2-4
d
Warm dark matter
Warm dark
matter comprises particles with an FSL comparable to the size
of a protogalaxy. Predictions based on warm dark matter are similar
to
those for cold dark matter on large scales, but with less small-scale
density
perturbations. This reduces the predicted abundance of dwarf galaxies
and may lead to lower density of dark matter in the central parts
of large
galaxies.
Some researchers consider this a better fit to observations. A
challenge for this model is the lack of particle candidates with
the required
mass ˜ 300 eV to 3000 eV.
No known particles can be categorized as warm dark matter. A postulated
candidate is the sterile neutrino: A heavier, slower form of neutrino
that
does not interact through the weak force, unlike other neutrinos.
Some
modified gravity theories, such as scalar–tensor–vector
gravity, require
“warm” dark matter to make their equations work.
2-4
e
Hot dark matter
Hot dark
matter consists of particles whose FSL is much larger than the
size of a protogalaxy. The neutrino qualifies as such particle.
They were
discovered independently, long before the hunt for dark matter:
they were
postulated in 1930, and detected in 1956. Neutrinos’ mass
is less than 10-6
that of an electron. Neutrinos interact with normal matter only
via gravity
and the weak force, making them difficult to detect (the weak
force only
works over a small distance, thus a neutrino triggers a weak force
event
only if it hits a nucleus head-on). This makes them ‘weakly
interacting
light particles’ (WILPs), as opposed to WIMPs.
The three known flavours of neutrinos are the electron, muon,
and tau. Their
masses are slightly different. Neutrinos oscillate among the flavours
as they
move. It is hard to determine an exact upper bound on the collective
average
mass of the three neutrinos (or for any of the three individually).
For
example, if the average neutrino mass were over 50 eV/c2 (less
than 10-5
of the mass of an electron), the universe would collapse. CMB
data and
other methods indicate that their average mass probably does not
exceed
0.3 eV/c2. Thus, observed neutrinos cannot explain dark matter.
Because galaxy-size density fluctuations get washed out by free-streaming,
hot dark matter implies the first objects that can form are huge
superclustersize
pancakes, which then fragment into galaxies. Deep-field observations
show instead that galaxies formed first, followed by clusters
and superclusters
as galaxies clump together.
2-5
Detection of dark matter particles
If dark
matter is made up of sub-atomic particles, then millions, possibly
billions, of such particles must pass through every square centimeter
of the
Earth each second. Many experiments aim to test this hypothesis.
Although
WIMPs are popular search candidates, the Axion Dark Matter Experiment
(ADMX) searches for axions. Another candidate is heavy hidden
sector
particles which only interact with ordinary matter via gravity.
These experiments can be divided into two classes: direct detection
experiments,
which search for the scattering of dark matter particles off atomic
nuclei within a detector; and indirect detection, which look for
the products
of dark matter particle annihilations or decays.
