Axionic black holes and wormholes

Theories in which gravity is coupled to a Kalb-Ramond field are known to have black hole solutions characterized by the value of the conserved axion charge. The Kalb-Ramond field configuration for these black holes has vanishing field strength. The axion charge may be measured by an analog of the Aharanov-Bohm interference effect. The axion-charge to mass ratio may be arbitrarily large, as contrasted to the case of the Reissner-Nordstrom black hole where the electric-charge to mass ratio has an upper bound of one. The generic endpoint of semiclassical evaporation of an axionic black hole would therefore be an object of very large axion charge with mass of order the Planck mass. Axion charge also couples to Giddings-Strominger type instantons (wormholes) present in these theories. Instead of evaporating completely, therefore, it is likely that an axionic black hole will be swallowed by a wormhole, avoiding the appearance of a naked singularity. The loss of quantum coherence is a more subtle issue.This essay received the third award from the Gravity Research Foundation for the year 1989-Ed.On leave from the Physics Department, Syracuse University, Syracuse NY 13244-1130, USA.