Black holes, holography and boundaries What remains of quantum gravity

This thesis explores various aspects of black holes, holography and boundaries, in the pursuit of a better understanding of a theory of quantum gravity. The chapters try to answer various aspects of the same question: what remains of quantum gravity? What remains after UV degrees of freedom are coarse-grained away, and how should the information paradox be understood from this perspective? How do aspects of near-extremal black holes leave an imprint on correlators, once zoomed into the horizon region and dimensionally reduced to two dimensions? And what changes in the study of three-dimensional hyperbolic gravity if part of spacetime is cut out by end-of-the-world branes?
Analogues of replica wormholes and a dynamical Page curves emerge after coarse-graining a simple system. A disccusion is offered on the coarse-graining of AdS black holes not coupled to a bath, keeping in mind the small black hole limit. Evaporating small black holes require coarse-graining below the black hole threshold, and simple boundary correlators might be able to distinguish black holes from thermal gas with the same energy. It is found that near-AdS2 corrections in JT gravity differ for massless and massive fields. The classical imprint of near-extremality on scalar two-point-functions is computed, agreeing with the appropriate limit of the well-controlled BTZ black hole. Lastly, this thesis discusses how Neumann boundary conditions on two-dimensional end-of-the-world branes correspond to gluing twisted Chern-Simons fields and how inner products of boundary conformal blocks involve timelike defect Liouville, depending on the tension of the brane.