- 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?
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.
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.
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.
Just an idea I'm playing with,
There is no dark energy/dark matter. The universe is not expanding into “infinite nothing”, but instead reducing into the amount of space it occupied at inception. Matter is reducing in scale because there is nothing in the universe requiring it to maintain a static size at the subatomic level. The gravitational effect of dark matter is actually the gravitational wake of previously larger matter that is presently reducing in scale. Due to local gravity galaxies are reducing in scale faster than they are moving towards each other so there appears to be space growing between them. Nothing is growing apart so no need for dark energy. NOT SHRINKING. Scale reduction / collapse of ratios, not densification of matter. We still see red shift because the matter of distant galaxies is “reducing” away from us even though it is not “moving” away from us. Although there are observations that protons, neutrons, and even quarks maintain ratios to each other, there is no evidence that they are arbitrarily fixed in size. Especially since the ruler is not exempt from the effect.
I would enjoy constructive criticism. Thanks.
protected by Qmechanic♦ May 15 '16 at 4:17
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