The Apparent (Gravitational) Horizon in Cosmology

TL;DR

This paper explores the nature and significance of the apparent (gravitational) horizon in cosmology, emphasizing its role in defining the observable universe and its implications for understanding cosmic data.

Contribution

It provides a detailed analysis of the apparent horizon in the Friedmann-Robertson-Walker metric and clarifies its physical reality and importance in cosmological models.

Findings

The apparent horizon is crucial for defining the size of the observable universe.

It is a dynamic feature that may evolve into an event horizon depending on cosmic conditions.

The apparent horizon has observable signatures that impact cosmological data interpretation.

Abstract

In general relativity, a gravitational horizon (more commonly known as the "apparent horizon") is an imaginary surface beyond which all null geodesics recede from the observer. The Universe has an apparent (gravitational) horizon, but unlike its counterpart in the Schwarzschild and Kerr metrics, it is not static. It may eventually turn into an event horizon---an asymptotically defined membrane that forever separates causally connected events from those that are not---depending on the equation of state of the cosmic fluid. In this paper, we examine how and why an apparent (gravitational) horizon is manifested in the Friedmann-Robertson-Walker metric, and why it is becoming so pivotal to our correct interpretation of the cosmological data. We discuss its observational signature and demonstrate how it alone defines the proper size of our visible Universe. In so doing, we affirm its physical reality and its impact on cosmological models.