Generalized Second Law for Cosmology

TL;DR

This paper proposes a new generalized second law of thermodynamics for cosmology, applicable even without event horizons, based on the behavior of past and future Q-screens and their generalized entropy.

Contribution

It introduces the concept of Q-screens and proves a generalized second law for spacetimes satisfying the Quantum Focussing Conjecture, extending thermodynamic principles to broader cosmological settings.

Findings

Generalized entropy increases monotonically outside past Q-screens.

Law applies to spacetimes obeying the Quantum Focussing Conjecture.

Analogous law for future Q-screens in black hole and collapsing regions.

Abstract

We conjecture a novel Generalized Second Law that can be applied in cosmology, regardless of whether an event horizon is present: the generalized entropy increases monotonically outside of certain hypersurfaces we call past Q-screens. A past Q-screen is foliated by surfaces whose generalized entropy (sum of area and entanglement entropy) is stationary along one future null direction and increasing along the other. We prove that our Generalized Second Law holds in spacetimes obeying the Quantum Focussing Conjecture. An analogous law applies to future Q-screens, which appear inside evaporating black holes and in collapsing regions.