The gravitational horizon for a Universe with phantom energy

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/ρ -1, where p and ρ 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.