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From comparing the mass estimates to the observed amount of light from
galaxies, and from the abundance of light elements, that there is a
problem with the fraction of the mass of the Universe that is in
normal matter or baryons. The fraction of light elements indicates
that the density of the Universe in baryons is only 2 to 4% what we
measure as the observed density. The rest of the mass appears to be
`missing', meaning unobserved or dark.
Exactly how much of the Universe is in the form of dark matter is a
mystery and difficult to determine, obviously because its not
visible. It has to be inferred by its gravitational effects on the
luminous matter in the Universe (stars and gas) and is usually
expressed as the mass-to-luminosity ratio (M/L). A high M/L indicates
lots of dark matter, a low M/L indicates that most of the matter is in
the form of baryonic matter, stars and stellar reminants plus gas.
A important point to the study of dark matter is how it is distributed.
If it is distributed like the luminous matter in the Universe, that most
of it is in galaxies. However, studies of M/L for a range of scales
shows that dark matter becomes more dominate on larger scales.
- dark matter forms the halos around galaxies and the intracluster space between galaxies
- it is increasly important on large scales, early hope was that dark matter would
be sufficient to close the Universe (Omega = 1), however, its value maxed out at 0.3
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Most importantly, on very large scales of 100 Mpc's (Mpc = megaparsec,
one million parsecs and kpc =
1000 parsecs) the amount of dark matter inferred is near the value needed
to close the Universe. Thus, it is for two reasons that the dark matter
problem is important, one to determine what is the nature of dark
matter, is it a new form of undiscovered matter?, the second is the
determine if the amount of dark matter is sufficient to close the
Universe. |
Baryonic Dark Matter:
- the key problem for the 21st century is to determine the nature of dark matter
- searches for dark matter have divided into two paths, one to look for a baryonic dark matter candidate, such
as old stars
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It is not too surprising to find that at least some of the matter in the
Universe is dark since it requires  energy to observe an object, and most
of space is cold and low in energy. Can dark matter be some form of
normal matter that is cold and does not radiate any energy? For example,
dead stars?
Once a star has used up its hydrogen fuel, it usually ends its life as a
white dwarf star, slowly cooling to become a black dwarf. However, the
timescale to cool to a black dwarf is thousands of times longer than the
age of the Universe. High mass stars will explode and their cores will
form neutron stars or black holes. However, this is rare and we would
need 90% of all stars to go supernova to explain all of the dark
matter. |
- while stellar reminants and low mass objects certainly exist, they do not appear to exist in numbers needed
to explain dark matter, thus a second path is to consider non-baryonic candidates
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Another avenue of thought is to consider low mass objects. Stars that
are very low in mass fail to produce their own light by thermonuclear
fusion. Thus, many, many brown dwarf stars could make up the dark
matter population. Or, even smaller, numerous Jupiter-sized planets, or
even plain rocks, would be completely dark outside the illumination of a
star. The problem here is that to make-up the mass of all the dark
matter requires huge numbers of brown dwarfs, and even more Jupiter's or
rocks. We do not find many of these objects nearby, so to presume they
exist in the dark matter halos is unsupported. |
Non-Baryonic Dark Matter:
- dark matter is so unusual that it seems plausible that it is not composed of normal matter
- known particles and new particles are considered
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An alternative idea is to consider forms of dark matter not composed of
quarks or leptons, rather made from some exotic material. If the
neutrino has mass, then it would make a good dark matter candidate since
it interacts weakly with matter and, therefore, is very hard to detect.
However, neutrinos formed in the early Universe would also have mass,
and that mass would have a predictable effect on the cluster of
galaxies, which is not seen.
Another suggestion is that some new particle exists similar to the
neutrino, but more massive and, therefore, more rare. This Weakly
Interacting Massive Particle (WIMP) would escape detection in our
modern particle accelerators, but no other evidence of its existence has
been found. |
- some solutions do not use new particles but instead consider exotic early Universe effects
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The more bizarre proposed solutions to the dark matter problem require
the use of little understood relics or defects from the early Universe. One
school of thought believes that topological defects may have appears
during the phase transition at the end of the GUT era. These defects
would have had a string-like form and, thus, are called cosmic strings.
Cosmic strings would contain the trapped remnants of the earlier dense
phase of the Universe. Being high density, they would also be high in
mass but are only detectable by their gravitational radiation.
Lastly, the dark matter problem may be an illusion. Rather than missing
matter, gravity may operate differently on scales the size of galaxies.
This would cause us to overestimate the amount of mass, when it is the
weaker gravity to blame. This is no evidence of modified gravity in our
laboratory experiments to date. |
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