Black hole shadows in nonminimally coupled Weyl connection gravity

TL;DR

This paper investigates black hole shadows within a Weyl connection gravity framework, deriving observational bounds on non-metricity parameters using Event Horizon Telescope data, thus linking extended gravity theories with astrophysical observations.

Contribution

It introduces a nonminimally coupled Weyl gravity model, computes black hole shadows, and constrains the theory's parameters using real observational data.

Findings

Weyl parameter bounds from shadow observations are established.

Modified black hole solutions are consistent with EHT constraints.

Black hole imaging constrains spacetime non-metricity effectively.

Abstract

We study black hole shadows in nonminimally coupled Weyl connection gravity, a metric-affine extension of general relativity in which spacetime is described by a metric and a Weyl vector field encoding non-metricity. Despite going beyond the Riemannian framework, the presence of a non-dynamical Weyl vector ensures second-order field equations. The theory admits Schwarzschild- and Reissner--Nordstr\"{o}m-like solutions modified by a Weyl integration constant that parametrizes deviations from General Relativity. By computing the corresponding shadow radii and confronting them with the Event Horizon Telescope constraints on Sgr A*, we place observational bounds on the Weyl parameter. Assuming an observer distance $r_O = 4.1\times 10^{10}M$ and requiring consistency at the $2\sigma$ level, we obtain $\omega \gtrsim 10^{11.7}M$ (model I), $\omega \gtrsim 10^{10.5}M$ (model II), and $\omega \sim 10^{12}M$ (model III). Our results show that present horizon-scale imaging already sets meaningful limits on spacetime non-metricity. This work highlights the power of black hole shadow observations as probes of extended gravitational dynamics and establishes a direct link between Weyl-based theories and current astrophysical data.