Physical Foundations of Cosmology

204 The very early universe

60

50

40

30

20

10

0

MSSM

q (GeV)

105 1010 1015

αs−^1

1

αw−^1

α 1 −^1

Fig. 4.20.

for this dark matter. These candidates can be classified according to whether the
dark matter particles originated via decoupling from a thermal bath or were created
in some nonthermal process.
In turn, thermal relics can be categorized further as to whether they were rel-
ativistic or nonrelativistic at the moment of decoupling. The relics which were
relativistic at this time constitute hot dark matter, while those which were nonrel-
ativistic constitute cold dark matter. The neutrino and the neutralino are examples
of hot and cold relics respectively.
The simplest example of a nonthermal relic is the condensate of a weakly inter-
acting massive scalar field. A well motivated particle physics candidate of this kind
is theaxion. Because the momentum of axions in the condensate is equal to zero,
axions can also serve as a cold dark matter component even although their masses
are very small. Below we describe some generic features characterizing different
dark matter candidates.

Hot relicsThe freeze-out of hot relics occurs when they are still relativistic. After
relics decouple from matter their number densitynψdecreases in inverse proportion
to the volume, that is, asa−^3 .The total entropy density of the remaining matter,s,
scales in the same way and hence the rationψ/shas remained constant until the
present time.