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I've been reading about dark matter. I understood that its existence is inferred by the discrepancy between the gravity exerted by a galaxy (total mass) and the movement of the stars that compose it.

In other words, the outer stars would not behave as they do if the total mass of the system was not much higher than can be seen.

I wondered if there are other possible explanations for this phenomenon instead of dark matter.

Qmechanic
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rnrneverdies
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    What have you looked at so far to try to find an answer? – David Z Sep 16 '14 at 05:03
  • What school are we talking about? Physics/astronomy grad school? If not, you may want to select a different topic. This one is very hard and simplification ad absurdum is not compatible with "doing a good job" for this one. – CuriousOne Sep 16 '14 at 05:08
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    http://physics.stackexchange.com/questions/tagged/dark-matter?sort=votes is not a bad place to start –  Sep 16 '14 at 05:10
  • Phew... for a moment I thought high school physics... Did you take a GR class? Do you feel comfortable discussing the differences between GR and tensor-vector-scalar gravity? – CuriousOne Sep 16 '14 at 05:14
  • OK... get yourself a different topic. This one is not for high school. – CuriousOne Sep 16 '14 at 05:48
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    @rnrneverdies You might like to look at "Warped Worlds" on my page http://www.wetsavannaanimals.net/wordpress/small-things-amuse-great-minds/ This is written for late primary school, but it might get you begun. You'll have a whiff of what General Relativity is about. – Selene Routley Sep 16 '14 at 06:30
  • Possible duplicates: http://physics.stackexchange.com/q/95/2451, http://physics.stackexchange.com/q/6561/2451 and links therein. – Qmechanic Sep 16 '14 at 12:17

2 Answers2

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The evidence for dark matter is extremely compelling provided that Einstein's theory of relativity is assumed. To get out of dark matter means dropping that assumption (or that we've SERIOUSLY botched some observations). Some people study alternate theories of gravity. The two alternate theories (or classes of theories? keywords?) I hear most about are f(R) gravity and MOND: MOdified Newtonian Dynamics.

Kyle Oman
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    One doesn't need GR to be compelled to assume that dark matter exists. The galactic rotation curve problem is Newtonian. – CuriousOne Sep 16 '14 at 05:11
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    @CuriousOne True, but GTR reduces to Newton and of course successfully explains other empirically witnessed phenomena (lensing, apsidal precession, ...) that Newton can't, so Kyle is saying that the galactic rotation curve is the first empirical effect that would falsify GTR if dark matter were repudiated. – Selene Routley Sep 16 '14 at 05:18
  • MOND doesn't even come close to being a physical theory and there are several extensions of GR that may be compatible with dark matter observations. Indeed, if I interpret some of what I have read correctly, dark matter may be a natural fit to GR's next non-trivial extension: Einstein–Cartan theory. Having said that, most of the direct evidence for dark matter is a Newtonian problem, and I wouldn't break out the death star for a problem that can be beaten into submission with a club. – CuriousOne Sep 16 '14 at 05:46
  • @CuriousOne Do you have a reference for EC theory and its relationship to dark matter: that sounds really interesting – Selene Routley Sep 16 '14 at 06:27
  • See e.g. http://arxiv.org/abs/1405.3435. If we want to explain dark matter by an extension of GR rather than trough a particle model, then EC seems a more natural fit than other ad-hoc extensions. Having said that, nature doesn't care about what "seems natural" to us, so EC may be wrong just as well. – CuriousOne Sep 16 '14 at 14:55
  • The other compelling evidence I'm thinking of (in addition to the rotation curves) is the matter abundance required to flatten the Universe compared to calculations of the baryon abundance (e.g. baryon fraction in galaxy clusters, baryogenesis calculations). Given that the curvature is a GR thing, I'd call this pretty GR dependent. – Kyle Oman Sep 16 '14 at 15:34
  • So where the 'bleep' is it? We can't find a particle. Given a uniform distribution in our galaxy could we see the effects of dark matter in our solar system? (By looking closely at the dynamics of comets or something.) – George Herold Sep 16 '14 at 16:28
  • @GeorgeHerold other than the romantic notion that we'd like to be able to confirm its existence, there is nothing compelling the DM particle to couple to the rest of the Universe by anything other than gravitation. Of course there's always the chance that we've just missed something in the observations, or that GR is wrong, but we think we've got the former under control, and there are some convincing tests of GR... The solar system is no good, unless we get exponentially better at accounting for perturbations from all the bodies in the SS. – Kyle Oman Sep 16 '14 at 18:19
  • In any dark matter particle scenario, the total mass of the DM inside Pluto's orbit is about 1/100,000th the mass of the Moon (or a Moon rock density asteroid with a radius of about 100 miles), but distributed more or less evenly throughout the solar system. Since you feel only the net pull of gravity from all directions, only the local inhomogenities of dark matter are observable to someone inside the DM distribution. So the solar system vicinity impact of DM is much, much smaller than the gravitational impact of one asteroid within the solar system -- too slight to detect experimentally. – ohwilleke Oct 14 '16 at 00:52
  • @WetSavannaAnimal Why do you think that Newtonian physics can't explain gravitational lensing? Newton did put gravitational light-bending in Opticks, and from there to lensing is trivial. Especially as he also worked out the numbers for the analogous case of atmospheric lensing due to the Earth's variable-density atmosphere. – Eric Baird Dec 09 '19 at 17:07
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While Milgrom's MOND theory and its relativistic generalization by Jacob Bekenstein called TeVeS provides one gravity modification approach which is the earliest and most famous, it is not the only one. A variety of proposals have been seriously considered in the last six years alone:

John W. Moffat has several others similar in concept such as MOG. There are f(R) and f(R,T) theories which have additional terms which are functions of the Ricci tensor and torsion scalar respectively. Non-local gravity formulations have been proposed to address dark matter phenomena. Then there are concepts like that of Alexandre Deur who reconsiders how the self-interaction of the graviton should be handled in quantum gravity (explored previously as early as 1981), and Lee Smolin who considers quantum gravity effects that could appear to be dark matter in loop quantum gravity approaches. Erik Verlinde's Emergent Gravity concept didn't set out to be a gravity modification approach but some versions of it provide an alternative to dark matter. Some running gravitational coupling constant solutions (also here) could also replace dark matter, possibly in connection with an asymptotically safe gravity scenario. Horndeski gravity was invented as an alternative dark energy realization but can be a dark matter alternative as well, as can be a generalisation of Brans-Dicke theory. A massive scalar graviton and tensor massive gravity (also here) have also been considered as an alternatives. Lubos Motl has advanced a variant called holographic modified dynamics.

Wave dark matter is strictly speaking a dark matter particle theory, but has some characteristics usually found in gravity modification theories.

ohwilleke
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