Black Hole Shadows Constrain Extended Gravity

Abstract

The first images of black hole shadows open new possibilities to constrain modern extended gravity theories. We present the method of shadow background calculation for black hole solutions in the form of Taylor series where $g_{11}=-g_{00}^{-1}$. The method is extended to general non-rotating case $g_{11}\ne -g_{00}^{-1}$. The results of the analysis are compared with the predictions of General relativity taking into account the Event Horizon Telescope data. The results for the Horndesky model with the Gauss-Bonnet invariant, loop quantum gravity, Bumblebee model and Gauss-Bonnet gravity are in full agreement with the observations of M87*. In conformal gravity, large values of $m_2$ and $Q_s$ must be excluded. In STEGR $f(Q)$ gravity the observational limits on the parameter $\alpha$ are: $-0.025<\alpha <0.04$. For an alternative generalization of the Bumblebee model with the Schwarzschild approximation: $-0.3<l<0.45$. These results demonstrate the maximum one can achieve without taking into account of the rotation of a black hole.

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• The ring-shaped shadow of rotating naked singularity with a complete photon sphere
• The surface geometry and shadow of a Schwarzschild black hole with halo
• Extended Gravity Constraints at Different Scales