Seminars of fiscal year 2022

Ultra-massive spacetimes and their universal properties

In spacetimes with a positive cosmological constant the area of (spatially) stable marginally trapped surfaces (MTS) has a finite bound. I will prove that any such spacetime containing stable MTSs with area approaching the bound acquire universal properties generically. In particular, they possess `holographic screens’ (i.e. marginally trapped tubes) foliated by MTS of spherical topology, composed of a dynamical horizon portion and a timelike membrane portion that meet at a preferred sphere S with constant Gaussian curvature and the maximal area. All holographic screens change signature precisely at S, and all of them develop towards the past with increasing area without limitation. A future singularity, which must be quasi-universal, also develops. Examples and implication of this result will be discussed.

Numerical simulations of vacuum bubbles and its consequences on the PBH scenario

There are several mechanisms for producing PBHs apart from the collapse of adiabatic fluctuations, for instance, from false vacuum bubbles, which can be generated if the inflaton becomes trapped during inflation. These localized bubbles will eventually end up forming black holes, also called baby Universes. In this work, we have numerically simulated the formation of such kind of vacuum bubbles. In particular, we have studied its dynamics, quantified the thresholds needed for its formation, studied its size and estimated the resulting PBH mass. Our results show that: i)The abundance of such PBHs dominates over those coming from the collapse of adiabatic fluctuations in the case of large-non gaussianities. ii) The mass distribution of the two channels of PBH production (collapse of adiabatic fluctuations--false vacuum bubbles) is different. iii) An analytical estimate previously used in the literature to predict the threshold for forming such bubbles seems inaccurate for large-non gaussianities, which leads to the conclusion that PBHs coming from false vacuum bubbles are more easily formed than previously found.