SPEAKER: Daniele Vernieri (IAP, Paris)

ABSTRACT: I will talk about black holes in Hořava gravity which has been proposed as an ultraviolet completion to general relativity. Adding to the gravitational action higher-order spatial derivatives, without adding higher-order time derivatives, leads to a modification of the graviton propagator. The resulting theory is power-counting renormalizable at the expense of violating Lorentz symmetry at all scales, and, perhaps surprisingly, it appears to be phenomenologically viable in its more general form. It is really challenging to study the consequences of Lorentz invariance breaking in the case of black holes. In fact, in a Lorentz-violating gravity theory with higher-order dispersion relations, the event horizon relinquishes its role as an absolute causal boundary, since modes propagating infinitely fast will be able to penetrate it. However, this role will be taken over by the so called “universal horizon”. Because of the causal properties of such a spacetime, no signal can escape the interior of the universal horizon propagating to the exterior, no matter how fast it moves. So, the existence of the universal horizon is a strong indication that the notion of a black hole still makes sense in Hořava gravity.