Abstract

The inflationary prediction of a flat Universe is at odds with current determinations of the matter density $({\ensuremath{\Omega}}_{M}\ensuremath{\simeq}0.2\ensuremath{-}0.4).$ This dilemma can be resolved if a smooth component contributes the remaining energy density $({\ensuremath{\Omega}}_{X}=1\ensuremath{-}{\ensuremath{\Omega}}_{M}).$ We parametrize the smooth component by its equation of state, ${p}_{X}=w{\ensuremath{\rho}}_{X},$ and show that $X$CDM with $w\ensuremath{\simeq}\ensuremath{-}0.6,$ ${\ensuremath{\Omega}}_{M}\ensuremath{\simeq}0.3,$ and $h\ensuremath{\simeq}0.7$ is the best fit to all present cosmological data. Together, the position of the peak in the CMB angular power spectrum and the type Ia supernova magnitude-redshift diagram provide a crucial test of $X$CDM.