Testing the weak equivalence principle with the cosmic microwave background spectrum

Abstract

We study the Sachs-Wolfe effect in a wide class of nonmetric theories of gravitation. It is found that in these theories an initially thermal photon energy spectrum would not remain thermal. The recent measurements of the cosmic microwave background radiation spectrum can therefore be used to constrain nonmetric gravitational couplings to electromagnetism. These constraints can be interpreted as a precise verification of the weak equivalence principle as applied to photons.