Rastall Gravity: A Thermodynamic Perspective

Abstract

We study the thermodynamic consistency of Rastall gravity in a spatially flat Friedmann–Robertson–Walker (FRW) universe. Beginning from the non-conservation ansatz originally proposed by Rastall, we derive the modified field and continuity equations and obtain expressions for horizon radii. Using the horizon temperature and a phenomenological form for the horizon entropy in Rastall gravity, we derive the generalized second law of thermodynamics (GSLT) and the thermodynamic equilibrium (TE) condition for both the apparent and event horizons. Numerical illustrations (ΛCDM background) demonstrate parameter ranges where GSLT and TE hold.