Shadows of black holes at cosmological distances in the co-varying physical couplings framework

ABSTRACT The co-varying physical couplings (CPC) framework states that physical parameters like the speed of light in vacuum c, the Newtonian constant G, and the cosmological constant Λ could indeed vary with the space–time coordinates xμ. Here, we assume a temporal variation, that is, c(t), G(t), and Λ(t). We show that the McVittie space–time, a black hole in an expanding universe, is a solution of the CPC framework providing naturally an important parameter of the model. Then, we calculate the shadow angular radius of this black hole at cosmological distances. A black hole shadow in the CPC context could be either larger or smaller than the same shadow in the standard cosmology. It depends on how the set {c, G, Λ} varies with time or with the cosmic expansion.