Hot DM refers to particles, such as neutrinos, that were moving at
nearly the speed of light at redshift z ~ 106 (or time
t ~ 1
yr), when the temperature T ~ 3 x 102 eV and the cosmic
horizon first encompassed 1012
M, the amount
of dark matter
contained in the halo of a large galaxy like the Milky Way. Hot DM
particles must also be still in thermal equilibrium after the last
phase transition in the hot early universe, the QCD confinement
transition, which presumably took place at
TQCD
102
MeV. Hot DM particles have a cosmological number density roughly
comparable to that of the microwave background photons, which (as we
will see shortly) implies an upper bound to their mass of a few tens
of eV. This then implies that free streaming of these relativistic
particles destroys any fluctuations smaller than supercluster size,
~ 1015 M
.
The ``hot,'' ``warm,'' ``cold'' DM terminology was introduced in 1983
[5,
6].
Warm DM particles interact much more weakly than
neutrinos. They decouple (i.e., their mean free path first exceeds the
horizon size) at T >> TQCD, and are not heated by the
subsequent annihilation of hadronic species. Consequently their
number density is roughly an order of magnitude lower, and their mass
an order of magnitude higher, than hot DM particles. Fluctuations
corresponding to sufficiently large galaxy halos,
1011
M
, could then
survive free streaming. In theories of
local supersymmetry broken at ~ 106 GeV, gravitinos could be DM
of the warm variety
[7,
8,
9]. Other warm
dark matter candidates are also possible, of course, such as
right-handed neutrinos
[10].
Warm DM does not fit the observations if
m = 1
[11], but for low
m some
have suggested that it may be worth reconsidering, to avoid some
possible problems of Cold DM
[12,
13]. However, the
cutoff in the power spectrum P(k) at large k implied by WDM will
also inhibit the formation of small dark matter halos at high
redshift. But such small halos are presumably where the first stars
form, which produce metals rather uniformly throughout the early
universe as indicated by observations of the Lyman
forest
(neutral hydrogen clouds seen in absorption in quasar spectra).
Cold DM consists of particles for which free streaming is of no
cosmological importance. Two different sorts of cold DM consisting of
elementary particles have been proposed, heavy thermal remnants of
annihilation such as supersymmetric neutralinos, and a cold Bose
condensate such as axions. A universe where the matter is mostly cold
DM and there is a large cosmological constant looks very much like the
one astronomers actually observe, and this
low-m
CDM
model [14]
is the current favorite model for structure
formation in the universe
[15,
16,
17].