Cosmological black holes and the direction of time

TL;DR

This paper explores how black hole horizons and cosmic expansion influence the directionality of time, linking cosmological, electromagnetic, gravitational, and thermodynamic arrows through horizon growth and radiation accretion.

Contribution

It introduces a model connecting black hole horizon growth due to cosmic expansion with the emergence of a preferred time direction across multiple physical domains.

Findings

Black hole horizon area increases over cosmic time in various cosmological models.

Accretion of cosmic microwave background radiation sets a lower limit on matter intake by black holes.

The asymmetry in horizon growth relates to the unidirectional flow of electromagnetic energy and entropy increase.

Abstract

Macroscopic irreversible processes emerge from fundamental physical laws of reversible character. The source of the local irreversibility seems to be not in the laws themselves but in the initial and boundary conditions of the equations that represent the laws. In this work we propose that the screening of currents by black hole event horizons determines, locally, a preferred direction for the flux of electromagnetic energy. We study the growth of black hole event horizons due to the cosmological expansion and accretion of cosmic microwave background radiation, for different cosmological models. We propose generalized McVittie co-moving metrics and integrate the rate of accretion of cosmic microwave background radiation onto a supermassive black hole over cosmic time. We find that for flat, open, and closed Friedmann cosmological models, the ratio of the total area of the black hole event horizons with respect to the area of a radial co-moving space-like hypersurface always increases. Since accretion of cosmic radiation sets an absolute lower limit to the total matter accreted by black holes, this implies that the causal past and future are not mirror symmetric for any spacetime event. The asymmetry causes a net Poynting flux in the global future direction; the latter is in turn related to the ever increasing thermodynamic entropy. Thus, we expose a connection between four different "time arrows": cosmological, electromagnetic, gravitational, and thermodynamic.