Cosmology: Black holes constrain varying constants

Abstract

There is evidence to suggest that the finestructure constant, a - a measure of the strength of the electromagnetic interaction between photons and electrons, is slowly increasing over cosmological timescales. As a=e2 /hc (where e is the electronic charge, h is Planck’s constant and c is the speed of light), this would call into question which of these fundamental quantities are truly constant. Here we consider black-hole thermodynamics as a test of which constants might actually be variable, discounting those that could lead to a violation of the generalized second law of thermodynamics.