First-order thermodynamics of Horndeski cosmology

• DOI: 10.48550/arxiv.2401.10351
• Publication Date: 2024-01-01
• Authors: Marcello Miranda; Serena Giardino; Andrea Giusti; Lavinia Heisenberg
• Keywords: Alternative gravity theories; Gravitation; FOS: Physical sciences; General Relativity and Quantum Cosmology (gr-qc); General Relativity and Quantum Cosmology

We delve into the first-order thermodynamics of Horndeski gravity, focusing on spatially flat, homogeneous, and isotropic cosmologies. Our exploration begins with a comprehensive review effective fluid representation within viable gravity. Notably, we uncover surprising alignment between constitutive relations governing "Horndeski fluid" those Eckart's thermodynamics. Narrowing our focus, specialize discussion to flat Friedmann-Lema{\^i}tre-Robertson-Walker spacetimes. Within this specific cosmological framework, systematically analyze two classes theories: shift-symmetric asymptotically shift-symmetric. These theories are characterized by non-vanishing braiding parameter, adding nuanced dimension investigation. On one hand, unlike case "traditional" scalar-tensor these peculiar subclasses gravity never relax General Relativity (seen formalism as an equilibrium state at zero temperature), but give rise additional states viscosity. other analysis further confirms previous findings according which curvature singularities "hot" exhibit diverging temperature, suggests that deviations from become extreme spacetime singularities. Furthermore, provide novel exact solution for theory toy model thermodynamic analysis.