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  1. Is there any physics theory that either groups together gravity and dark energy/dark matter or eliminates dark energy/dark matter by modifying standard understanding of gravity or any force? If so, is there any theory that still matches with all correct predictions of standard physics theories?
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Related: physics.stackexchange.com/q/6561/2451 –  Qmechanic Jun 4 '12 at 19:19
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3 Answers

There are theories that make such claims, like MOND and f(R) gravity, but they are not compelling theoretically. One must remember that the dark-matter was detected through it's effect on gravitationally bound clusters long before it was confirmed through cosmology. It isn't a fake--- it's really there. There are 3 different pieces of evidence for dark-matter clouds around galaxies:

  • Galaxy shapes and rotation curves: this confirms that galaxies are surrounded by a roughly spherical cloud of dark matter with more mass than the visible matter.
  • Gravitationally bound clusters: the average velocity of gravitationally bound clusters of galaxies is too big for them to stay bound, unless there is about 5 times more dark matter than ordinary matter in the galaxy clusters. This is an argument using escape velocity and statistics, and it predicts that dark matter is about 25% of critical density.
  • The cosmological data: this shows that the universe is 70% dark-energy and 30% dark-matter/ordinary matter, and we know ordinary matter is 5%. So this measurement matches the cluster data, acquired independently.

The dark energy is confirmed independently by the cosmology data (microwave background fluctuations) and the accelerated expansion of the universe. It also is required to make the simulations of structure formation in the early universe match observations.

The fact that different threads of experiment and modelling converge to give the same answer confirms that the dark matter and dark-energy are real things, and that they are described by General Relativity. One could still make up theories which reject dark-matter and dark-energy, but such attempts are generally misguided. We now have a pretty good grip on why GR looks the way it does, and it is not arbitrary. Modifications like f(R) gravity and MOND generally are impossible to embed in string theory, or even in an effective action type quantum gravity theory.

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Is there a proof you can't embed something like TeVeS in string theory? The dilaton is easy to put into string theory, and then you just have to have a string model that adds a vector field with a particular coupling to matter. Doesn't seem any more contrived than a large number of the string models I've seen. –  Jerry Schirmer Jun 4 '12 at 13:07
    
@JerrySchirmer: I believe that TeVeS allows a massive graviton at long distances, so that it can violate energy conditions classically, and so should be ruled out on general principles. But of course there is no proof. –  Ron Maimon Jun 11 '12 at 12:40
    
there's no massive graviton in TeVeS. –  Jerry Schirmer Jun 12 '12 at 0:54
    
@JerrySchirmer: I don't know TeVeS beyond some sketchy high level summary, but it has the proper multiplet 0,1,2 to allow a massive graviton, so there might be a way to finnagle it. Are you sure there's no jiggling of TeVeS parameters to make a massive graviton? Generally string theories don't produce a spin-1 gravity component, I don't know any case at least where this happens. –  Ron Maimon Jun 12 '12 at 2:46
    
TeveS is crazy on many levels. It has a free function in its Lagrangian that governs how the dynamical particles couple to each other. But it is explicitly designed to avoid fininte-range gravity effects, and the tensor term comes from an ordinary ricci scalar in the lagrangian. There is a spin-1 gravity component in the theory, but that is explicitly added by hand, by the addition of a new vector field, not by splitting the spectrum of gravitational waves. –  Jerry Schirmer Jun 12 '12 at 12:24
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Void models can eliminate dark energy. The key trick in constructing void models abandon homogeneity but retain isotropy as assumptions, are currently not ruled out by observation, but many people feel that they are somewhat contrived, since it requires an Earth near, but not at, the center of the universe. (in the void model, the universe has a center, but no edge). Also, a void model will still require dark matter, since the evidence for dark matter is generated mostly from the dynamics of galaxies and clusters of galaxies, rather than from the cosmology of the universe as a whole.

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Not an answer, but an important point about this class of theories.

A serious challenge for MOND and associated theories is to explain the Bullet cluster.

The weak gravitational lensing map of that part of the sky provides fairly direct evidence of gravitational mass that has been stripped away from its associated visible mass, and the distinction is that the invisible mass behaves as if it is largely collisionless compared to the visible mass. That's a nice match to the expected properties of dark matter, and not at all easy to explain in a dark matter-less theory.

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