In light of the failure of the LUX experiment to find dark matter I was wondering if the whole idea of dark matter possibly indicates some problem with GR rather than simply being as yet undiscovered. Is this likely?
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$\begingroup$ We cannot prove that something does not exist, only that something does exist. Unless we find evidence which invalidates dark matter, it may just be a case of not having enough sensitivity on our instruments to detect dark matter. $\endgroup$– HorusCommented Sep 5, 2016 at 17:56
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$\begingroup$ It's a logical possibility. Looking forward to an answer about why it's considered implausible relative to e.g, WIMP DM, especially in light of null results in DD searches $\endgroup$– innisfreeCommented Sep 5, 2016 at 18:53
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$\begingroup$ LUX isn't a axion search, it's a WIMP detector. And it is barely big enough to hope for a detection under optimal conditions (just the right dark matter mass). The really full-scale stage is to be called LUX-ZEPLIN, and has funding approval. $\endgroup$– dmckee --- ex-moderator kittenCommented Sep 5, 2016 at 19:05
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$\begingroup$ Axions are generally considered more experimentally challenging to get up to scale on, and so far only technology development scale instruments have been built and operated. $\endgroup$– dmckee --- ex-moderator kittenCommented Sep 5, 2016 at 19:07
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$\begingroup$ @dmckee LUX probed a lot of previously favored WIMP parameter space, especially SUSY WIMP. It's strange to say it's barely big enough $\endgroup$– innisfreeCommented Sep 5, 2016 at 19:19
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3 Answers
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The problem is that there is lots of evidence that dark matter (whatever we mean by that) exists. We have observations like the Bullet Cluster, where the dark matter can be detected by its lensing, and galaxies like Dragonfly 44 where 99.99% of its matter is dark. Trying to explain away such extreme examples by fiddling with theories of gravitation brings a whole new meaning to the word contrived.
On the other hand general relativity works extremely well in the low field limit where we've been able to test it. Gravitational lensing and gravitational time dilation agree spectacularly well with the predictions of GR.
It is possible to modify GR because it's just one of a range of theories collectively known as metric theories. In the context of dark matter the most obvious alternative is TeVeS since that was specifically designed as a covariant form of the MOND theory that purports to explain away dark matter. The problem is that any modified theories have to give the same predictions as GR in the regimes where we have experimental data, and it's hard to get them to agree with current experimental evidence for GR and still explain away dark matter.
So at the end of the day, unpalatable though it may be to some, dark matter remains the best explanation for the behaviour of the universe. It's not that physicists love the idea but that right now there aren't any convincing alternatives.
answered Sep 6, 2016 at 14:47
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4$\begingroup$ I would add the following point: Lovelock theorem tells us that GR is the only theory in 4 dimensions which conserves locally the stress-energy tensor and involves only second derivatives of the metric. This means that any conservative metric modification of GR will involve third and higher derivatives and introduces new degrees of freedom to the theory (e.g. f(R) theories and the Einstein frame). Non-metric modifications like TeVeS introduce new degrees of freedom explicitly. I.e. no matter how hard you try, you introduce new degrees of freedom. ... $\endgroup$– VoidCommented Sep 6, 2016 at 15:48
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3$\begingroup$ ... You can as well call these new DOF "dark matter", the only difference is how this new degree of freedom couples to other physics. LUX just tells us that dark matter doesnt couple through the electroweak force in the way the most prominent theoretical physicists suggested. But there is still the possibility that it does not couple at all, it is just a completely sterile particle coupling only to gravity in the very usual way. And this is the most elegant possibility. $\endgroup$– VoidCommented Sep 6, 2016 at 15:50
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$\begingroup$ Trying to explain away the mismatch between Einstein's GR and the actual data by inventing vast quantities of matter of unknown type, that's conveniently invisible, is also, perhaps, a little contrived. At one point, we explained the Mercury perihelion anomaly by suggesting the existence of a new planet, Vulcan. The longer we went without finding Vulcan, the more we downgraded the hypothesis. We didn't say, "We have ample proof that Vulcan exists, so it must be invisible!" $\endgroup$ Commented Dec 9, 2019 at 14:53
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The MOND (Modified Newton Dynamic) theory as always been active and might be the closest to what you are questioning. The idea of particles that we can't detect is quite simple and beautiful but there is another way to solve the problem of the missing mass. You can modified the law of gravitation, or Newton's law of acceleration :
https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
It is said here that the attempt to get rid of dark matter is not successful :
The most serious problem facing Milgrom's law is that it cannot completely eliminate the need for dark matter in all astrophysical systems: galaxy clusters show a residual mass discrepancy even when analysed using MOND. The fact that some form of unseen mass must exist in these systems detracts from the elegance of MOND as a solution to the missing mass problem, although the amount of extra mass required is 5 times less than in a Newtonian analysis, and there is no requirement that the missing mass be non-baryonic. It has been speculated that 2 eV neutrinos could account for the cluster observations in MOND while preserving the theory's successes at the galaxy scale
I have to say that there is still a lot of experiments looking for dark matter (like Xenon100 and maybe soon Xenon1T)
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The problem is not with GR, it's with the current incorrect understanding of dark matter.
The notion of dark matter as a weakly interacting clump of stuff that travels with the matter is incorrect.
Dark matter fills ‘empty’ space, strongly interacts with matter and is displaced by matter.
The dark matter displaced by the Earth pushing back and exerting pressure toward the Earth is gravity.
Curved spacetime is the state of displacement of the dark matter.
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1$\begingroup$ Do you have any proof for your claims other than these heuristical arguments? $\endgroup$ Commented Sep 5, 2016 at 22:55
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$\begingroup$ If the dark matter fills 'empty' space, strongly interacts with matter and is displaced by matter then the Milky Way's halo should be lopsided as it moves through and displaces the dark matter, analogous to a submarine moving through and displacing the water. $\endgroup$– Yin LangCommented Sep 5, 2016 at 23:17
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$\begingroup$ The Milky Way's dark matter halo appears to be lopsided $\endgroup$– Yin LangCommented Sep 5, 2016 at 23:17
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$\begingroup$ If the dark matter fills 'empty' space, strongly interacts with matter and is displaced by matter then there should be an offset between the light lensing through the space neighboring galaxy clusters and the center of the galaxy clusters themselves as the galaxy clusters move through and displace the dark matter, analogous to submarines moving through and displacing the water. $\endgroup$– Yin LangCommented Sep 5, 2016 at 23:18
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1$\begingroup$ This is absolute nonsense, and the preprints you link to are totally irrelevant. Furthermore, this same nonsense has been put forth in the past, with extremely similar citations: see here, here, here, and here, as well as in other deleted answers. Stop trolling. $\endgroup$ Commented Sep 5, 2016 at 23:43