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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.
Two major conundrums have arisen in astrophysics 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 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 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, special cases of various other gravitational theories are reproduced by dark fluid, e.g. inflation, quintessence, k-essence, f(R), Generalized Einstein-Aether f(K), MOND, TeVeS, BSTV, etc. The dark fluid model also suggests new models such as a certain f(K+R) model, which suggests interesting corrections to MOND that depend on redshift and density.
Dark fluid theory is not treated like a standard fluid mechanics 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.
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, 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.
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