Helder Velez brought the following paper to my attention: Carnero et al., "Clustering of Photometric Luminous Red Galaxies II: Cosmological Implications from the Baryon Acoustic Scale," http://arxiv.org/abs/1104.5426. They use a survey of galactic redshifts to measure baryon acoustic oscillations (BAO), and they also combine their results statistically with other people's BAO measurements based on the CMB. As far as I know, this gives the best current determination on the equation of state of dark energy: [itex]w=-1.03\pm .16[/itex], which is consistent with a cosmological constant (w=-1) but also consistent with a Big Rip (w<-1). They also present a constraint on the rate of change of the equation of state, parametrized as [itex]w=w_0+w_a(1-a)[/itex], where a is the scale factor. The result is [itex]w_a=.06 \pm .22[/itex], i.e., consistent with a constant equation of state. I don't know if there's any strong theoretical motivation for or against time-variation of w...? Would it violate local mass-energy conservation (giving a nonzero divergence of the stress-energy tensor)? Even a decade after the initial claims of accelerated expansion, I've still felt a little skeptical about the empirical evidence. It is interesting to find out that BAO can be measured in two independent ways (galactic redshifts as well as CMB), since that seems to make it less likely that there is just a systematic error in the measurements. It seems like "Hubble bubble" explanations are also ruled out these days. So maybe I'm finally ready to believe in dark energy wholeheartedly.