# I Questions regarding DM and particle physics

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1. Feb 9, 2017

### Buzz Bloom

I am hoping and I would most appreciate that one or more PF participants will be able to help me find out the following, either by posting answers or citing useful references.

1. What types of particles are candidates for being DM stuff?
2. For any of these candidates, are there estimates of the mass of the particle type.
3. For any of these candidates, are there estimates of a temperature for which the type would be in equilibrium with photons?

2. Feb 9, 2017

### ChrisVer

Known dark matter particles are cosmic neutrinos.
The rest of DM composition is unknown... in general neutral stable baryonic matter could be part of the DM (based on how the DM was varied in this paper by the existence of the hyperons https://arxiv.org/pdf/hep-ph/0604027.pdf).
Axions, WIMPs (coming from the Lightest Supersymmetric Particles), sterile neutrinos can also be part of the DM content.
Maybe I am missing stuff...

No idea about "estimates of the mass"... There are searches which exclude several areas of the parameter space...

If I recall well, equilibrium holds as long as the particles are relativistic? (i.e. $T \sim E_{kin} \gg m$)

3. Feb 9, 2017

### Staff: Mentor

All dark matter candidates are uncharged. They do not interact with photons, so there is no equilibrium with photons either.

As usual, Wikipedia has a good list of candidates and their rough mass ranges and properties, and searching for "dark matter candidates" produces many additional hits, like this overview.

4. Feb 9, 2017

### ChrisVer

This doesn't mean of course that they cannot interact effectively with photons... from known particles the neutral pions or Higgs decay to diphoton... From DM candidates which could decay to diphoton is the axion.
So you immediately get for this scenario two free parameters : the mass $m_a$ and the effective coupling constant to photons $g_{\alpha \gamma \gamma}$... and plots like Fig3 in : https://arxiv.org/pdf/0811.3347.pdf

5. Feb 9, 2017

### Staff: Mentor

Fine: They do not interact with photons on scales where it would lead to observable consequences on the photons in the universe within the current experimental precision.

You can never rule out very small coupling constants.

6. Feb 9, 2017

### Buzz Bloom

Hi @mfb:

I understand that at the present time DM and photons do not interact. However, isn't it expected that during the very early stages of there universe the temperature was hot enough for the physics to be different, for example some symmetries may not yet be broken. At such a high temperature it might well be that whatever the DM particle is, it would be able to exchange energy with photons and be in a state of thermal equilibrium with photons. When the temperature cooled enough, the DM particle and photons would not longer exchange energy. Does this make sense? If so, at what order of magnitude temperature would this transition be expected to occur?

Regards,,
Buzz

7. Feb 9, 2017

### Buzz Bloom

Hi @ChrisVer:
Thanks for the link.

I would much appreciate a citation to a reference that confirms what I underlined. Can you provide one?

Re your quote from post #2: "Known dark matter particles are cosmic neutrinos."
The following is a quote by Chalnoth in post #2 the thread
https://www.physicsforums.com/threa...m-in-friedmanns-equation.903173/#post-5686926
Neutrinos as a significant component of the dark matter are definitely out, as they are way too light, and wouldn't be able to form structures early enough in the universe to explain observations.​
I asume that you meant that although neutrinos are a part of dark matter, they are not a significant part.
Regards,
Buzz

Last edited: Feb 9, 2017
8. Feb 9, 2017

### Buzz Bloom

Hi @mfb:
Thanks for the links. I have looked through the Wikipedia article, but I had assumed that the list of particles there might be incomplete.

I understand that at the present time there is no way for photons and DM to exchange energy, either directly or indirectly by means of other intermediary baryonic matter. However, at some early time the universe may have been hot enough for the physics to be different. For example, (1) some symmetries may not yet have been broken, or (2) there were some particles which no longer exist that could interact both with photons and DM so that the DM particles and photons were at the same temperature. Does this make sense? If so, where might I find an order-of-magnitude estimate for a temperature at which photons and DM would directly or indirectly interact so that they would be in thermal equilibrium?

Regards,
Buzz

Last edited: Feb 9, 2017
9. Feb 9, 2017

### Staff: Mentor

The coupling of dark matter to photons doesn't change over time. The densities change, of course. At the time of production, the dark matter was in thermal equilibrium (at least in models I know of), but this equilibrium was not necessarily by a DM/photon coupling: At that temperature we also have the weak interaction. As the universe cooled, DM and photons got decoupled, and nonrelativistic dark matter today will have a temperature different from the CMB.

All this can be found in the introductions of the relevant papers, and probably at Wikipedia as well.

