📝 SummarXiv

Abstract

Yes and maybe. In contrast to the fluid particles of this perfect fluid spacetime which follow non-geodesic world-lines and escape to infinity, we prove that freely-falling test particles of McVittie spacetime can reach the black hole horizon in finite proper time. We review the relevant evidence and argue that the fate of an extended test body is less clear. More precisely: we consider expanding McVittie spacetimes with a non-negative cosmological constant. In each member of this class, we identify a region of the spacetime such that an observer following an initially-ingoing timelike geodesic crosses the black hole horizon of the spacetime in a finite amount of proper time. The curvature behaves in interesting ways along these geodesics. In the case of a positive cosmological constant, curvature scalars (of zero, first and second order in derivatives), Jacobi fields and parallel propagated (p.p.) frame components of the curvature remain finite along timelike geodesics running into the black hole horizon. For a vanishing cosmological constant, scalar curvature terms of zero and first order as well as Jacobi fields remain finite in this limit. However, scalar curvature terms of second order diverge, and we show that there are p.p.\ frame components of the curvature tensor that also diverge in this limit. We argue that this casts a doubt as to whether or not an extended test body can survive crossing the black hole horizon in this case.