Symmetry Reduced Loop Quantum Gravity: A Bird's Eye View

TL;DR

This paper reviews recent progress in symmetry reduced Loop Quantum Gravity models, highlighting their potential observational signatures in the Cosmic Microwave Background and critically examining the reduction procedures used in quantum cosmology.

Contribution

It provides a comprehensive overview of how symmetry reduction in LQG connects quantum geometry with observable cosmological phenomena and discusses methodological issues in the reduction process.

Findings

Planck scale effects of LQG can influence CMB at large scales

Ultraviolet quantum geometry features interact with infrared cosmological perturbations

Symmetry reduction procedures in quantum cosmology are critically examined

Abstract

This is a brief overview of the current status of symmetry reduced models in Loop Quantum Gravity (LQG), focussing on the very early universe. Over the last 3 years or so the subject has matured sufficiently to make direct contact with observations of the Cosmic Microwave Background (CMB). In particular, thanks to an unforeseen interplay between the ultraviolet features of quantum geometry and the infrared properties of quantum fields representing cosmological perturbations, \emph{Planck scale effects} of LQG can leave imprints on CMB \emph{at the largest angular scales}. In addition to a summary of these results, the article also contains a critical discussion of the symmetry reduction procedure used in discussions of quantum cosmology (and quantum black holes).