Identifying cosmological perturbations in group field theory condensates

TL;DR

This paper explores how group field theory condensates can model the early universe's homogeneous and inhomogeneous geometric features, linking quantum gravity to observable cosmological phenomena.

Contribution

It clarifies the mean-field approximation in GFT, showing how it encodes classical statistical distributions of geometry and connects to cosmological observations.

Findings

GFT condensates represent quantum homogeneous geometries with statistical inhomogeneity information.

The statistical description aligns with observed universe homogeneity if the distribution scale exceeds the Planck length.

Derived effective cosmological equations match loop quantum cosmology's improved dynamics.

Abstract

One proposal for deriving effective cosmological models from theories of quantum gravity is to view the former as a mean-field (hydrodynamic) description of the latter, which describes a universe formed by a 'condensate' of quanta of geometry. This idea has been successfully applied within the setting of group field theory (GFT), a quantum field theory of 'atoms of space' which can form such a condensate. We further clarify the interpretation of this mean-field approximation, and show how it can be used to obtain a semiclassical description of the GFT, in which the mean field encodes a classical statistical distribution of geometric data. In this sense, GFT condensates are quantum homogeneous geometries that also contain statistical information about cosmological inhomogeneities. We show in the isotropic case how this information can be extracted from geometric GFT observables and mapped to quantities of observational interest. Basic uncertainty relations of (non-commutative) Fourier transforms imply that this statistical description can only be compatible with the observed near-homogeneity of the Universe if the typical length scale associated to the distribution is much larger than the fundamental 'Planck' scale. As an example of effective cosmological equations derived from the GFT dynamics, we then use a simple approximation in one class of GFT models to derive the 'improved dynamics' prescription of holonomy corrections in loop quantum cosmology.