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.