The state of the dark energy equation of state

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 wQ to be -1.45<wQ<-0.74 at the 95% confidence level. Although our limit on wQ is improved with respect to previous analyses, cosmological data do not rule out the possibility that the equation of state parameter wQ of the dark energy Q is less than -1. We present a tracking model that ensures wQ<~-1 at recent times and discuss the observational consequences.