4.6.2. Exotic Baryonic Dark Matter
Big Stellar Remnants: From
time to time there has been
speculation that a population of stars must have existed in the
Galactic Halo prior to the formation of Globular clusters (Pop II.
objects). This population is referred to as Population III (Carr
et al. 1984) and
is theorized to have provided the initial compliment of heavy elements
that are observed to exist in Pop. II objects.
The candidate Population III objects are known
as Very Massive Objects (VMOs) which are stars of mass 103 -
106 M
. These
objects would be incredibly luminous for short
periods of time and therefore have to turn on at very high redshift
in order to escape detection. They would produce a substantial yield
of metals and quickly collapse into a very large stellar remnant.
Clusters of these remnants might gravitationally coalesce into one
large mass black hole perhaps forming the central engine to power a
QSO at high redshift. If we assume an average mass
of this coalesced stellar remnants of 106
M, then the required
space density is 10-8 pc-3. As these objects are in the
halo, their orbits carry them in and out of the galactic plane and
their velocity dispersion would be set by the halo mass. By our adopted
halo parameters, the expected velocity dispersion would be
300 km s-1 and on average each massive remnant would pass
through the
galactic plane every 108 years. As 106 of them are
required to
form the halo mass, we expect a galactic plane crossing every 100 years
or so. The passage of a 106
M black hole through the
gaseous plane of our disk would probably not go unnoticed.
Quantum Black Holes: An
intersection between General Relativity and the precepts of
quantum mechanics allows for the existence of a very unusual particle -
a mini black hole. A mini black hole has a mass equivalent to that
of a large terrestrial mountain, about 1015 grams and a radius
of 10-13 cm. Such an object could only be created by tremendous
compressional forces which might have been present in the very
early Universe. As the radius of a mini black hole is like that of
a nucleon, it is a quantum mechanical system. As there are no energy
barriers in a quantum mechanical system, tunneling will allow energy
to leak out from the event horizon of the mini black hole thus causing
the system to shrink which increases the rate of energy leakage.
Mini black holes are then destined to evaporate with the last stage
being a sudden release of high energy photons (gamma rays). For a mass
of 1015 grams the evaporation time scale is 10 billion years.
While the existence of mini black holes may appear to be preposterous
as well as a desperate attempt to understand the dark matter problem,
it is no more so than some of the particle schemes proposed below.
At least in this case, there is a prediction. If mini black holes
of mass 1015 grams exist, the Universe now should exhibit gamma
ray bursters which are isotropically distributed. Amazingly,
this population has now been observed (see
Chapter 6) although no
one really believes that the population is due to evaporating mini
black holes.