"Twisted" black holes are unphysical

TL;DR

The paper critically examines 'twisted' black holes, showing they are unphysical due to lack of asymptotic flatness and the presence of unhidden closed timelike curves, despite satisfying vacuum Einstein equations.

Contribution

It demonstrates that 'twisted' black holes are unphysical variants of Taub--NUT spacetimes with problematic features such as non-asymptotic flatness and closed timelike curves.

Findings

They are not globally asymptotically flat.

They contain unhidden closed timelike curves.

They are minor variants of Taub--NUT spacetimes.

Abstract

So-called "twisted" black holes have recently been proposed by Zhang (1609.09721 [gr-qc]), and further considered by Chen and Jing (1610.00886 [gr-qc]), and more recently by Ong (1610.05757 [gr-qc]). While these spacetimes are certainly Ricci-flat, and so mathematically satisfy the vacuum Einstein equations, they are also merely minor variants on Taub--NUT spacetimes. Consequently they exhibit several unphysical features that make them quite unreasonable as realistic astrophysical objects. Specifically, these "twisted" black holes are not (globally) asymptotically flat. Furthermore, they contain closed timelike curves that are not hidden behind any event horizon --- the most obvious of these closed timelike curves are small azimuthal circles around the rotation axis, but the effect is more general. The entire region outside the horizon is infested with closed timelike curves.