First-order thermodynamics of scalar-tensor cosmology

TL;DR

This paper develops a thermodynamic framework for scalar-tensor cosmology, analyzing how the universe's expansion and singularities influence its effective temperature and equilibrium states.

Contribution

It introduces a novel thermodynamic approach to scalar-tensor gravity and applies it to cosmological solutions, revealing the universe's thermodynamic behavior near singularities and at late times.

Findings

Universe approaches equilibrium with zero temperature at late times

Effective temperature diverges near the Big Rip for conformally coupled scalar fields

Expansion cools the universe while singularities heat it, competing near singularities

Abstract

A new thermodynamics of scalar-tensor gravity is applied to spatially homogeneous and isotropic cosmologies in this class of theories and tested on analytical solutions. A forever-expanding universe approaches the Einstein "state of equilibrium" with zero effective temperature at late times and departs from it near spacetime singularities. "Cooling" by expansion and "heating" by singularities compete near the Big Rip, where it is found that the effective temperature diverges in the case of a conformally coupled scalar field.