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Toward a causal effective thermodynamics of scalar-tensor gravity

Narayan Banerjee1,*, Andrea Giusti2,3,4,†, Valerio Faraoni5,‡, Luca Gallerani6,3,§, and Alain Maltais-Gosselin5,∥

  • *Contact author: narayan@iiserkol.ac.in
  • †Contact author: andrea.giusti9@unibo.it
  • ‡Contact author: vfaraoni@ubishops.ca
  • §Contact author: luca.gallerani6@unibo.it
  • ∥Contact author: amaltais26@ubishops.ca
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Phys. Rev. D 114, 064018 – Published 8 September, 2026

DOI: https://doi.org/10.1103/m92d-xy54

Abstract

The thermal analogy between the effective fluid of scalar-tensor gravity and Eckart’s irreversible thermodynamics is extended to the causal Israel-Stewart model, adopting the minimal ansatz of promoting the heat flux density to a timelike vector. This choice yields analytically manageable constitutive equations, allowing for the first consistent decoupling of the effective temperature T and the effective thermal conductivity K of scalar-tensor gravity. Crucially, this new framework preserves the interpretation of general relativity as the equilibrium state approached via a dynamical relaxation process in the vanishing-KT limit. This new causal formalism is applied to cosmology.

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Outline

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Phys. Rev. D 114, 064018– Published 8 September, 2026
  • Received 28 May 2026
  • Accepted 22 July 2026
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