Emergence of a low spin phase in group field theory condensates

TL;DR

This paper demonstrates that group field theory condensates naturally evolve into a low spin phase, reproducing classical Friedmann cosmology and linking quantum gravity models with loop quantum cosmology.

Contribution

It shows that GFT condensates reach a low spin phase with a dominant spin, leading to classical cosmological dynamics, strengthening the connection between GFT and LQC.

Findings

GFT condensates dynamically reach a low spin phase.

Total volume evolution follows classical Friedmann equations.

Dominant spin can be the lowest or next lowest value.

Abstract

Recent results have shown how quantum cosmology models can be derived from the effective dynamics of condensate states in group field theory (GFT), where 'cosmology is the hydrodynamics of quantum gravity': the classical Friedmann dynamics for homogeneous, isotropic universes, as well as loop quantum cosmology (LQC) corrections to general relativity have been shown to emerge from fundamental quantum gravity. We take one further step towards strengthening the link with LQC and show, in a class of GFT models for gravity coupled to a free massless scalar field and for generic initial conditions, that GFT condensates dynamically reach a low spin phase of many quanta of geometry, in which all but an exponentially small number of quanta are characterised by a single spin $j_0$ (i.e. by a constant volume per quantum). As the low spin regime is reached, GFT condensates expand to exponentially large volumes, and the dynamics of the total volume follows precisely the classical Friedmann equations. This behaviour follows from a single requirement on the couplings in the GFT model under study. We present one particular simple case in which the dominant spin is the lowest one: $j_0=0$ or, if this is excluded, $j_0=1/2$. The type of quantum state usually assumed in the derivation of LQC is hence derived from the quantum dynamics of GFT. These results confirm and extend recent results by Oriti, Sindoni and Wilson-Ewing in the same setting.