01484nas a2200157   4500000000100000000000100001008004100002100001500043700001900058700002100077700002300098700001700121245006400138856003700202520108700239                                       d1 aJesse Daas1 aKolja Kuijpers1 aFrank Saueressig1 aMichael F. Wondrak1 aHeino Falcke00aProbing Quadratic Gravity with the Event Horizon Telescope   uhttps://arxiv.org/abs/2204.084803 aQuadratic 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. It is found 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 which 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.