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The self-interaction of dark matter may be small but it cannot be negligible if it is able to dissipate energy to relax into galactic clumps (necessary to explain galaxy rotation curves).

According to some answers in this old question: How Does Dark Matter Form Lumps?, the gravitational self-interaction alone is enough to allow dark matter clumping (via n-body interactions). Although two answers suggest something other than gravity is needed (one states considering the weak force is necessary, while another answer argues for why gravity alone doesn't explain how in cosmology dark matter could clump first).

I am curious about:

  • Have the measurements of dark matter profiles of galaxies become good enough to provide indirect measurements of dark matter self-interactions?

  • Can this self-interaction be used to say anything about the mass of the dark matter particles? At the very least, can we say with certainty they have mass above some threshold (ruling out very light particles such as axions or neutrinos, and ruling out some kind of unseen massless particles)?

  • Since the strength and radial distribution of the gravitational force vs the weak force differ so strongly, is it possible to determine from the self interaction whether dark matter interacts via the weak force?

Kyle Oman
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PPenguin
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3 Answers3

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First of all, let me clarify that a non-gravitational interaction is absolutely not required to allow dark matter to clump.

There are many attempts to constrain the self-interaction cross section of dark matter. Qualitatively speaking, if dark matter scatters off of itself, this would tend to put an upper limit on the local density of dark matter. Measurements sensitive to this density (e.g. rotation curves, gravitational lensing) can therefore constrain the cross section.

There are several examples in the literature of such constraints (this being only a small selection). As it stands, there is still debate over whether a claim of a self-interaction detection (or really, an interestingly constraining limit) can be made, so learning much about the particle mass or the force mediating the interaction is still rather premature.

Kyle Oman
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There is currently no strong evidence that dark matter interacts with ordinary matter via anything besides gravity. There have been proposals for non-gravitational interactions to explain discrepancies with small scale observations. But these discrepancies can also be explained by the effects of ordinary baryonic physics.

Virgo
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  • Any proof of non-gravitational interaction means successful detection of dark matter. Till that point, it remains hypothetical. – kpv Apr 12 '17 at 03:43
  • @kpv your comment is unrelated to my answer. – Virgo Apr 12 '17 at 04:04
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Dark matter is still hypothetical - name given to excess gravity that can not be explained in terms of known bayionic matter.

Your question assumes proven existence of dark matter, which is not necessarily true.

Properties attributed to dark matter are - transparent, cold, and non-interactive (except via gravity). These properties resemble those of empty space more than anything else.

kpv
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  • This is not a well informed response, and it completely does not answer the question. – DilithiumMatrix Apr 12 '17 at 01:38
  • This is mentioning fringe ideas in a way that misleadingly represents them as main stream widely accepted views. – Virgo Apr 12 '17 at 03:03
  • @Virgo: Which fringe/idea is suggested here? – kpv Apr 12 '17 at 03:36
  • @kpv "These properties resemble those of empty space more than anything else." Relating dark matter properties to empty space is a very fringe science statement. – Virgo Apr 12 '17 at 04:03
  • @Virgo: Can you name anything that resembles more? Empty space is cold, transparent, and it only interacts via gravity/inertia. I do not say it is science. But till the time dark matter is detected, there is nothing else more closely described by these properties. If you think it is fringe science, it is your opinion, I have stated what the properties appear to be most closely related. Even if/when DM is detected, these properties will continue to resemble those of empty space. It is not science, but there is nothing wrong in saying so because the characteristics do match those of space. – kpv Apr 12 '17 at 05:35
  • @kpv saying "empty space interacts via gravity inertia" sounds a very strange statement to me and is very far from any mainstream scientific view in my opinion. – Virgo Apr 12 '17 at 05:37
  • @Virgo: How else space interacts (with mass/energy)? It is not documented in exactly these words but it is what main stream science says. "Mass tells space how to curve, and space tells mass how to move". This is the interaction I refer to. – kpv Apr 12 '17 at 05:41
  • @kpv I don't see the relation to dark matter at all. I think you are mentioning some confusing half thought out ideas and presenting them as though they were the standard view. I don't think this web site is a good forum for doing that. I do not wish to continue this discussion. – Virgo Apr 12 '17 at 06:17
  • @Virgo: well you started it, not me. If you really meant to end the discussion, you did not have to make your expert "half thought" comments just before ending the discussion. I am not interested either in continuing this. – kpv Apr 12 '17 at 06:29