XII BLACK HOLES WORKSHOP

Guimarães 19-20^th December 2019

The so-called hybrid metric-Palatini theory of gravity (HMPG), proposed in 2012 by T. Harko et al., contains an arbitrary function of the Palatini curvature ($ \cal R$) and is known to successfully describe both local (solar-system) and cosmological observations. We describe static, spherically symmetric vacuum solutions of HMPG in the simplest case where its scalar-tensor representation has a zero scalar field potential (\phi)$, and both Riemannian ($) and Palatini ($\cal R$) Ricci scalars are zero. Such a scalar-tensor theory coincides with general relativity with a phantom conformally coupled scalar field as a source of gravity. Generic asymptotically flat solutions either contain a naked central singularity or describe traversable wormholes, and there is a special two-parameter family of globally regular black hole solutions with extremal horizons. In addition, there is a one-parameter family of solutions with an infinite number of extremal horizons. It is argued that the obtained black hole and wormhole solutions are unstable under monopole perturbations. A similar analysis is performed for the extended HMPG with an arbitrary function of two variables ($ and $\cal R$), whose scalar-tensor representation contains two scalar fields.

Oral presentation: yes. Poster: no.