Cosmological scalar perturbations for a metric reconstructed from group field theory

TL;DR

This paper develops a method to describe scalar cosmological perturbations within group field theory (GFT), revealing differences from general relativity and highlighting the need for further refinement for realistic cosmological modeling.

Contribution

It introduces a novel approach to reconstruct an effective spacetime metric in GFT that includes scalar perturbations and analyzes their dynamics, which differ from classical GR predictions.

Findings

Perturbations include all scalar components of standard theory.

Inhomogeneities do not follow GR dynamics in the studied simple GFT model.

Squeezed and oscillating modes exhibit non-GR signatures and different signatures.

Abstract

While homogeneous cosmologies have long been studied in the group field theory (GFT) approach to quantum gravity, including a quantum description of cosmological perturbations is highly non-trivial. Here we apply a recent proposal for reconstructing an effective spacetime metric in GFT to the case of a metric with small inhomogeneities over a homogeneous background. We detail the procedure and give general expressions for cosmological scalar perturbations defined in terms of the GFT energy-momentum tensor. These include all the scalar components of standard perturbation theory and hence can be used to define gauge-invariant quantities. This is a major advantage of the effective metric approach compared to previous GFT studies limited to volume perturbations. We compute these perturbations explicitly for a particular Fock coherent state. While it was previously shown that such a state can be interpreted as an approximately flat homogeneous cosmology at late times, here we find that, in a very simple example, inhomogeneities do not follow the dynamics of general relativity in the semiclassical regime. More specifically, restricting ourselves to a specific coherent state in a simple (free) GFT, we study two types of perturbative GFT modes, squeezed and oscillating modes. For squeezed modes we find perturbation equations with Euclidean signature and a late-time limit that differs from general relativistic perturbation equations. Oscillating modes satisfy different dynamical equations that also differ from those of general relativity, but show a Lorentzian signature. Considering that our results were obtained within a number of simplifying assumptions [...], we discuss how going beyond these assumptions could lead to a more desirable phenomenology. Overall, our analysis should be understood as a first step in understanding cosmological perturbations within the effective GFT metric.