicts the relative scale of the Universe in factors of ten . The film is an adaptation of the book Cosmic View by Dutch educator Kees Boeke, and more recently is the basis of a new...