Editorial: Coarse graining in quantum gravity -- Bridging the gap between microscopic models and spacetime physics

Abstract

The Renormalization Group encodes three concepts that could be key to accelerate progress in quantum gravity. First, it provides a micro-macro connection that could connect microscopic spacetime physics to phenomenology at observationally accessible scales. Second, it enables a search for universality classes that could link diverse quantum-gravity approaches and allow us to discover that distinct approaches could encode the same physics in mathematically distinct structures. Third, it enables the emergence of symmetries at fixed points of the Renormalization Group flow, providing a way for spacetime symmetries to emerge from settings in which these are broken at intermediate steps of the construction. These three concepts make the Renormalization Group an attractive method and conceptual underpinning of quantum gravity. Yet, in its traditional setup as a local coarse-graining, it could appear at odds with concepts like background independence that are expected of quantum gravity. Within the last years, several approaches to quantum gravity have found ways how these seeming contradictions could be reconciled and the power of the Renormalization Group approach unleashed in quantum gravity. This special issue brings together research papers and reviews from a broad range of quantum gravity approaches, providing a partial snapshot of this evolving field.

Phase transitions in TGFT: a Landau-Ginzburg analysis of Lorentzian quantum geometric models

“…To understand the phase structure of such theories better and in particular to determine under which conditions well-defined continuum spacetime geometries can emerge, one can apply coarse graining methods and functional renormalization group (FRG) techniques, as well as powerful approximation techniques such as mean-field theory, well known from the local field theory context and quantum many-body physics [27][28][29]. However, neither the application of these methods to TGFT nor the interpretation of their results is immediate, for two main reasons: first, because of the combinatorial non-locality of TGFT interactions, which requires to adapt standard RG techniques; second, because quantum gravity requires a manifestly background-independent form of coarse graining prescription [30,31], that, in particular, does not refer directly to spatiotemporal scales (distances, energies) and are necessarily of a more abstract nature. Despite these challenges, much progress has been achieved in recent years and these techniques have successfully been extended to the context of matrix, tensor and group field theory models [7,.…”