4
$\begingroup$

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.

$\endgroup$
  • 1
    $\begingroup$ What have you looked at so far to try to find an answer? $\endgroup$ – David Z Sep 16 '14 at 5:03
  • $\begingroup$ 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. $\endgroup$ – CuriousOne Sep 16 '14 at 5:08
  • 2
    $\begingroup$ physics.stackexchange.com/questions/tagged/… is not a bad place to start $\endgroup$ – user10851 Sep 16 '14 at 5:10
  • $\begingroup$ 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? $\endgroup$ – CuriousOne Sep 16 '14 at 5:14
  • 1
    $\begingroup$ @rnrneverdies You might like to look at "Warped Worlds" on my page 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. $\endgroup$ – WetSavannaAnimal Sep 16 '14 at 6:30
6
$\begingroup$

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.

$\endgroup$
  • 1
    $\begingroup$ One doesn't need GR to be compelled to assume that dark matter exists. The galactic rotation curve problem is Newtonian. $\endgroup$ – CuriousOne Sep 16 '14 at 5:11
  • 1
    $\begingroup$ @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. $\endgroup$ – WetSavannaAnimal Sep 16 '14 at 5:18
  • $\begingroup$ 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. $\endgroup$ – CuriousOne Sep 16 '14 at 5:46
  • $\begingroup$ @CuriousOne Do you have a reference for EC theory and its relationship to dark matter: that sounds really interesting $\endgroup$ – WetSavannaAnimal Sep 16 '14 at 6:27
  • $\begingroup$ See e.g. 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. $\endgroup$ – CuriousOne Sep 16 '14 at 14:55
3
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ +1 for the most number of references I've seen in a Stackexchange answer. $\endgroup$ – PNDA May 16 '18 at 18:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.