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/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.
