Compact objects with spin parameter $a_* > 1$

TL;DR

This paper discusses the possibility of compact objects with spin parameters exceeding the Kerr bound, challenging the traditional black hole description in General Relativity and exploring implications for future observational tests.

Contribution

It introduces the theoretical possibility of objects with spin parameter greater than one, beyond the Kerr black hole limit, and discusses potential observational consequences.

Findings

Objects with $a_* > 1$ could exist if they are not black holes.

Future experiments may detect deviations from Kerr black hole geometry.

Accretion processes might spin up objects beyond the Kerr bound.

Abstract

In 4-dimensional General Relativity, black holes are described by the Kerr solution and are completely specified by their mass $M$ and by their spin angular momentum $J$. A fundamental limit for a black hole in General Relativity is the Kerr bound $|a_*| \le 1$, where $a_* = J/M^2$ is the spin parameter. Future experiments will be able to probe the geometry around these objects and test the Kerr black hole hypothesis. Interestingly, if these objects are not black holes, the accretion process may spin them up to $a_* > 1$.