There
are many reasons to believe that the universe is full of "dark matter", matter
that influences the evolution of the universe gravitationally, but is not seen
directly in our present observations.
The adjacent image exhibits one recent piece of evidence for undetected
matter: the hot gas seen in the X-ray spectrum would have dispersed if it were
held in place only the by gravity of the mass that is producing light in this
image (the so-called "luminous mass"). The nature of this dark matter, and the
associated "missing mass problem", is one of the fundamental cosmological issues
of modern astrophysics.
Hot Dark Matter and Cold Dark MatterDiscussions of dark matter
typically consider two extremes
- Hot Dark Matter
- Cold Dark Matter
Hot dark matter is composed of particles that
have zero or near-zero mass (the neutrinos are a prime example). The Special
Theory of Relativity requires that massless particles move at the speed of light
and that nearly massless particles move at nearly the speed of light. Thus, such
very low mass particles must move at very high velocities and thus form (by the
kinetic theory of gases) very hot gases.
On the other hand, cold dark matter is composed of objects sufficiently
massive that they move at sub-relativistic velocities. They thus form much
colder gases. The difference between cold dark matter and hot dark matter is
significant in the formation of structure, because the high velocities of hot
dark matter cause it to wipe out structure on small scales.
Tutorial on Current Status of Dark MatterThe following is a brief
tutorial on this issue:
- If inflation
is correct the density of the Universe should be exactly the closure density.
Luminous stars and galaxies contribute only about 0.5% of the closure density,
so 99% of the Universe is in the form of dark matter. We may speculate on what
particles could make up this dark matter.
- The known neutrinoes have problems as candidates for dark matter because
they are relativistic (hot dark matter) and therefore they erase fluctuations
on small scales. Thus, relativistic neutrinos could form large structures like
superclusters, but would have trouble forming smaller structures like
galaxies. These arguments might be at least partially invalidated if one of
the types of neutrinos (the tau neutrino is the obvious candidate) is
considerably more massive than the electron or muon neutrino.
- On smaller scales such as galaxies and clusters of galaxies, dynamical
estimates of the mass based on rotation curves or velocity dispersions of
galaxies indicate that 90% (not 99%) of the total mass is not seen
("sub-luminous"). This implies that the mass density of the Universe is 10% of
the closure density. In this case, the sub-luminous mass could be normal
(baryonic) and be locked up in stellar remnants (white dwarfs, neutron stars,
black holes) or just in very dim stars called "Brown Dwarfs". There is recent
evidence for possible observation of one of these very dim Brown Dwarfs.
- Although inflation demands that the Universe have a density equal to its
critical density (and inflation is necessary to solve certain problems of the
standard big bang model like the horizon problem) there has never been any
observational evidence to support this high of mass density. Most dynamical
studies suggest values of 10-20% of closure density. These studies are based
on large scale deviations from Hubble expansion velocities (peculiar
velocities).
- Large scale structure (e.g. the distribution of galaxies) is very hard to
understand, particularly in light of the relatively smooth microwave
background as measured by the COBE satellite. One way to accomodate this is to
go to a mixed dark matter model in which you have some hot dark matter (for
the large scale) and some cold dark matter to act as a seed for galaxy
formation. None of those models, however, fit the data using the critical
density. The best models to date suggest mixed dark matter and an overall
cosmological mass density of 20-30% of closure. Hence, to retain inflation,
with its inescapable prediction that the Universe must be flat, requires
re-invoking Einstein's cosmological constant - meaning the universe has vacuum
energy (negative pressure) and is currently accelerating. This makes our
cosmology complicated but much data is pointing this way.
- Supernova 1987a neutrino time of flight studies as well as the Solar
Neutrino experiment are consistent with the neutrino having a mass, but a very
small mass, not one that can cosmologically dominate. We cannot currently test
for various supersymmetric particles which would only be created at very high
energy (e.g. the early universe) - so there remain many viable potential
particles that are consistent with the Standard Model of particle physics,
that would remain unnoticed in any accelerator experiments.
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