Copernican principle

Copernican principle

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In 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...

, the Copernican principle, named after Nicolaus Copernicus
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....

, states that the Earth is not in a central, specially favored position. More recently, the principle has been generalized to the relativistic
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....

 concept that humans are not privileged observers of the universe. In this sense, it is equivalent to the mediocrity principle
Mediocrity principle
The mediocrity principle is the notion in philosophy of science that there is nothing very unusual about the evolution of our solar system, the Earth, any one nation, or humans. It is a heuristic in the vein of the Copernican principle, and is sometimes used as a philosophical statement about the...

, with important implications for the philosophy of science
Philosophy of science
The philosophy of science is concerned with the assumptions, foundations, methods and implications of science. It is also concerned with the use and merit of science and sometimes overlaps metaphysics and epistemology by exploring whether scientific results are actually a study of truth...

.

Since the 1990s the term has been used (interchangeably with "the Copernicus method") for J. Richard Gott
J. Richard Gott
John Richard Gott III is a professor of astrophysical sciences at Princeton University. He is known for developing and advocating two cosmological theories with the flavor of science fiction: Time travel and the Doomsday argument.- Exotic matter time travel theories :Paul Davies's bestseller How...

's Bayesian-inference
Bayesian inference
In statistics, Bayesian inference is a method of statistical inference. It is often used in science and engineering to determine model parameters, make predictions about unknown variables, and to perform model selection...

-based prediction of duration of ongoing events, a generalized version of the Doomsday argument
Doomsday argument
The Doomsday argument is a probabilistic argument that claims to predict the number of future members of the human species given only an estimate of the total number of humans born so far...

.

Origin and implications


Michael Rowan-Robinson
Michael Rowan-Robinson
Michael Rowan-Robinson is an astronomer and astrophysicist. He was educated at Pembroke College, Cambridge and is Professor of Astrophysics and until May 2007 was Head of the Astrophysics Group at Imperial College London. From 1981 to 1982, he gave public lectures as professor of astronomy at...

 emphasizes the importance of the Copernican principle: "It is evident that in the post-Copernican era of human history, no well-informed and rational person can imagine that the Earth occupies a unique position in the universe."

Hermann Bondi
Hermann Bondi
Sir Hermann Bondi, KCB, FRS was an Anglo-Austrian mathematician and cosmologist. He is best known for developing the steady-state theory of the universe with Fred Hoyle and Thomas Gold as an alternative to the Big Bang theory, but his most lasting legacy will probably be his important...

 named the principle after Copernicus in the mid-20th century, although the principle itself dates back to the 16th-17th century paradigm shift
Paradigm shift
A Paradigm shift is, according to Thomas Kuhn in his influential book The Structure of Scientific Revolutions , a change in the basic assumptions, or paradigms, within the ruling theory of science...

 away from the Ptolemaic system, which placed 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...

 at the center of the Universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

. Copernicus demonstrated the motion of the planets can be explained without the assumption that Earth is centrally located and stationary. He argued that the apparent retrograde motion of the planets is an illusion caused by Earth's movement around 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...

, which the Copernican model
Copernican heliocentrism
Copernican heliocentrism is the name given to the astronomical model developed by Nicolaus Copernicus and published in 1543. It positioned the Sun near the center of the Universe, motionless, with Earth and the other planets rotating around it in circular paths modified by epicycles and at uniform...

 placed at the centre of the Universe. Copernicus himself was mainly motivated by technical dissatisfaction with the earlier system and not by support for any mediocrity principle. In fact, although the Copernican heliocentric model is often described as "demoting" Earth from its central role it had in the Ptolemaic geocentric model, neither Copernicus nor other 15th- and 16th-century scientists and philosophers viewed it as such.

In cosmology, if one assumes the Copernican principle and observes that the universe appears isotropic from our vantage-point on Earth, then one can prove that the Universe is generally homogeneous (at any given time) and is also isotropic about any given point. These two conditions comprise 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...

.

