2-4
Free streaming length
Dark matter
can be divided into cold, warm, and hot categories. These
categories refer to velocity rather than an actual temperature,
indicating
how far corresponding objects moved due to random motions in the
early
universe, before they slowed due to cosmic expansion – this
is an important
distance called the free streaming length (FSL). Primordial density
fluctuations
smaller than this length get washed out as particles spread from
overdense to underdense regions, while larger fluctuations are
unaffected;
therefore this length sets a minimum scale for later structure
formation.
The categories are set with respect to the size of a protogalaxy
(an object
that later evolves into a dwarf galaxy): Dark matter particles
are classified
as cold, warm, or hot according to their FSL; much smaller (cold),
similar
to (warm), or much larger (hot) than a protogalaxy. Mixtures of
the above
are also possible: a theory of mixed dark matter was popular in
the mid-
1990s, but was rejected following the discovery of dark energy.
Cold dark matter leads to a bottom-up formation of structure with
galaxies
forming first and galaxy clusters at a latter stage, while hot
dark matter
would result in a top-down formation scenario with large matter
aggregations
forming early, later fragmenting into separate galaxies; the latter
is
excluded by high-redshift galaxy observations.
2-4
a
Fluctuation spectrum effects
These
categories also correspond to fluctuation spectrum effects and
the
interval following the Big Bang at which each type became non-relativistic.
Davis et al. wrote in 1985:
Candidate particles can be grouped into three categories on the
basis of their
effect on the fluctuation spectrum (Bond et al. 1983). If the
dark matter is
composed of abundant light particles which remain relativistic
until shortly
before recombination, then it may be termed “hot”. The
best candidate for
hot dark matter is a neutrino ... A second possibility is for
the dark matter
particles to interact more weakly than neutrinos, to be less abundant,
and
to have a mass of order 1 keV. Such particles are termed “warm
dark matter”,
because they have lower thermal velocities than massive neutrinos
... there are at present few candidate particles which fit this
description.
Gravitinos and photinos have been suggested (Pagels and Primack
1982;
Bond, Szalay and Turner 1982) ... Any particles which became nonrelativistic
very early, and so were able to diffuse a negligible distance,
are
termed “cold” dark matter (CDM). There are many candidates
for CDM
including supersymmetric particles.
