Applicability of the Newman-Janis Algorithm to Black Hole Solutions of Modified Gravity Theories

TL;DR

The paper investigates the use of the Newman-Janis algorithm in modified gravity theories and finds that it generally introduces pathologies, suggesting it should not be used outside of General Relativity for constructing rotating black holes.

Contribution

It demonstrates that applying the Newman-Janis algorithm to non-GR solutions often leads to problematic metrics, highlighting limitations of the method in modified gravity contexts.

Findings

Application of the Newman-Janis algorithm causes pathologies in modified gravity solutions.

The algorithm is unreliable for generating rotating black holes outside General Relativity.

It is recommended not to use the Newman-Janis algorithm in non-GR theories.

Abstract

The Newman-Janis algorithm has been widely used to construct rotating black hole solutions from non-rotating counterparts. While this algorithm was developed within General Relativity, it has more recently been applied to non-rotating solutions in modified gravity theories. We find that the application of the Newman-Janis algorithm to an arbitrary non-GR spherically-symmetric solution introduces pathologies in the resulting axially-symmetric metric. This then establishes that, in general, the Newman-Janis algorithm should not used to construct rotating black hole solutions outside of General Relativity.