Probing quadratic gravity with the Event Horizon Telescope

¹ Department of High Energy Physics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
² Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
e-mail: m.wondrak@astro.ru.nl
³ Department of Mathematics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands

Received: 21 May 2022
Accepted: 24 November 2022

Abstract

Quadratic gravity constitutes a prototypical example of a perturbatively renormalizable quantum theory of the gravitational interactions. In this work, we construct the associated phase space of static, spherically symmetric, and asymptotically flat spacetimes. We find that the Schwarzschild geometry is embedded in a rich solution space comprising horizonless, naked singularities and wormhole solutions. Characteristically, the deformed solutions follow the Schwarzschild solution up outside of the photon sphere, while they differ substantially close to the center of gravity. We then carry out an analytic analysis of observable signatures accessible to the Event Horizon Telescope, comprising the size of the black hole shadow as well as the radiation emitted by infalling matter. On this basis, we argue that it is the brightness within the shadow region that constrains the phase space of solutions. Our work constitutes the first step towards bounding the phase space of black-hole-type solutions with a clear quantum gravity interpretation based on observational data.