Hawking speculated (perhaps 30 years ago) that substellar "mini" black holes could have been created in the big bang. He even thought that they might persist to the present epoch, and be "evaporating" through a radiative process. Subsequent inflation theory makes this considerably less likely, but not completely impossible. Presume for a moment that evaporating mini black holes do exist. At what range would they be visible to GLAST [Fermi]?
Thanks for your interesting question. You have a good understanding of the issues. Two different features of a hypothetical black hole evaporation would determine its visibility to GLAST [Fermi]. One is the distance and the other is the intrinsic brightness. In addition to the uncertainty surrounding primordial production of these small black holes, as you mentioned, the brightness as a function of time is highly model-dependent. Different models would therefore predict different ranges of distances of visibility.
Here's why the brightness is model dependent. According to Hawking's idea, as the mass of the black hole decreases (due to evaporation), the effective temperature increases. As the temperature increases, the evaporation rate increases. So, you can see that a sort of runaway process occurs at the end of life as the mass gets small, and the end should be quite dramatic. For any particular temperature (or mass), the evaporation rate depends on the number of different ways energy can be carried away, which corresponds with the number of types of particles that can be produced. We've only explored particle physics up to an energy scale of a few TeV (limited by the size of the accelerators we can build), and, although there is a theory that describes to good precision all the interactions observed below a few TeV, any extrapolation to higher energy (and therefore to the high temperatures at the end of the black hole's life) is a guess. There is good reason to believe the current model of particles and their interactions is incomplete, and extensions of the model all predict new particles at higher masses. The details of those different models will determine the brightness profile of the burst of energy as the black hole disappears.
The EGRET experiment (GLAST's [Fermi's] predecessor) on the Compton Gamma Ray Observatory put significant limits on primordial black hole evaporation, assuming a particular particle model. That paper was published in the Astrophysical Journal, Volume 434, pp557-559, in 1994. You might have access to it by clicking on the link below.
Dr. Steven Ritz, NASA-GSFC
Fermi Deputy Project Scientist and LAT Instrument Scientist
