Emergent Friedmann dynamics with a quantum bounce from quantum gravity condensates

TL;DR

This paper derives effective cosmological equations from group field theory quantum gravity, revealing a quantum bounce that replaces the classical Big Bang singularity, with results aligning with loop quantum cosmology in certain regimes.

Contribution

It introduces a condensate-based approach to derive emergent Friedmann dynamics with quantum gravity effects, including a non-singular bounce, from group field theory.

Findings

Quantum bounce replaces classical singularity.

Modified Friedmann equations resemble loop quantum cosmology.

Classical equations recovered in semi-classical limit.

Abstract

We study the effective cosmological dynamics, emerging as the hydrodynamics of simple condensate states, of a group field theory model for quantum gravity coupled to a massless scalar field and reduced to its isotropic sector. The quantum equations of motion for these group field theory condensate states are given in relational terms with respect to the scalar field, from which effective dynamics for spatially flat, homogeneous and isotropic space-times can be extracted. The result is a generalization of the Friedmann equations, including quantum gravity modifications, in a specific regime of the theory corresponding to a Gross-Pitaevskii approximation where interactions are subdominant. The classical Friedmann equations of general relativity are recovered in a suitable semi-classical limit for some range of parameters of the microscopic dynamics. An important result is that the quantum geometries associated with these GFT condensate states are non-singular: a bounce generically occurs in the Planck regime. For some choices of condensate states, these modified Friedmann equations are very similar to those of loop quantum cosmology.