Field space parametrization in quantum gravity and the identification of a unitary conformal field theory at the heart of 2D Asymptotic Safety

Abstract

In this thesis we investigate various fundamental aspects of asymptotically safe quantum gravity, in particular the compatibility of Asymptotic Safety with the requirements for background independence and unitarity. The first part contains a detailed analysis of the space of metrics. We unveil a novel, specifically designed connection on this space and use the corresponding geodesics to connect all metrics having the same signature. In the second part we study the consequences of different metric parametrizations, i.e. of different connections on the space of metrics, for the Asymptotic Safety scenario by exploring the respective properties of the decisive renormalization group fixed points. Based on a bimetric investigation we show that Asymptotic Safety can be reconciled with background independence. The third part of this work focuses on the 2D limit of quantum gravity. To this end, we show that the 2D limit of the Einstein--Hilbert action at a nontrivial fixed point becomes Polyakov's induced gravity action which describes a conformal field theory. In this way, we can prove that the fixed point theory is unitary. The last part concerns the reconstruction of the functional integral out of a given effective theory, where we reconstruct the bare action for an Einstein--Hilbert-type effective average action and for a Liouville action.