4.1. Overview
Objects which emit light, whether they are cigars, light bulbs,
stars or galaxies can be characterized by their
emitted energy per unit mass. This is parameterized as
the Mass to Luminosity ratio or M / L.
For cosmological purposes, it is most
convenient to express M / L in terms of solar masses and
luminosities. For main sequence stars it can be shown that
L 
Mn
where n 
3.5-4. Thus a
10 M
star has M / L
10-3, a 1
M star has M / L =1
and a 0.1 M star has
M / L
1000.
The term "dark
matter" refers to the the existence of objects which
have extreme M / L.
Identifying the nature and extent of the dark matter component
in the Universe is, arguably, the
most significant unsolved problem in all of cosmology.
The cosmological parameter 
,
of course, is insensitive to the nature of the mass in the
Universe. The only requirement is that this mass gravitates.
This requirement can be meet if the dark matter is composed
of a small space density of very massive objects or a large
space density of very low mass objects. It is the total
integration of this mass-density over spacetime that determines
and hence the
curvature of the Universe.
Some clues about the nature of dark matter can be obtained by
determining its distribution. For instance, if it can be shown
that the galaxy distribution is an unbiased tracer of the mass
distribution, then we can conclude that the dark matter is
exclusively associated with galaxies and is not found between
galaxies. In this case, the dark matter must be of a form which
allows it to dissipate into small scale gravitational potentials.
At the other extreme, if there is significant bias such that most
of the mass is distributed between the galaxies, then it is not
prone to clumping on small scales and therefore is distributed
more diffusely.
In general, the evidence for dark matter is a result of analyzing the
motions of test particles on some scale and then applying the virial
theorem to estimate the mass. If the virial mass is larger than
the mass estimated from the "light" then this indicates the presence
of gravitating mass which has no corresponding light and is
therefore "dark". The ability of dark matter to gravitate is independent
of its nature. A test particle under the influence of gravity will not
care whether the gravotating mass is baryonic or nonbaryonic.
Figure 4-1
provides an overview of the possible kinds of dark matter which could
exist and provide most of the gravitational mass in the Universe.
|
Figure 4-1: Flow chart overview of the Dark Matter situation. Possible candidates are driven by what the actual value of Omega is. The role of unknown physics in all of this is indicated by the dashed lines and boxes. |