Testing Horndeski Gravity from EHT Observational Results for Rotating Black Holes

TL;DR

This paper uses EHT observations of M87* to test Horndeski gravity black hole models, constraining their parameters and exploring the potential to distinguish them from Kerr black holes, thus probing deviations from general relativity.

Contribution

It provides the first observational constraints on rotating Horndeski black holes using EHT data, analyzing their shadows and potential deviations from Kerr black holes.

Findings

EHT data constrains Horndeski black hole parameters

Rotating Horndeski shadows can mimic Kerr shadows depending on parameters

Potential to distinguish Horndeski gravity from general relativity in certain regimes

Abstract

The Event Horizon Telescope (EHT) collaboration recently unveiled the first image of the supermassive black hole M87*, which exhibited a ring of angular diameter $\theta_{d}=42 \pm 3 \mu as$, a circularity deviation $\Delta C \leq 0.1$, and also inferred a black hole mass of $M=(6.5 \pm 0.7) \times 10^9 M_\odot $. This provides a new window onto tests of theories of gravity in the strong-field regime, including probes of violations of the no-hair theorem. It is widely believed that the Kerr metric describes the astrophysical black holes, as encapsulated in the critical but untested no-hair theorem. Modeling Horndeski gravity black holes -- with additional hair parameter $h$ besides the mass $M$ and spin $a$ of the Kerr black hole -- as the supermassive black hole M87*, we observe that to be a viable astrophysical black hole candidate, the EHT result constrains ($a$, $h$) parameter space. However, a systematic bias analysis indicates rotating Horndeski black hole shadows may or may not capture Kerr black hole shadows, depending on the parameter values; the latter is the case over a substantial part of the constrained parameter space, allowing Horndeski gravity and general relativity to be distinguishable in the said space, and opening up the possibility of potential modifications to the Kerr metric.