We discuss several issues bearing on the observed asymmetry between matter and antimatter in the content of the universe, in particular, the possible role in this of Hawking radiation from black holes, with allowance for weak $C$- and $T$-violating interactions. We show that the radiation, species by species, can be asymmetric between baryons and antibaryons. However, if baryon number is microscopically conserved there cannot be a net flux of baryon number in the radiation. Black-hole absorption from a medium with net baryon number zero can drive the medium to an asymmetric state. On the other hand, if baryon conservation is violated, a net asymmetry can develop. This can arise through asymmetric gravitational interactions of the radiated particles, and conceivably, by radiation of long-lived particles which decay asymmetrically. In the absence of microscopic baryon conservation, asymmetries can also arise from collision processes generally, say in the early stages of the universe as a whole. However, no asymmetries can develop (indeed any "initial" ones are erased) insofar as the baryon-violating interactions are in thermal equilibrium, as they might well be in the dense, high-temperature stages of the very early universe. Thus particle collisions can generate asymmetries only when nonequilibrium effects driven by cosmological expansion come into play. A scenario for baryon-number generation suggested by superunified theories is discussed in some detail. Black-hole radiation is another highly nonequilibrium process which is very efficient in producing asymmetry, given microscopic $C$, $T$, and baryon-number violation.

• Received 16 August 1978

DOI:https://doi.org/10.1103/PhysRevD.19.1036