MadSci Network: Astronomy |
Dark
matter and its nature have a long and complicated history. Some of the
best evidence of the existence of dark matter is found in the rotation
curves of galaxies and the dynamics of clusters of galaxies.
However, this dark matter could be baryonic or non-baryonic. To resolve
this, scientists have performed detailed calculations of nucleosynthesis in
the early universe. These calculations combined with observations of the
abundance of deuterium relative to hydrogen indicate that the amount of
mass in baryons in the universe is about 3 percent of the critical density
(the desnity beyond which th universe would eventually recollapse into a
Big Crunch). Other observations of galaxies and galaxy clusters indicate
that the total amount of matter in the universe is closer to 10-30 percent
of the critical value.
Another piece of evidence pointing towards non-baryonic dark matter is the
hot gas
surrounding many clusters of galaxies. This gas contains a few times more
mass than the stars in the galaxies in the cluster, but the total amount of
baryonic matter (stars and hot gas) is only 10-30 percent of the total
amount of matter. This rules out diffuse gas as a possible reservoir of
baryonic dark matter. Could there be baryonic dark matter? Sure, and a
large collaboration has been searching for it via gravitational
microlensing.
Finally, an Italian group has announced tentative evidence of the direct detection of non-baryonic dark
matter. An American group
performing a
similar study disputes
their claim, and it's fair to say that the jury is still out on whether the
claim is valid or not. Hopefully these teams along with the GLAST
satellite will decipher the nature of dark matter with sufficient evidence
that debates will be forgotten.
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