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I have read into some depth about the various candidates for dark matter, the merits and flaws of each.

However what I have been unable to attain from texts alone is what is the most likely explanation of dark matter right now?

Obviously, some are more likely than others, but at the present day, which ones 'seem' to be the most likely based on the contributions to them at the present.

I am going to be writing an EPQ and wish to focus on the most likely theories, as there are too many to go over them all.

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  • $\begingroup$ There's like two seriously-sought models that I'm aware, not sure that that is "too many" $\endgroup$ – Kyle Kanos Nov 18 '16 at 19:28
  • $\begingroup$ @KyleKanos which two, out of curiosity? Things that spring to mind are WIMPs, sterile neutrinos, axions and a newcomer, ultralight bosons. $\endgroup$ – Kyle Oman Nov 18 '16 at 19:59
  • $\begingroup$ @KyleOman: WIMPs and axions. I tend to forget about sterile neutrinos regularly & have not heard of ultralight bosons. $\endgroup$ – Kyle Kanos Nov 18 '16 at 20:00
  • $\begingroup$ It's very very subjective what the most likely one is. What it seens you are asking is what the most popular candidates are. A better question imo is to ask for a list of the different candidates and their strengths / weaknesses. $\endgroup$ – Winther Nov 18 '16 at 22:13
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As it was said in the comments, "most likely" is impossible to tell, all we can tell is what is most popular at the moment.

I would say WIMPs (Weakly Interacting Massive Particles) are, or at least were, the favorite candidate. There are many reasons for this :

  • First of, we do believe dark matter can be formulated in term of a new particle rather than lets say an alternative gravity model that could naturally account for it, since those models fail to account for some observations (CMB power spectrum, Bullet Cluster)
  • WIMPs seem to require minimal hypotheses (no electric charge, stable, wide allowed mass range).
  • As such, it is easy to conceive a theory that predicts WIMPs. It is the case of supersymmetry, a rather popular model, which predicts the neutralino, a particle that could very well play the role of dark matter.
  • WIMPs should have their annihilation cross-section lying in a restricted range in order to be compatible with dark matter. Indeed, if this cross-section was too large, too much of it would have annihilated before the freeze-out and it would not account for all of the dark matter. If it was too low, not enough would have annihilated and it would be too abundant. As a result, for WIMP of mass $\sim$ electroweak scale (with a few orders of magnitude margin) the expected annihilation cross-section turns out to be roughly what you'd expect for a particle interacting through the weak interaction. This is the so-called "WIMP miracle". This also means that their cross-section might be high enough for them to be detected in ground experiments.
  • WIMPs cannot be too massive; because of unitarity, WIMPs mass cannot exceed a few hundreds TeVs (their annihilation cross-section have to be lesser than $\mathcal{O}(1/m^2)$, and for large masses, this cross-section would be too small and the WIMP density would exceed the observed density of matter in the universe). So, they might very well be light enough to be produced in colliders.

TL;DR: WIMPs are attractive for theoretical reasons (well motivated) as well as experimental reasons (there is reasonable hope to detect them if they do exist). However, direct detection experiments have not seen anything so far, and we have not found supersymmetry at the LHC (yet?), making WIMPs less appealing then they used to be.

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  • $\begingroup$ See Woit's recent blog, Lucas. He mentions supersymmetry. $\endgroup$ – John Duffield Nov 19 '16 at 15:26
  • $\begingroup$ I know Woit and his views on the matter; I have also read this post. It does not really contradict what I wrote though $\endgroup$ – Lucas Gautheron Nov 19 '16 at 16:06
  • $\begingroup$ Thank you for this, this has given me a lot more to look into for my EPQ. $\endgroup$ – Benjamin Rogers-Newsome Nov 22 '16 at 20:12
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I believe that trillions of neutrinos are being generated by every star every where and even thought each one these neutrinos are nearly mass less all of them add to a very considerable mass. Source: https://www.youtube.com/watch?v=md1CKUQp04Q

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    $\begingroup$ Neutrinos make up, at most, a few percent of the dark matter in the universe, cf. this Physics.SE post. $\endgroup$ – Kyle Kanos Nov 18 '16 at 19:31
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    $\begingroup$ See also this Physics.SE post $\endgroup$ – Kyle Kanos Nov 18 '16 at 19:46
  • $\begingroup$ @KyleKanos in the context of this post, I agree, but sterile neutrinos are a currently viable candidate for all the DM. $\endgroup$ – Kyle Oman Nov 18 '16 at 19:56
  • $\begingroup$ Why did you give my post negative when the quest ask for a theroy. I give one? $\endgroup$ – Riley Cook Nov 18 '16 at 20:28
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    $\begingroup$ Riley, I haven't yet voted on this post but I will suggest two reason that it is not a good post. Firstly it has very little content here—you expect readers to go elsewhere (and worse, to a video) to find out what your argument is. Link only answers are bad answers by definition. We're not building a link-farm here. Second, the most natural interpretation of what you suggest is already well ruled out. The three known flavors of neutrino make a barely noticeable contribution to the dark-matter density. Yes, sterile neutrinos remain a viable candidate, but that isn't what you wrote here. $\endgroup$ – dmckee Nov 18 '16 at 23:44
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Most likely candidate for dark matter?

Space.

I have read into some depth about the various candidates for dark matter, the merits and flaws of each.

I don't know of any merits for WIMPs.

However what I have been unable to attain from texts alone is what is the most likely explanation of dark matter right now? Obviously, some are more likely than others, but at the present day, which ones 'seem' to be the most likely based on the contributions to them at the present.

IMHO when you understand general relativity you will appreciate that the most likely explanation for dark matter is space.

I am going to be writing an EPQ and wish to focus on the most likely theories, as there are too many to go over them all.

General relativity is a likely theory. In fact it's just about the best-tested theory we've got. See Clifford M Will's confrontation between general relativity and experiment. Also see Einstein's Leyden Address where he described a gravitational field as space which was "neither homogeneous nor isotropic". He also said this "has finally disposed of the view that space is physically empty". Space is a something rather than nothing. The crucial point is in The Foundation of the General Theory of Relativity: "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy". That's spatial energy causing gravity, and it isn't made up of particles. Whenever the energy-density of space is not homogeneous, there's a gravitational effect. Why this isn't common knowledge, at least amongst physicists and cosmologists, I just don't know.

You'll be aware of the raisin cake analogy and conservation of energy, so you can appreciate that space expands between the galaxies, but not within the galaxies. So every galaxy is embedded in space with a higher energy-density than the surrounding space. So there's a gravitational effect, and it isn't caused by matter. See Inhomogeneous and interacting vacuum energy and Inhomogeneous Vacuum: An Alternative Interpretation of Curved Spacetime. They don't say space is dark matter, but they do say space isn't nothing. Remember this:

Space has its vacuum energy. When it's inhomogeneous it has a mass equivalence and a gravitational effect. And space, of course, is dark.

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protected by Qmechanic Nov 18 '16 at 21:54

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