Dark fluid
Encyclopedia
In astronomy
and cosmology
, dark fluid is an alternative theory to both dark matter
and dark energy
and attempts to explain both phenomena in a single framework.
Dark fluid proposes that dark matter and dark energy are not separate physical phenomena as previously thought, nor do they have separate origins, but that they are linked together and are really specific sub-effects of new extended laws of gravity at very large scales. Other alternative theories of extended gravity, such as Modified Newtonian dynamics
(MOND), also show up as specific sub-effects. Our current laws of gravity modeled on observations within the scales of the Earth
and the Solar system
might be insufficient to explain gravity at these larger scales.
and cosmology
in recent times, both dealing with the laws of gravity
. The first was the realization that there aren't enough visible stars or gas inside galaxies
to account for their high rate of rotation
. The theory of dark matter
was created to explain this phenomenon. It theorizes that the galaxies are spinning as fast as they are because there is more matter in those galaxies (including our own Milky Way
) than can be seen by counting the mass of stars and gas alone, and that this unseen (dark) matter is invisible because it doesn't interact with the electromagnetic force from which all forms of light comes, which we use to see things.
The second conundrum came from the observations of a very specific kind of supernova
, known as a Type Ia supernova
: when they were compared in distant vs. nearby galaxies, it was found that the distant supernova were fainter, and thus farther away than they expected. This implied that the universe
was not only expanding, but accelerating its expansion
. The theory of dark energy
was created to explain this phenomenon.
In the traditional approach to modeling effects of gravity, general relativity
is assumed to be valid at cosmological scales as well as in the solar system where its predictions have been more accurately tested. Not changing the rules of gravity, however, implies the presence of dark matter
and dark energy
in parts of the universe where the curvature of the space-time manifold is far less than that of in the solar system. It is phenomenologically possible to alter the equations of gravity in regions of low space-time curvature such that the dynamics of the space-time causes what we assign to the presence of dark matter
and dark energy
. Dark fluid even goes one step beyond the standpoint of the generally covariant modified theories of gravity. It hypothesizes that the fabric of space acts much like a fluid. So dark fluid currently provides a general and powerful model for altering the dynamics of the space-time manifold. In this theory, space would flow, coagulate, compress, or expand just like any other fluid. The idea is that when space is in the presence of matter, it slows down and coagulates around it; this then attracts more space to coagulate around it, thus amplifying the force of gravity near it. This description is similar to theories of gravitational back-reaction
. The effect is always present, but only becomes noticeable in the presence of a really large mass such as a galaxy. If this effect sounds very much like a description of dark matter, then that's not a coincidence, as a special case of the equations of dark fluid reproduces dark matter. But the theory of dark fluid does not hold that actual particles of dark matter exist, but rather that this is just an illusionary effect of space bunching up on itself.
On the other extreme, in places where there is relatively little matter, as in the voids
between galactic superclusters, the theory of dark fluid predicts that space relaxes, and starts stretching away from itself. Thus dark fluid becomes a repulsive force, with the same effect as dark energy.
Dark fluid goes beyond dark matter and dark energy in that it predicts a continuous range of attractive and repulsive qualities, under various matter density cases. Indeed, dark fluid reproduces various other gravitational theories as special cases within it, e.g. inflation, quintessence
, k-essence, f(R)
, Generalized Einstein-Aether f(K), MOND
, TeVeS
, BSTV
, etc. It also suggests new models such as a certain f(K+R) model, which suggests intriguing corrections to MOND depending on redshift and density.
model, because many of the fluid mechanics equations are too difficult to solve completely. A formalized fluid mechanical approach, like the generalized Chaplygin gas model
, would be an ideal method for modeling this theory, but it requires too many observational data points to work properly, and there aren't enough such data points available to cosmologists yet. So a simplification step was undertaken by modeling the theory through scalar field models instead, as is done in other alternative approaches to dark energy and dark matter.
Parameters must be chosen in dark fluid theory to satisfy the conditions and constraints of Big Bang nucleosynthesis
, parameterized post-Newtonian formalism
, and causality
.
years, one giga-year), at which point it begins to match MOND. Interestingly, dark fluid predicts that due to time-dependent variables, the current universe is not old enough for all galaxies to conform to MOND yet, but that after about 100 Gyr, the galaxies in the universe would mostly resemble MOND.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
and 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...
