Speaker: Alberto Salvio (U. Rome and INFN)

Title: Inflation and ultracompact objects in quadratic gravity

Abstract:

An extension of general relativity obtained by adding quadratic terms to the action will be considered. Such theory is a UV completion of general relativity: the additional terms soften gravity above a certain scale and render gravity renormalizable. The presence of 4 derivatives implies via the Ostrogradsky theorem that the classical Hamiltonian is unbounded from below. Nevertheless, I will argue that the relevant solutions are not unstable, but metastable: when the energies are much below a threshold (that is high enough to describe the whole cosmology) runaways are avoided. Remarkably, the chaotic inflation theory of initial conditions ensures that such bound is satisfied and testable implications for the early universe will be discussed. I will also argue that the basic unitarity condition is satisfied. Moreover, thanks to the UV softening of gravity in this theory, sufficiently light objects must be horizonless and explicit analytic examples of horizonless ultracompact objects (UCOs) will be discussed. These UCOs are more compact than Schwarzschild black holes and their spacetime is regular everywhere. Due to the lack of a horizon, light UCOs do not evaporate and, therefore, they may play the role of dark matter.

Room: Zoom videoconference (contact justin.feng@tecnico.ulisboa.pt for URL)