In practice, astronomers observe that the Universe has heterogeneous structures up to the scale of galactic superclusters, filaments
Galaxy filament
In physical cosmology, galaxy filaments, also called supercluster complexes or great walls, are, so far, the largest known cosmic structures in the universe. They are massive, thread-like structures with a typical length of 50 to 80 megaparsecs h-1 that form the boundaries between large voids in...

 and great 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...

s, but becomes more and more homogeneous and isotropic when observed on larger and larger scales, with little detectable structure on scales of more than about 200 million parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....

s. However, on scales comparable to the radius of the observable universe, we see systematic changes with distance from the Earth. For instance, galaxies contain more young stars and are less clustered, and quasars appear more numerous. While this might suggest that the Earth is at the center of the Universe, the Copernican principle requires us to interpret it as evidence for the evolution of the Universe with time: this distant light has taken most of the age of the Universe to reach and shows us the Universe when it was young. The most distant light of all, cosmic microwave background radiation
Cosmic microwave background radiation
In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....

, is isotropic to at least one part in a thousand.

Modern mathematical cosmology is based on the assumption that the Cosmological principle is almost, but not exactly, true on the largest scales. The Copernican principle represents the irreducible philosophical assumption needed to justify this, when combined with the observations.

Bondi and Thomas Gold
Thomas Gold
Thomas Gold was an Austrian-born astrophysicist, a professor of astronomy at Cornell University, a member of the U.S. National Academy of Sciences, and a Fellow of the Royal Society . Gold was one of three young Cambridge scientists who in the 1950s proposed the now mostly abandoned 'steady...

 used the Copernican principle to argue for the perfect cosmological principle
Perfect Cosmological Principle
The Perfect Cosmological Principle states that the Universe is homogenous and isotropic in space and time. In this view the universe looks the same everywhere , the same as it always has and always will...

 which maintains that the universe is also homogeneous in time, and is the basis for the steady-state cosmology. However, this strongly conflicts with the evidence for cosmological evolution mentioned earlier: the Universe has progressed from extremely different conditions at 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...

, and will continue to progress toward extremely different conditions, particularly under the rising influence 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...

, apparently toward the Big Freeze or 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...

.

Confirmation


Measurements of the effects 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....

 in the dynamics of distant astrophysical systems in 2000 proved the Copernican principle on a cosmological
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...

 scale. The radiation that pervades the universe was demonstrably warmer at earlier times. Uniform cooling of the cosmic microwave background over billions of years is explainable only if the universe is experiencing a metric 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...

.

Ecliptic alignment of cosmic microwave background anisotropy


Results from 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) appear to run counter to Copernican expectations. The motion of the solar system, and the orientation of the plane of the ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...

 are aligned with features of the microwave sky, which on conventional thinking are caused by structure at the edge of the observable universe

Lawrence Krauss is quoted as follows in the referenced Edge.org article:
It would be somewhat surprising if the WMAP alignments were a complete coincidence, but the anti-Copernican implications suggested by Krauss would be far more surprising, if true. Other possibilities are (i) that residual instrumental errors in WMAP cause the effect (ii) that unexpected microwave emission from within the solar system is contaminating the maps.

Modern tests


From the PhysicsWorld.org news article "New tests of the Copernican Principle proposed,"

See also

  • 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...

  • Copernican heliocentrism
    Copernican heliocentrism
    Copernican heliocentrism is the name given to the astronomical model developed by Nicolaus Copernicus and published in 1543. It positioned the Sun near the center of the Universe, motionless, with Earth and the other planets rotating around it in circular paths modified by epicycles and at uniform...

  • Doomsday argument
    Doomsday argument
    The Doomsday argument is a probabilistic argument that claims to predict the number of future members of the human species given only an estimate of the total number of humans born so far...

  • Geocentric model
    Geocentric model
    In astronomy, the geocentric model , is the superseded theory that the Earth is the center of the universe, and that all other objects orbit around it. This geocentric model served as the predominant cosmological system in many ancient civilizations such as ancient Greece...

  • Hubble Bubble (astronomy)
    Hubble Bubble (astronomy)
    In astronomy, a Hubble Bubble would be "a departure of the local value of the Hubble constant from its globally averaged value," or "a local monopole in the peculiar velocity field, perhaps caused by a local void in the mass density."The Hubble Constant, named for astronomer Edwin Hubble, whose...

  • Mediocrity principle
    Mediocrity principle
    The mediocrity principle is the notion in philosophy of science that there is nothing very unusual about the evolution of our solar system, the Earth, any one nation, or humans. It is a heuristic in the vein of the Copernican principle, and is sometimes used as a philosophical statement about the...


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