, dark fluid is an alternative theory to both 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...
and 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 attempts to explain both phenomena in a single framework.
Dark fluid proposes that dark matter and dark energy are not separate physical phenomena as previously thought, nor do they have separate origins, but that they are linked together and are really specific sub-effects of new extended laws of gravity at very large scales. Other alternative theories of extended gravity, such as Modified Newtonian dynamics
Modified Newtonian dynamics
In physics, Modified Newtonian dynamics is a hypothesis that proposes a modification of Newton's law of gravity to explain the galaxy rotation problem. When the uniform velocity of rotation of galaxies was first observed, it was unexpected because Newtonian theory of gravity predicts that objects...
(MOND), also show up as specific sub-effects. Our current laws of gravity modeled on observations within the scales of 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...
and 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...
might be insufficient to explain gravity at these larger scales.
Overview
Two major conundrums have arisen in astrophysicsAstrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
and 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...
in recent times, both dealing with the laws of gravity
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...
. The first was the realization that there aren't enough visible stars or gas inside 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...
to account for their high rate of rotation
Galaxy rotation curve
The rotation curve of a galaxy can be represented by a graph that plots the orbital velocity of the stars or gas in the galaxy on the y-axis against the distance from the center of the galaxy on the x-axis....
. The theory of 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...
was created to explain this phenomenon. It theorizes that the galaxies are spinning as fast as they are because there is more matter in those galaxies (including our own 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...
) than can be seen by counting the mass of stars and gas alone, and that this unseen (dark) matter is invisible because it doesn't interact with the electromagnetic force from which all forms of light comes, which we use to see things.
The second conundrum came from the observations of a very specific kind of supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
, known as a 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...
: when they were compared in distant vs. nearby galaxies, it was found that the distant supernova were fainter, and thus farther away than they expected. This implied that 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...
was not only expanding, but accelerating 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 theory 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...
was created to explain this phenomenon.
In the traditional approach to modeling effects of gravity, 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...
is assumed to be valid at cosmological scales as well as in the solar system where its predictions have been more accurately tested. Not changing the rules of gravity, however, implies the presence of 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...
and 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...
in parts of the universe where the curvature of the space-time manifold is far less than that of in the solar system. It is phenomenologically possible to alter the equations of gravity in regions of low space-time curvature such that the dynamics of the space-time causes what we assign to the presence of 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...
and 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...
. Dark fluid even goes one step beyond the standpoint of the generally covariant modified theories of gravity. It hypothesizes that the fabric of space acts much like a fluid. So dark fluid currently provides a general and powerful model for altering the dynamics of the space-time manifold. In this theory, space would flow, coagulate, compress, or expand just like any other fluid. The idea is that when space is in the presence of matter, it slows down and coagulates around it; this then attracts more space to coagulate around it, thus amplifying the force of gravity near it. This description is similar to theories of gravitational back-reaction
Back-reaction
In theoretical physics, Back-reaction is often necessary to calculate the behavior of a particle or an object in an external field.When the particle is considered to be infinitely light or have an infinitesimal charge, it is said that we deal with a probe and the back-reaction is neglected...
. The effect is always present, but only becomes noticeable in the presence of a really large mass such as a galaxy. If this effect sounds very much like a description of dark matter, then that's not a coincidence, as a special case of the equations of dark fluid reproduces dark matter. But the theory of dark fluid does not hold that actual particles of dark matter exist, but rather that this is just an illusionary effect of space bunching up on itself.
On the other extreme, in places where there is relatively little matter, as in the voids
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...
between galactic superclusters, the theory of dark fluid predicts that space relaxes, and starts stretching away from itself. Thus dark fluid becomes a repulsive force, with the same effect as dark energy.
Dark fluid goes beyond dark matter and dark energy in that it predicts a continuous range of attractive and repulsive qualities, under various matter density cases. Indeed, dark fluid reproduces various other gravitational theories as special cases within it, e.g. inflation, quintessence
Quintessence (physics)
In physics, quintessence is a hypothetical form of dark energy postulated as an explanation of observations of an accelerating universe. It has been proposed by some physicists to be a fifth fundamental force...
, k-essence, f(R)
F(R) gravity
f gravity is a type of modified gravity theory first proposed in 1970by Buchdahl as a generalisation of Einstein's General Relativity. Although it is an active field of research, there are known problems with the theory...
