Beyond Comoving Volume: Horizon Flux and Matter Creation in Entropic Cosmology

TL;DR

This paper derives cosmological equations from thermodynamics, incorporating horizon flux and matter creation, revealing how entropy evolves through bulk production and boundary fluxes in an open universe.

Contribution

It extends thermodynamic derivations of Friedmann equations to include horizon flux and matter creation, highlighting the non-comoving nature of the apparent horizon.

Findings

Derived the GSL considering horizon flux and matter creation.

Identified the two physical contributions to matter entropy: bulk production and horizon flux.

Showed the total entropy is balanced by horizon thermodynamics, bulk creation, and boundary fluxes.

Abstract

We explore the derivation of the Friedmann equations from a thermodynamic perspective, applying the unified first law of thermodynamics to the apparent horizon of a flat Friedmann-Lema\^itre-Robertson-Walker (FLRW) universe. We extend this framework to incorporate gravitationally induced particle creation, treating the region enclosed by the apparent horizon as an open thermodynamic system. A crucial aspect of our analysis is the recognition that the apparent horizon volume is not comoving; this requires a consistent accounting of particle exchange across the moving boundary. We demonstrate that the evolution of the particle number, and explicitly the matter entropy, can be decomposed into two distinct physical contributions: genuine bulk particle production and a net flux induced by the dynamics of the horizon itself. Finally, we derive the Generalized Second Law (GSL) in this setting, showing transparently how the total entropy budget is balanced by horizon thermodynamics, bulk creation, and boundary fluxes.