The Gravitational Horizon for a Universe with Phantom Energy

TL;DR

This paper defends the view that the gravitational horizon, coinciding with the Hubble sphere, remains the limit of our observable universe even in cosmologies with phantom energy, clarifying misconceptions about its significance.

Contribution

It clarifies the role of the gravitational horizon as the true observational limit in all cosmological models, including those with phantom energy, correcting previous misinterpretations.

Findings

The gravitational horizon is the limit of observability regardless of the equation of state.

Claims that R_h is not a true horizon in phantom energy cosmologies are based on misinterpretations.

The paper reaffirms the importance of null geodesics in understanding cosmic horizons.

Abstract

The Universe has a gravitational horizon, coincident with the Hubble sphere, that plays an important role in how we interpret the cosmological data. Recently, however, its significance as a true horizon has been called into question, even for cosmologies with an equation-of-state w = p/rho > -1, where p and rho are the total pressure and energy density, respectively. The claim behind this argument is that its radius R_h does not constitute a limit to our observability when the Universe contains phantom energy, i.e., when w < -1, as if somehow that mitigates the relevance of R_h to the observations when w > -1. In this paper, we reaffirm the role of R_h as the limit to how far we can see sources in the cosmos, regardless of the Universe's equation of state, and point out that claims to the contrary are simply based on an improper interpretation of the null geodesics.