If dark energy contributes mass-energy density ρ and pressure p to the stress-energy tensor, then you can define w=p/ρ, where w=−1 gives a cosmological constant, w<−1 gives a big rip, and w<−1/3 if we want to use dark energy to explain cosmological acceleration. The WP "Big Rip" article cites a paper that dates back to 2003 http://arxiv.org/abs/astro-ph/0302506 , which states that the empirical evidence was at that time only good enough to give -2 \lesssim w \lesssim -.5.
Have observations constrained w any more tightly since 2003?
I've heard many people express the opinion that w<-1 is silly or poorly motivated, or that "no one believes it." What are the reasons for this? Other than mathematical simplicity, I don't know of any reason to prefer w=-1 over w\ne -1. Considering that attempts to calculate \Lambda using QFT are off by 120 orders of magnitude, is it reasonable to depend on theory to give any input into what values of w are believable?
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