Hot dark matter (HDM) is a theoretical form
of dark matter which consists of particles
that travel with ultrarelativistic velocities.
Dark matter is a form of matter that neither
emits nor absorbs light.
Within physics, this behavior is characterized
by dark matter not interacting with electromagnetic
radiation, hence making it dark and rendering
it undetectable via conventional instruments
in physics.
Data from galaxy rotation curves indicate
that approximately 80% of the mass of a galaxy
cannot be seen, forcing researchers to innovate
ways that indirectly detect it through dark
matter's effects on gravitational fluctuations.
There exists no consensus in the theoretical
physics community as to whether dark matter
is divisible into various 'types', but there
exists evidence for differentiating dark matter
into "hot" (HDM) and "cold" (CDM) types–some
even suggesting a middle-ground of "warm"
dark matter (WDM).
The terminology is not meant to invoke any
association with temperature, but instead
refer to the size of the purported dark matter
particles (WIMPs).
In turn, the size of the particles determines
the velocities at which they travel at in
an inverse relationship: HDM travels faster
than CDM because the HDM particles are theorized
to be of lower mass.
== Role in Galaxy Formation ==
In terms of its application, the distribution
of Hot dark matter could also help explain
how clusters and superclusters of galaxies
formed after the Big Bang.
Theorists claim that there exist two classes
of dark matter: 1) those that "congregate
around individual members of a cluster of
visible galaxies" and 2) those that encompass
"the clusters as a whole."
Because Cold dark matter possesses a lower
velocity, it could be the source of "smaller,
galaxy-sized lumps," as shown in the image.
Hot dark matter, then, should correspond to
the formation of larger mass aggregates that
surround whole galaxy clusters.
However, data from the cosmic microwave background
radiation, as measured by the COBE satellite,
is highly uniform, and such high-velocity
Hot dark matter particles cannot form clumps
as small as galaxies beginning from such a
smooth initial state, highlighting a discrepancy
in what dark matter theory and the actual
data are saying.
Theoretically, in order to explain relatively
small-scale structures in the observable Universe,
it is necessary to invoke Cold dark matter
or WDM.
In other words, Hot dark matter being the
sole substance in explaining cosmic galaxy
formation is no longer viable, placing Hot
dark matter under the larger umbrella of mixed
dark matter (MDM) theory.
== Neutrinos ==
An example of a hot dark matter particle is
the neutrino.
Neutrinos have very small masses, and do not
take part in two of the four fundamental forces,
the electromagnetic interaction and the strong
interaction.
They interact by the weak interaction, and
gravity, but due to the feeble strength of
these forces, they are difficult to detect.
A number of projects, such as the Super-Kamiokande
neutrino observatory, in Gifu, Japan are currently
studying these neutrinos.
== See also ==
Lambda-CDM model
Modified Newtonian dynamics
