We have reanalyzed constraints on the equation of state parameter, wQ≡P/ρ, of the dark energy, using several cosmological data sets and relaxing the usual constraint wQ>~-1. We find that combining cosmic microwave background, large scale structure, and type Ia supernova data yields a nontrivial lower bound on wQ. At 95.4% confidence we find, assuming a flat geometry of the universe, a bound of -2.68<wQ<-0.78 if wQ is taken to be a completely free parameter. Reassuringly we also find that the constraint wQ>~-1 does not significantly bias the overall allowed region for wQ. When this constraint is imposed the 95.4% confidence bound is -1<~wQ<-0.71. Also, a pure cosmological constant (w=-1) is an excellent fit to all available data. Based on simulations of future data from the Planck CMB experiment and the SNAP and SNfactory supernova experiments we estimate that it will be possible to constrain wQ at the 5% level in the future.
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Hannestad et al. (2002) studied this question.