Speaker
Description
We present evidence that semiclassical gravity can give place to ultracompact stars, indistinguishable from black holes up to current observations. We integrate the semiclassical equations of (spherically symmetric) stellar equilibrium for a constant-density classical fluid. The semiclassical contribution is modelled by a quantum massless scalar field in a genuinely-static vacuum state compatible with asymptotic flatness (Boulware vacuum). The Renormalized Stress-Energy Tensor (RSET) is firstly approximated by a cut-off version of the analytic Polyakov approximation. This approximation reveals a crucial difference with respect to purely classical solutions: stars whose compactness is nearing that of a black hole exhibit bounded pressures and curvatures up to central core of a very small relative size. For a subfamily of these ultracompact configurations, their mass can be made arbitrarily close to zero at the boundary of the core, just before the solution enters a singular regime. Our analysis suggests the absence of a Buchdahl limit in semiclasical gravity, while indicating that the cut-off regularized Polyakov approximation must be improved to describe equilibrium configurations of arbitrary compactness that remain regular at the center of spherical symmetry.
