Breaking Buchdahl: Ultracompact stars in semiclassical gravity

The Buchdahl theorem imposes a bound on the maximum compactness that stars in equilibrium can achieve in general relativity. This bound is one of the theoretical motivations behind the belief that the dark and compact objects that we observe must correspond to black holes. In this talk, I will revise the hypotheses behind the Buchdahl theorem and address the consequences of individually relaxing its assumptions.


In particular, if the energy density of the stellar fluid is allowed to become negative, it is possible to obtain regular stars as compact as black holes. To conclude, I will show how similar models of black hole mimickers are found within semiclassical theories of gravity that consider the backreaction effects of the quantum vacuum.