Those gamma rays coming out of galactic center, flagging the presence of an antimatter cloud of enormous extent, have spawned few explanations more exotic than the one we consider today: Black holes. Primordial black holes, in fact, produced in their trillions at the time of the Big Bang and left evaporating through so-called ‘Hawking radiation’ ever since. That’s the theory of Cosimo Bambi (Wayne State University) and colleagues, who are studying the same antimatter cloud we recently examined here in terms of its possible connection with low mass X-ray binary stars.
Hawking radiation offers a mechanism for small black holes to lose mass over time. But since the phenomenon has never been observed, the upcoming launch of the GLAST (Gamma-ray Large Area Space Telescope) satellite again looms large in significance. GLAST should be able to find evaporating black holes, assuming they are there, and there is even some possibility that the Pierre Auger Observatory may eventually detect tiny black holes created when high-energy cosmic rays slam into the upper atmosphere. If so, we would have a window into any evaporative effects associated with these enigmatic events.
But assuming that black holes do evaporate, the trick is to figure out how fast, and that rate depends upon mass, with more massive black holes producing fewer evaporated particles. What Bambi’s team argues is that a mass of about 1016 grams, roughly that of a fairly common asteroid, will produce the right amount of antimatter to explain the detections. Theoretically, the signature radiation from black holes of this particular size should be observable given the right equipment, but neither the GLAST mission or ESA’s INTEGRAL satellite seems well suited for that task (more on the latter problem in this New Scientist story).
All of which is interesting it itself, but the paper offers a bonus:
We have considered evaporating primordial BHs [black holes], as a possible source of positrons to generate the observed photon 511 keV line from the Galactic Bulge. The analysis of the accompanying continuous photon background produced, in particular, by the same evaporating BHs, allows to ﬁx the mass of the evaporating BHs near 1016 g. It is interesting that the necessary amount of BHs could be of the same order of magnitude as the amount of dark matter in the Galactic Bulge. This opens a possibility that such primordial BHs may form all cosmological dark matter. The background MeV photons created by these primordial BHs can be registered in the near future, while the neutrino ﬂux may be still beyond observation. The signiﬁcance of this model would be difficult to overestimate, because these BHs would present a unique link connecting early universe and particle physics.
So there’s a theory for you: Primordial black holes as the explanation for dark matter itself. But bear in mind that along with the x-ray binaries so recently considered in relation to galactic antimatter, other explanations are still in play, including type Ia supernovae and a host of far more exotic possibilities outlined in the introduction to the paper. GLAST should help, but the suspicion grows that the antimatter cloud at galactic center may remain enigmatic for some time to come.
The paper is Bambi et al., “Primordial black holes and the observed Galactic 511 keV line,” available online.