Mass-to-Luminosity Ratios

Mass-to-Luminosity Ratios:
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.

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:
We know of the presence of dark matter from dynamical studies. But we also know from the
abundance of light elements that there is also a problem in our understanding of the fraction of the
mass of the Universe that is in normal matter or baryons. The fraction of light elements (hydrogen,
helium, lithium, boron) indicates that the density of the Universe in baryons is only 2 to 4% what
we measure as the observed density.
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 normal 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.

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.