, Generalized Einstein-Aether f(K), MOND
Modified Newtonian dynamics
In physics, Modified Newtonian dynamics is a hypothesis that proposes a modification of Newton's law of gravity to explain the galaxy rotation problem. When the uniform velocity of rotation of galaxies was first observed, it was unexpected because Newtonian theory of gravity predicts that objects...
, TeVeS
Teves
Teves may refer to* Continental Teves, an automotive brake component manufacturer based in Wales, now part of Continental AG* TVes, a Venezuelan TV station * Tensor-vector-scalar gravity theory by Jacob Bekenstein...
, BSTV
Bi-scalar tensor vector gravity
Bi-scalar tensor vector gravity theory is an extension of the tensor–vector–scalar gravity theory . TeVeS was proposed by Jacob Bekenstein, BSTV was proposed by R.H.Sanders. BSTV makes TeVeS more flexible by making a non-dynamical scalar field in TeVeS into a dynamical one....
, etc. It also suggests new models such as a certain f(K+R) model, which suggests intriguing corrections to MOND depending on redshift and density.
Simplifying assumptions
Dark fluid theory is not treated like a standard fluid mechanicsFluid mechanics
Fluid mechanics is the study of fluids and the forces on them. Fluid mechanics can be divided into fluid statics, the study of fluids at rest; fluid kinematics, the study of fluids in motion; and fluid dynamics, the study of the effect of forces on fluid motion...
model, because many of the fluid mechanics equations are too difficult to solve completely. A formalized fluid mechanical approach, like the generalized Chaplygin gas model
Chaplygin gas
Chaplygin gas, which occurs in certain theories of cosmology, is a hypothetical substance that satisfies an exotic equation of state in the formp=-A/\rho^\alphawhere p is the pressure, \rho is the density, with \alpha=1 and A a positive constant...
, would be an ideal method for modeling this theory, but it requires too many observational data points to work properly, and there aren't enough such data points available to cosmologists yet. So a simplification step was undertaken by modeling the theory through scalar field models instead, as is done in other alternative approaches to dark energy and dark matter.
Parameters must be chosen in dark fluid theory to satisfy the conditions and constraints of Big Bang nucleosynthesis
Big Bang nucleosynthesis
In physical cosmology, Big Bang nucleosynthesis refers to the production of nuclei other than those of H-1 during the early phases of the universe...
, parameterized post-Newtonian formalism
Parameterized post-Newtonian formalism
Post-Newtonian formalism is a calculational tool that expresses Einstein's equations of gravity in terms of the lowest-order deviations from Newton's theory. This allows approximations to Einstein's equations to be made in the case of weak fields...
, and causality
Causality (physics)
Causality is the relationship between causes and effects. It is considered to be fundamental to all natural science, especially physics. Causality is also a topic studied from the perspectives of philosophy and statistics....
.
Modified Newtonian dynamics
Modified Newtonian dynamics (MOND) is very good at explaining the rotational curves of spiral galaxies that are in equilibrium (i.e. ones which haven't undergone any recent mergers). Analysing MOND using generalized dark fluid equations, one finds that where MOND falls short is in merging systems like galactic clusters, where time-dependent equations are important, but MOND doesn't take these into account adequately. The MOND theory is actually the dark fluid theory's static special case when galaxy structures are in equilibrium. It is estimated that the average time for oscillations inside a galaxy to settle down after a disturbance event is about 1 Gyr ( years, one giga-year), at which point it begins to match MOND. Interestingly, dark fluid predicts that due to time-dependent variables, the current universe is not old enough for all galaxies to conform to MOND yet, but that after about 100 Gyr, the galaxies in the universe would mostly resemble MOND.External links
- "Cosmological constraints on unifying Dark Fluid models." By A. Arbey, Dec. 2008. Link to published version.
- "Is it possible to consider Dark Energy and Dark Matter as a same and unique Dark Fluid?" Unpublished manuscript by A. Arbey, 2005.
- A. Arbey and F. Mahmoudi, "One-loop quantum corrections to cosmological scalar field potentials" Phys. Rev. D 75, 063513 (2007).
- "Cosmology with a Variable Chaplygin Gas". By Zong-Kuan Guo, Yuan-Zhong Zhang (2005, 2007).