Daniel Guariento (Perimeter Institute for Theoretical Physics, Canada)
The generalized McVittie solution, representing a central time-dependent mass in an expanding cosmological background, has been shown to be an exact solution of a self-gravitating subset of the Horndeski class of scalar field actions, and constitutes an important example of an analytic solution for hairy black holes. Following the analysis performed on its fixed-mass counterpart, we demonstrate that a time-dependent central mass may have a significant impact on the overall causal structure of the spacetime. The metric always has an event horizon at future cosmological time infinity in the appropriate limits, but the character of the horizon depends on the accretion and cosmological histories in the bulk. The possible limits are a black-hole horizon, a pair of black- and white-hole horizons separated by a bifurcating 2-sphere, or an entirely white-hole horizon. The last case is only possible if there is accretion onto the central mass.