2-4
b
Alternative definitions
Another
approximate dividing line is warm dark matter became non-relativistic
when the universe was approximately 1 year old and 1 millionth
of
its present size and in the radiation-dominated era (photons and
neutrinos),
with a photon temperature 2.7 million Kelvins. Standard physical
cosmology
gives the particle horizon size as 2 c t (speed of light multiplied
by
time) in the radiation-dominated era, thus 2 light-years. A region
of this size
would expand to 2 million light-years today (absent structure
formation).
The actual FSL is approximately 5 times the above length, since
it continues
to grow slowly as particle velocities decrease inversely with
the scale
factor after they become non-relativistic. In this example the
FSL would
correspond to 10 million light-years, or 3 megaparsecs, today,
around the
size containing an average large galaxy.
The 2.7 million K photon temperature gives a typical photon energy
of 250
electronvolts, thereby setting a typical mass scale for warm dark
matter:
particles much more massive than this, such as GeV–TeV mass
WIMPs,
would become non-relativistic much earlier than one year after
the Big
Bang and thus have FSLs much smaller than a protogalaxy, making
them
cold. Conversely, much lighter particles, such as neutrinos with
masses of
only a few eV, have FSLs much larger than a protogalaxy, thus
qualifying
them as hot.
2-4
c
Cold dark matter
Cold dark
matter offers the simplest explanation for most cosmological
observations. It is dark matter composed of constituents with
an FSL much
smaller than a protogalaxy. This is the focus for dark matter
research, as hot
dark matter does not seem capable of supporting galaxy or galaxy
cluster
formation, and most particle candidates slowed early.
The constituents of cold dark matter are unknown. Possibilities
range
from large objects like MACHOs (such as black holes and Preon
stars
or RAMBOs (such as clusters of brown dwarfs), to new particles
such as
WIMPs and axions.
Studies of Big Bang nucleosynthesis and gravitational lensing
convinced
most cosmologist that MACHOs cannot make up more than a small
fraction
of dark matter. According to A. Peter: “... the only really
plausible darkmatter
candidates are new particles.”
The 1997 DAMA/NaI experiment and its successor DAMA/LIBRA in
2013, claimed to directly detect dark matter particles passing
through the
Earth, but many researchers remain skeptical, as negative results
from
similar experiments seem incompatible with the DAMA results.
Many supersymmetric models offer dark matter candidates in the
form of
the WIMPy Lightest Supersymmetric Particle (LSP). Separately,
heavy sterile
neutrinos exist in non-supersymmetric extensions to the standard
model
which explain the small neutrino mass through the seesaw mechanism.
