Does dark matter have to be one particle

[KeplerJunior]

Whenever physicist talk about dark matter they always seem to be implying that dark matter is one particle. Does this have to be the case? Could there not be many particles that are heavy, slow and with no electric charge that could all be varieties of dark matter.

 

[Vanadium 50]

There could be, but why add a bunch of epicycles unnecessarily?

 

[DaveC426913]

Whenever physicist talk about dark matter they always seem to be implying that dark matter is one particle. Does this have to be the case? Could there not be many particles that are heavy, slow and with no electric charge that could all be varieties of dark matter.

Of course there could be. Remember though, particles come in families. Hadrons (baryons and mesons), leptons, fermions, etc.

Saying Dark Matter is a particle is essentially saying Dark Matter is a family of particles, defined by some common principle.

 

[kurros]

I seem to recall some rather outlandish ideas about "mirror" dark matter floating around some time ago, in which there was a full "copy" of all the Standard Model particles and forces in a hidden dark sector. You could then have a whole dark universe, complete with stars and planets. So there are lots of wild ideas out there. This amount of "clumpiness" in the dark sector is somewhat ruled out these days though, I think.

 

[twofish-quant]

Whenever physicist talk about dark matter they always seem to be implying that dark matter is one particle.

I don't think they are. We do know that whatever forms dark matter has the same *types* of characteristics (it's cold and dark), but there is nothing that says that it's only one type of cold dark matter.

In fact, one of the candidates for dark matter are supersymmetric particles and there are tons of different types of those particles.

Could there not be many particles that are heavy, slow and with no electric charge that could all be varieties of dark matter.

Yes.

 

[Chronos]

Most particle physicists would agree there is no reason to believe only one species of DM particles exists. As already mentioned, there are a number of species of ordinary matter particles, and little to suggest the DM realm is not similarly populated.

 

[Nabeshin]

Most particle physicists would agree there is no reason to believe only one species of DM particles exists. As already mentioned, there are a number of species of ordinary matter particles, and little to suggest the DM realm is not similarly populated.

While this is true, since we are just scratching the tip of the iceberg for direct detection of dark matter particles, we are starting with the simplest cases where there is just one. A common example is that DM could be the lightest supersymmetric particle, which is prevented from decay by R-parity. In such a model, even if other supersymmetric particles possesses qualities like that of dark matter, they quickly decay into the lightest one and you're left with essentially one DM particle.

Similarly, things like axions are only one particle rather than a whole dark sector. Ultimately, we understand these simple physical scenarios the best and they are perhaps the best motivated by theoretical concerns. If it turns out that we do find some DM particles, but only a small fraction of what we should have found, this could provide good evidence for a much larger dark sector.

 

[DaveC426913]

While this is true, since we are just scratching the tip of the iceberg for direct detection of dark matter particles, we are starting with the simplest cases where there is just one.

Personally, I suspect it will be more a matter of finding one defining property of dark matter particles - so it will essentially be one particle to us - but as we get more sophisticated, we will discover subtleties starting to emerge, so that the first one particle will be differentiated into more and more, until we have a whole family.

We've found a dark matter particle, which we'll call a darkepton.
Now we're finding subtle differences in darkeptons, we'll call these ones up darkeptons and those ones down darkeptons.

 

[mfb]

I seem to recall some rather outlandish ideas about "mirror" dark matter floating around some time ago, in which there was a full "copy" of all the Standard Model particles and forces in a hidden dark sector. You could then have a whole dark universe, complete with stars and planets. So there are lots of wild ideas out there. This amount of "clumpiness" in the dark sector is somewhat ruled out these days though, I think.

The "clumpiness" is the problem with these models. Compact dark objects would give a lot of gravitational lensing, which would have been detected by the exoplanet searches. In addition, it would give some strange systems like stars orbiting "nothing" (not even a black hole).