Cosmological perturbations from full quantum gravity

TL;DR

This paper extends group field theory (GFT) quantum gravity models by incorporating scalar fields as reference frames, enabling the derivation of cosmological inhomogeneities and quantum fluctuations that resemble early universe conditions.

Contribution

It introduces scalar degrees of freedom into GFT condensates, allowing the extraction of correlation functions of inhomogeneities and demonstrating scale-invariant quantum fluctuations.

Findings

Approximately scale-invariant initial quantum fluctuations in local volume.

Quantum fluctuations have naturally small amplitude.

Results support GFT condensate cosmology as a quantum gravity foundation for early universe studies.

Abstract

The early universe provides an opportunity for quantum gravity to connect to observation by explaining the large-scale structure of the Universe. In the group field theory (GFT) approach, a macroscopic universe is described as a GFT condensate; this idea has already been shown to reproduce a semiclassical large universe under generic conditions, and to replace the cosmological singularity by a quantum bounce. Here we extend the GFT formalism by introducing additional scalar degrees of freedom that can be used as a physical reference frame for space and time. This allows, for the first time, the extraction of correlation functions of inhomogeneities in GFT condensates: in a way conceptually similar to inflation, but within a quantum field theory of both geometry and matter, quantum fluctuations of a homogeneous background geometry become the seeds of cosmological inhomogeneities. We find approximately scale-invariant initial quantum fluctuations in the local volume, with naturally small amplitude; this behaviour extends to other quantities such as the matter density. These results confirm the potential of GFT condensate cosmology to provide a purely quantum gravitational foundation for the understanding of the early universe.