Cosmological apparent and trapping horizons

TL;DR

This paper investigates the properties and dynamics of various cosmological horizons in FLRW space, establishing criteria for their trapping nature, temperature positivity, and physical viability within extended gravity theories.

Contribution

It introduces a simple criterion for apparent horizon trapping based on Ricci curvature and explores horizon entropy and thermodynamics in extended gravity models.

Findings

Apparent horizon trapping occurs with positive Ricci curvature.

Horizon temperature positivity aligns with trapping conditions.

Some solutions in Brans-Dicke gravity are deemed unphysical based on entropy considerations.

Abstract

The dynamics of particle, event, and apparent horizons in FLRW space are discussed. The apparent horizon is trapping when the Ricci curvature is positive. This simple criterion coincides with the condition for the Kodama-Hayward apparent horizon temperature to be positive, and also discriminates between timelike and spacelike character of the apparent horizon. We discuss also the entropy of apparent cosmological horizons in extended theories of gravity and we use the generalized 2nd law to discard an exact solution of Brans-Dicke gravity as unphysical.