Relating Follicly-Challenged Compact Stars to Bald Black Holes

TL;DR

This paper explores the relationship between no-hair relations of compact stars and black holes, demonstrating that as stars become more compact and anisotropic, their properties approach those of black holes, with both numerical and analytical evidence.

Contribution

It provides the first detailed numerical and analytical analysis showing how compact star no-hair relations converge to black hole relations as compactness increases.

Findings

No-hair relations approach black hole relations with increasing compactness.

Analytical proof that the current dipole moment reaches the black hole limit quadratically.

Compact stars become more oblate as they approach the black hole limit.

Abstract

Compact stars satisfy certain no-hair relations through which their multipole moments are given by their mass, spin and quadrupole moment. These relations are approximately independent of their equation of state, relating pressure to density. Such relations are similar to the black hole no-hair theorems, but these possess event horizons inside which information that led to their formation can hide. Compact stars do not possess horizons, so whether their no-hair relations are related to the black hole ones is unclear. We investigate how the two relations are related by studying relations among multipole moments for compact stars with anisotropic pressure as a toy model, which allows such stars to be more compact than those with isotropic pressure. We here show numerically that the compact star no-hair relations approach the black hole ones as the compactness approaches that of a black hole. We also prove analytically that the current dipole moment exactly reaches the black hole limit quadratically in compactness as strongly-anisotropic stars approach the black hole limit. We moreover show that compact stars become progressively oblate in this limit, even if prolate at low compactness due to strong anisotropies.