Investigating the interplay of the braneworld gravity and the plasma environment on the black hole shadow

TL;DR

This study models the shadow of rotating braneworld black holes in plasma environments, using EHT data to constrain parameters like tidal charge and plasma density, revealing how plasma and spacetime geometry influence shadow size.

Contribution

It introduces a comprehensive analysis of black hole shadows considering both braneworld gravity effects and plasma environments, providing new constraints on tidal charge and plasma parameters from EHT observations.

Findings

Inhomogeneous plasma decreases shadow size with increasing density.

Homogeneous plasma enlarges the black hole shadow.

EHT data constrains tidal charge within specific ranges for M87* and Sgr A*.

Abstract

We investigate the shadow of a rotating braneworld black hole in dispersive plasma environments and assess the potential of the Event Horizon Telescope (EHT) observations to constrain braneworld gravity. The spacetime around a rotating braneworld black hole is modelled by a Kerr-Newman-like metric determined by its mass $M$, spin $a$, and tidal charge $q$, which encodes the gravitational effects of the bulk spacetime. We consider both inhomogeneous and homogeneous plasma environments characterised by plasma parameters $\alpha_i$ ($i=1,2\text{ and }3$) to study light propagation and the interplay of the background spacetime and the plasma environment in influencing the shadow size and shape. We find that as the plasma density increases, inhomogeneous plasma environments decrease the shadow size, however homogeneous plasma enlarges it. On studying the effect due to the background spacetime, we find that $q<0$ (negative tidal charge) increases the shadow diameter, while $q>0$ decreases it. Using the EHT measurements of M87* and Sgr A*, we constrain the $(q,\alpha_i)$ parameter space. The EHT data constrains the tidal charge in the range $-1.15 \lesssim q \lesssim 0.45$ for M87* and $-0.65 \lesssim q \lesssim 0.8$ for Sgr A* in the low density plasma limit, which is indeed the case for M87* and Sgr A*. However, for black holes surrounded by high density plasma, the shadow size is governed both by the background geometry as well as by the plasma environment. In such cases, joint constraints from plasma density estimates and observed shadow angular diameters can provide valuable insights into the underlying spacetime geometry.