Black Holes Might Survive a Cosmological Bounce

Researchers have developed a theoretical solution suggesting the persistence of black holes through a hypothetical cosmological bounce. This study is framed within scalar-tensor theories of gravity, which provide a natural framework for modeling bouncing cosmologies, where the universe undergoes a contraction followed by an expansion, avoiding an initial singularity. The complexity of incorporating a localized inhomogeneity, such as a black hole, into an evolving cosmological background led the authors to employ a perturbative scheme. The model begins with a leading-order approximation describing a spatially flat bouncing FLRW spacetime sourced by a radiation perfect fluid. Next, a central inhomogeneity is introduced through a generalized McVittie geometry, with perturbations encoded in the corresponding first-order metric and scalar-field functions. Calculations were performed as a series expansion up to $\mathcal{O}(\eta^4)$ near the bounce at $\eta=0$, considering an anisotropic fluid with radial and tangential pressures. An integration constant, $d_0$, was identified as the true perturbative parameter, where all perturbations vanish as $d_0 \to 0$. The key result is the emergence of a small evolving horizon of size $r_h \sim d_0$, which is interpreted as the horizon of the central inhomogeneity. The persistence of this horizon through the cosmological bounce supports the idea that a black hole could survive such a cosmological transition. Furthermore, the evolution of this horizon is not symmetric about the bounce time, $\eta=0$. This work opens avenues for a better understanding of black hole dynamics in non-standard cosmological scenarios and the implications of modified gravity theories.