Vacuum semiclassical gravity does not leave space for safe singularities

TL;DR

This paper argues that semiclassical gravity predicts the formation of visible singularities affecting predictability, challenging the notion that such singularities are safely hidden behind horizons in classical general relativity.

Contribution

It demonstrates that semiclassical effects lead to visible singularities in black hole geometries and explores potential resolutions to preserve predictability.

Findings

Semiclassical Schwarzschild and Reissner-Nordström geometries have naked singularities.

Dynamical scenarios suggest only three possible outcomes avoiding singularities.

The paper discusses the timescales and viability of these outcomes within semiclassical gravity.

Abstract

General relativity predicts its own demise at singularities, but also appears to conveniently shield itself from the catastrophic consequences of such singularities, making them safe. For instance, if strong cosmic censorship were ultimately satisfied, spacetime singularities, although present, would not pose any practical problems to predictability. Here we argue that under semiclassical effects the situation should be rather different: the potential singularities which could appear in the theory will generically affect predictability and so one will be forced to analyse whether there is a way to regularise them. For these possible regularisations, the presence and behaviour of matter during gravitational collapse and stabilisation into new structures will play a key role. First we show that the static semiclassical counterparts to the Schwarzschild and Reissner-Nordstr\"om geometries have singularities which are no longer hidden behind horizons. Then we argue that in dynamical scenarios of formation and evaporation of black holes, we are left with only three possible outcomes which could avoid singularities and eventual predictability issues. We briefly analyse the viability of each one of them within semiclassical gravity, and discuss the expected characteristic timescales of their evolution.