Dark Matter Candidates 215level of particle physics. At the cosmological level there are two new parameters, the
ratio푥of the temperatures of the ordinary and mirror cosmic background radiations,
and the relative amount훽of mirror baryons compared to the ordinary ones. In the
presence of mirror matter, the matter energy density parameter is
훺m=훺b( 1 +훽). (9.14)Scalar fieldsSuppose that the dark matter particles are described by a spin-0 scalar
field like the QCD axion. A variety of unification theories have proposed other scalar
field candidates, the bosonic particles being typically ultralight with an ultrahigh
phase-space density. This may lead to aBose–Einstein Condensate(BEC), a macro-
scopic occupancy of the many-body ground state [19]. In principle, for a fixed number
of thermalized identical bosons, a BEC will form if a푛휆^3 deB≫1where푛is the num-
ber density and휆deBis the de Broglie wavelength. This is equivalent to there being a
critical temperature푇c, below which a BEC can form. For small boson masses the cor-
responding critical temperature of condensation is so high (≫TeV), that the bosons
are fully condensed very early on, that is, almost all of the bosons occupy the low-
est available energy state. Hence, the cosmological scalar field dark matter can be
described by a single coherent, classical scalar field.
If the thermal decoupling within the bosonic dark matter occurs when the expan-
sion rate exceeds its thermalization rate well after condensation, most of the bosons
will stay in the ground state (BEC) and the classical field remains a good description,
analogous to the fact that CMB photons after decoupling still follow a black-body
distribution. Given the huge critical temperature at hand, one may effectively con-
sider the BEC state as an initial condition. On the other hand, one may also envisage
a scenario in which the coherent scalar field is created gravitationally at the end of
inflation. A prime motivation for studying scalar field dark matter has been its abil-
ity to suppress small-scale clustering and hence potentially resolve some dark matter
problems.
Supersymmetric Cold Dark Matter (CDM). Particles which were very slow at time
푡eqwhen galaxy formation started are candidates for CDM. If these particles are
massive and have weak interactions, so called WIMPs (Weakly Interacting Massive
Particles), they became nonrelativistic much earlier than the leptons and become
decoupled from the hot plasma. For instance, the supersymmetric models (SUSY)
contain a very large number of new particles, of which the lightest ones would be
stable. At least three such neutral SUSY ‘sparticles’—thephotino,theZinoand the
Higgsino—or a linear combination of them (theneutralino) could serve. However, neg-
ative results from laboratory searches now appear to rule out the minimal supersym-
metric model as a remedy for CDM. Heavier nonminimally supersymmetric particles
are no longer obvious CDM candidates.
Sterile Neutrinos. Very heavy sterile neutrinos could also be CDM candidates, or
other cold thermal relics of mass up to some 300TeV. All this is very speculative.
Alternatively, the CDM particles may be very light if they have some superweak