Boltzmann's Htheorem, entropy and the strength of gravity in theories with a nonminimal coupling between matter and geometry
Abstract
In this paper we demonstrate that Boltzmann's Htheorem does not necessarily hold in the context of theories of gravity with nonminimally coupled matter fields. We find sufficient conditions for the violation of Boltzmann's Htheorem and derive an expression for the evolution of Boltzmann's H in terms of the nonminimal coupling function, valid in the case of a collisionless gas in a homogeneous and isotropic FriedmannLemaîtreRobertsonWalker universe. We highlight the implications of this result for the evolution of the entropy of the matter fields, briefly discussing the role played by collisions between particles whenever they are relevant. We also suggest a possible link between the high entropy of the Universe and the weakness of gravity in the context of these theories.
 Publication:

Physics Letters B
 Pub Date:
 September 2020
 DOI:
 10.1016/j.physletb.2020.135641
 arXiv:
 arXiv:2003.10154
 Bibcode:
 2020PhLB..80835641A
 Keywords:

 General Relativity and Quantum Cosmology;
 Astrophysics  Cosmology and Nongalactic Astrophysics;
 High Energy Physics  Theory
 EPrint:
 7 pages, no figures. Clarifications added, typos corrected. Results unchanged