A phantom menace? Cosmological consequences of a dark energy component with super-negative equation of state

It is extraordinary that a number of observations indicate that we live in a spatially flat, low matter density Universe, which is currently undergoing a period of accelerating expansion. The effort to explain this current state has focused attention on cosmological models in which the dominant component of the cosmic energy density has negative pressure, with an equation of state /w>=-1. Remarking that most observations are consistent with models right up to the /w=-1 or cosmological constant (/Λ) limit, it is natural to ask what lies on the other side, at /w<-1. In this regard, we construct a toy model of a ``phantom'' energy component which possesses an equation of state /w<-1. Such a component is found to be compatible with most classical tests of cosmology based on current data, including the recent type 1a SNe data as well as the cosmic microwave background anisotropy and mass power spectrum. If the future observations continue to allow /w<-1, then barring unanticipated systematic effects, the dominant component of the cosmic energy density may be stranger than anything expected.