Could hot dark matter be cold dark matter?

[Rothiemurchus]

Could hot dark matter be cold dark matter?
In other words did hdm cool and become cdm?
And could hdm moving at or close to the speed of light
exist beyond the most distant detected galaxies?

 

[meteor]

wrt hot dark matter, the only candidates that I know that are considered are light neutrinos, and they travel very close to c, and I can't imagine any mechanism that made them to slow to low velocities to become CDM and return to their actual velocities. But this is not a "black or white", there are models in which is postulate the coexistence of cold and hot dark matter, for example the cold+hot dark matter model (CHDM model)

 

[Nereid]

Neutrinos certainly are HDM, and there were plenty around well before galaxies formed.

Neutrinos decoupled from the rest of the universe (except for gravity, of course) well before photons did, so the relict neutrinos are considerably colder than the CMBR, ~1.8K IIRC. Even if at least one flavour of neutrino has the maximum mass allowed by the best observations to date, these 'cold' relict neutrinos are still moving around at very high speeds.

At some point, the universe will have expanded enough for the relict neutrinos to become 'warm'. While there is still some interest in neutrinos cosmologically, they don't attract the interest they did as they are such a minor constituent of the universe.

 

[mathman]

Non-baryonic ("dark") matter is estimated to be about 23% of the universe. Neutrinos (part of the non-baryonic) seems to be at most 5% of the universe so there is a lot of "dark" matter unaccounted for.

 

[hellfire]

The term hot dark matter refers to particles which were relativistic at the time they decoupled from the rest of the components of the universe, whereas cold dark matter refers to particles which were non-relativistic at the time they decoupled.

In order to explain structure formation it is currently believed that the main component of dark matter must be cold. The relevant quantity is the size of the horizon when the particles become non-relativistic and the mass of these particles contained within this horizon size, since this mass will determine the minimum size in the density fluctuations (cutoff scale) -- i.e. the way these particles change the promordial power spectrum suppressing low powers.

Cosmic neutrinos (the usual canditate for hot dark matter) remained relativistic until late (low temperatures) because of their low mass. The cutoff corresponds to a scale similar to galaxy clusters. A subsequent mechanism for fragmentation of structures would be necessary to explain the observations.