10. Feb 9, 2017

### ChrisVer

That's what I meant, obviously I am not trying to question what % of DM each candidate can be... obviously you can have more than 1 "candidates" filling the gap (eg you could potentially have neutralinos and axions at different densities each to fit the observations)...

I don't have a reference. I said it because I was thinking about the decoupling of the several SM particles from the primodial soup, so for example processes like $e^- e^+ \leftrightarrow \gamma \gamma$ went out of equilibrium when the temperature dropped below what was needed to produce back the two electrons.
Of course this is not a general rule, and the way DM particles decoupled from normal matter strongly depends on the model. So for example, you can decouple and freeze axions even during the inflation era.
http://pdg.lbl.gov/2012/reviews/rpp2012-rev-axions.pdf

11. Feb 9, 2017

### Buzz Bloom

ν
Hi:@mfb:

I think I have understood all of your post from other readings, including Wikipedia. What I am still seeking is the temperature at which there is thermal equilibrium. If I choose one of the candidate particles, and there is an associated estimated mass, I can then calculate the temperature at which the average photon energy hν equals this particle mass-energy mc2. What I am not sure about is whether this is an reasonable estimate of an equilibrium temperature. Can you advise me about this?

Regards,
Buzz

Last edited: Feb 9, 2017
12. Feb 9, 2017

### ChrisVer

13. Feb 10, 2017

### Staff: Mentor

Forget the photons. At the time of decoupling, whatever is decoupling (dark matter, or particles decaying to dark matter later) will be in thermal equilibrium with whatever couples to it - probably the electroweak interaction before symmetry breaking or W/Z afterwards depending on the temperature, or other things, it all depends on the dark matter type. Afterwards its temperature changes in a different way than the temperature of regular matter.

Specific values depend on specific models.

14. Feb 10, 2017

### ChrisVer

15. Feb 10, 2017

### Buzz Bloom

Hi @mfb:
I must not be phrasing my question very well.

The reason I am seeking the photon temperature at the time when a DM particle has the same temperature, is because as I understand it, the photon temperature is always proportional to 1/a. It is irrelevant whether or not the DM particle actually interacts directly with photons. If photons interact with something that interacts with something , etc., that interacts with photons, then, as I understand it, this means the average photon energy at this temperature will equal the kinetic energy of the DM particle. Am I wrong about this? If this is correct, I can then calculate the ratio of Q = vaverage/c for the DM particle. From that I can calculate the corresponding value of Q0 for a=1. Q0 is used in the adjusted form of the Friedmann Equation I derived in the thread
This calculation of Q0 does not depend on the temperature of the DM particle remaining the same as the photon temperature. I understand it will not remain the same after the equilibrium ends.

Regards,
Buzz

Last edited: Feb 10, 2017
16. Feb 10, 2017

### Staff: Mentor

It is not, although that is a reasonable approximation most of the time. If nonrelativistic particles or non-equilibrium processes contribute notably, the temperature will not scale with 1/a.
Before electroweak symmetry breaking, that relation becomes meaningless.

This is also model-dependent. The early universe could have particles that decayed to dark matter particles later, for example. The decays can produce particles at any speed.

17. Feb 10, 2017

### Buzz Bloom

Hi @mfb:
I much appreciate your educating me about ideas I have that are incorrect.

Am I correct that this is a truth about physics that is not reflected in the FE?
If so, can you help me understand what kind of change to the FE would correct this for each of the two cases you mentioned?

I now see that I was mistaken about overlooking the case of delayed particle decays. Do you have any suggestions about how an estimate can be made regarding the current very small residual kinetic energy of such created DM particles?

Regards,
Buzz

18. Feb 10, 2017

### Staff: Mentor

The Friedmann equations tell you how the size of the universe changes. They don't tell you what the matter in the universe does. And the form you posted (which is only an approximation anyway) does not work nicely if there are conversions between the different types. It works in the universe after a few minutes, when the amount of matter, radiation, and potentially dark matter, doesn't change much any more.
It depends on the specific model. This is getting somewhat repetitive.

19. Feb 10, 2017

### Buzz Bloom

Hi @mfb:
I apologize for being repetitive. I much appreciate all your help and your patience.

Regards,
Buzz

20. Feb 11, 2017

### ChrisVer

Can you read what this equation says?

The photon temperature didn't always scale like 1/a, there were deviations from that scaling for example everytime a particle turned non-relativistic. That's because the photons at that time got heated (entropy from the no-more relativistic particles is transfered to the photon bath). You can read about that in the damtp script I posted above.

what is the residual kinetic energy of those particles?

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