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 \equiv p / \rho \geq - 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 \geq - 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 .