Questions regarding DM and particle physics

[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?

 

[ChrisVer]

1. What types of particles are candidates for being DM stuff?

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...

2. For any of these candidates, are there estimates of the mass of the particle type.

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

3. For any of these candidates, are there estimates of a temperature for which the type would be in equilibrium with photons?

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

 

[mfb]

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.

 

[ChrisVer]

They do not interact with photons

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

 

[mfb]

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.

 

[Buzz Bloom]

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.

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

 

[Buzz Bloom]

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

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

 

[Buzz Bloom]

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.

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

 

[mfb]

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.

 

[ChrisVer]

I asume that you meant that although neutrinos are a part of dark matter, they are not a significant part.

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

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

 

[Buzz Bloom]

ν

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.

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

 

[ChrisVer]

http://www.slac.stanford.edu/econf/C040802/papers/L002.PDF
also for a way to put bounds to the masses, check this script

 

[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?

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.

 

[ChrisVer]

Also the following may help, eg 3.3.2
http://www.damtp.cam.ac.uk/user/db275/Cosmology/Chapter3.pdf

but the whole script can help you understand a whole lot more, so you should read it all

 

[Buzz Bloom]

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.

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 = v_average/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

https://www.physicsforums.com/threads/q-re-the-matter-term-in-friedmanns-equation.903173 .​

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

 

[mfb]

because as I understand it, the photon temperature is always proportional to 1/a

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.

If this is correct, I can then calculate the ratio of Q = v_average/c for the DM particle. From that I can calculate the corresponding value of Q₀ for a=1.

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.

 

[Buzz Bloom]

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

If nonrelativistic particles or non-equilibrium processes contribute notably, the temperature will not scale with 1/a.

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?

The early universe could have particles that decayed to dark matter particles later, for example. The decays can produce particles at any speed.

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

 

[mfb]

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.

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?

It depends on the specific model. This is getting somewhat repetitive.

 

[Buzz Bloom]

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

Regards,
Buzz

 

[ChrisVer]

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?

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 transferred to the photon bath). You can read about that in the damtp script I posted above.

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?

what is the residual kinetic energy of those particles?

 

[Buzz Bloom]

Hi @ChrisVer:

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 transferred to the photon bath). You can read about that in the damtp script I posted above.

I apologize that I have not yet read the article your post #14. I will do that before asking any more questions in this thread,

ADDED
except perhaps some which seek clarification about the article.

what is the residual kinetic energy of those particles?

My understanding (very possibly incorrect) of the physics is that a mass particle with kinetic energy (KE) and momentum (possibly relativistic) at some time in the past has less as time passes and the universe expands. The momentum varies as 1/a, and the KE has a corresponding reduction. What I meant by "residual" KE is that KE which is still present as a property of the particle even when KE becomes very small at the present time.

Regards,
Buzz