On the Bartnik mass of apparent horizons

TL;DR

This paper characterizes the geometry of apparent horizons in asymptotically flat spacetimes, computes their Bartnik mass, and explores implications for conjectures in gravitational physics.

Contribution

It provides a complete characterization of apparent horizons' geometry and computes their Bartnik mass, linking it to Hawking mass and disproving a related conjecture.

Findings

Bartnik mass equals Hawking mass for non-degenerate apparent horizons

Disproved Gibbons' conjecture related to apparent horizons

Constructed examples with arbitrarily large negative curvature integral

Abstract

In this paper we characterize the intrinsic geometry of apparent horizons (outermost marginally outer trapped surfaces) in asymptotically flat spacetimes; that is, the Riemannian metrics on the two sphere which can arise. Furthermore we determine the minimal ADM mass of a spacetime containing such an apparent horizon. The results are conveniently formulated in terms of the quasi-local mass introduced by Bartnik in 1989. The Hawking mass provides a lower bound for Bartnik's quasilocal mass on apparent horizons by way of Penrose's conjecture on time symmetric slices, proven in 1997 by Huisken and Ilmanen and in full generality in 1999 by Bray. We compute Bartnik's mass for all non-degenerate apparent horizons and show that it coincides with the Hawking mass. As a corollary we disprove a conjecture due to Gibbons in the spirit of Thorne's hoop conjecture, and construct a new large class of examples of apparent horizons with the integral of the negative part of the Gauss curvature arbitrarily large.