What is Dark Matter?
The story of dark matter is best divided into two parts. First
we have the reasons that we know that it exists. Second is the
collection of possible explanations as to what it is.
Why the Universe Needs Dark Matter
We believe that that the Universe is critically balanced between
being open and closed. We derive this fact from the observation
of the large scale structure of the Universe. It requires a certain
amount of matter to accomplish this result. Call it M.
We can estimate the total baryonic matter of the universe
by studying Big Bang nucleosynthesis. This is done by connecting
the observed He/H ratio of the Universe today to the amount of
baryonic matter present during the early hot phase when most
of the helium was produced. Once the temperature of the Universe
dropped below the neutron-proton mass difference, neutrons began
decaying into protons. If the early baryon density was low, then
it was hard for a proton to find a neutron with which to make
helium before too many of the neutrons decayed away to account
for the amount of helium we see today. So by measuring the He/H
ratio today, we can estimate the necessary baryon density shortly
after the Big Bang, and, consequently, the total number of baryons
today. It turns out that you need about 0.05 M total baryonic
matter to account for the known ratio of light isotopes. So only
1/20 of the total mass of the Universe is baryonic matter.
Unfortunately, the best estimates of the total mass of everything
that we can see with our telescopes is roughly 0.01 M. Where
is the other 99% of the stuff of the Universe? Dark Matter!
So there are two conclusions. We only see 0.01 M out of 0.05
M baryonic matter in the Universe. The rest must be in baryonic
dark matter halos surrounding galaxies. And there must be some
non-baryonic dark matter to account for the remaining 95% of
the matter required to give omega, the mass of the Universe,
in units of critical mass, equal to unity.
For those who distrust the conventional Big Bang models, and
don't want to rely upon fancy cosmology to derive the presence
of dark matter, there are other more direct means. It has been
observed in clusters of galaxies that the motion of galaxies
within a cluster suggests that they are bound by a total gravitational
force due to about 5-10 times as much matter as can be accounted
for from luminous matter in said galaxies. And within an individual
galaxy, you can measure the rate of rotation of the stars about
the galactic center of rotation. The resultant "rotation
curve" is simply related to the distribution of matter in
the galaxy. The outer stars in galaxies seem to rotate too fast
for the amount of matter that we see in the galaxy. Again, we
need about 5 times more matter than we can see via electromagnetic
radiation. These results can be explained by assuming that there
is a "dark matter halo" surrounding every galaxy.
What is Dark Matter?
This is the open question. There are many possibilities, and
nobody really knows much about this yet. Here are a few of the
many published suggestions, which are being currently hunted
for by experimentalists all over the world. Remember, you need
at least one baryonic candidate and one non-baryonic candidate
to make everything work out, so there there may be more than
one correct choice among the possibilities given here.
· Normal matter which has so far eluded our gaze, such
· dark galaxies
· brown dwarfs
· planetary material (rock, dust, etc.)
· Massive Standard Model neutrinos. If any of the neutrinos
are massive, then this could be the missing mass. On the other
hand, if they are too heavy, as the purported 17 KeV neutrino
would have been, massive neutrinos create almost as many problems
as they solve in this regard.
· Exotica (See the "Particle Zoo" FAQ entry
for some details)
Massive exotica would provide the missing mass. For our purposes,
these fall into two classes: those which have been proposed for
other reasons but happen to solve the dark matter problem, and
those which have been proposed specifically to provide the missing
Examples of objects in the first class are axions, additional
neutrinos, supersymmetric particles, and a host of others. Their
properties are constrained by the theory which predicts them,
but by virtue of their mass, they solve the dark matter problem
if they exist in the correct abundance.
Particles in the second class are generally classed in loose
groups. Their properties are not specified, but they are merely
required to be massive and have other properties such that they
would so far have eluded discovery in the many experiments which
have looked for new particles. These include WIMPS (Weakly Interacting
Massive Particles), CHAMPS, and a host of others.
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