Black holes at a crossroads: late-stage evaporation in quadratic gravity

In General Relativity black hole evaporation leads to sudden bursts of energy and loss of information. It can be argued that these phenomena happen in the final stages of evaporation, where the semiclassical approximation needs to be refined with quantum corrections also for the gravitational interaction. A natural way to describe gravity at high energies is to add quadratic curvature terms to the Einstein-Hilbert action, i.e. quadratic gravity. At the cosmological level it is known that its classical solutions can give rise to a model of inflation that matches observations strikingly well, while in an astrophysical context it allows for the possibility of non-Schwarzschild black holes at small masses. These solutions have very peculiar properties, due to the presence of a massive spin-2 particle corresponding to a ghost at the quantum level. The branch of non-Schwarzschild solutions crosses the one of Schwarzschild ones at a specific mass which could be between the one of an asteroid and the Planck mass, depending on the value of a slightly constrained free parameter. By analyzing their dynamical stability and thermodynamical properties, we investigate what could happen to a black hole that has evaporated to this crossing point. While this investigation might not solve the problem of the endpoint of evaporation, it can shed light on the directions it might take during its last moments.