The state of the dark energy equation of state

Abstract

By combining data from seven cosmic microwave background experiments (including the latest WMAP results) with the Hubble parameter measurement from the Hubble space telescope and luminosity measurements of type Ia supernovae, we demonstrate the bounds on the dark energy equation of state ${w}_{Q}$ to be $\ensuremath{-}1.45<{w}_{Q}<\ensuremath{-}0.74$ at the 95% confidence level. Although our limit on ${w}_{Q}$ is improved with respect to previous analyses, cosmological data do not rule out the possibility that the equation of state parameter ${w}_{Q}$ of the dark energy Q is less than $\ensuremath{-}1.$ We present a tracking model that ensures ${w}_{Q}<~\ensuremath{-}1$ at recent times and discuss the observational consequences.