No evidence for dark energy----------

[wolram]

http://arxiv.org/PS_cache/arxiv/pdf/0908/0908.3186v1.pdf

We use a variant of principal component analysis to investigate the possible temporal evolution of the dark energy equation of state, w(z). We constrain w(z) in multiple redshift bins, utilizing the most recent data from Type Ia supernovae, the cosmic microwave background, baryon acoustic oscillations, the integrated Sachs-Wolfe effect, galaxy clustering, and weak lensing data. Unlike other recent analyses, we find no significant evidence for evolving dark energy; the data remains completely consistent with a cosmological constant. We also study the extent to which the time-evolution of the equation of state would be constrained by a combination of current- and future-generation surveys, such as Planck and the Joint Dark Energy Mission.

[George Jones]

Careful with the title of the thread; the title of the paper is "No Evidence for Dark Energy Dynamics ...," and the abstract says "we find no significant evidence for evolving dark energy ..."

[wolram]

Careful with the title of the thread; the title of the paper is "No Evidence for Dark Energy Dynamics ...," and the abstract says "we find no significant evidence for evolving dark energy ..."

Sorry George.

[marcus]

Thanks to both, for pointing paper out and truing up its interpretation.
The important thing seems to be what they say in their abstract summary which fortunately Wolram quoted in original post

...the data remains completely consistent with a cosmological constant. We also study the extent to which...

The cosmo constant is a special case of "dark energy" where the density of dark energy does not change over time and the relation of that to pressure also does not change. In this particular special case (which seems the simplest and most appealing to a lot of people) the constant ratio between the density and pressure is -1.

In the cosmo constant case of "dark energy" it is always true that

pressure = - energy density.

So if you like metric units that would mean that if the energy density (as an unreal example) were to be one joule per cubic meter, then the pressure would be exactly
minus one pascal.
The ratio between the two is minus one.

This ratio between the density and the pressure is often notated by a "w" and called the "equation of state" parameter.
If you have a cosmo model with dark energy in it and the energy density is constant and
w = -1, then that corresponds to a cosmological constant.

So what these people are doing is reassuring us that so far the data is still consistent with a steady w = -1. That the number w does not somehow change with time or redshift. Personally I am glad they are checking this repeatedly and stringently, and I am also very glad that it keeps coming out to be w = -1.
I like calm.

Other people, who like nervousness and excitement, probably are wishing that it would come out that w is not -1, and even that w changes over time. This to me would be the pits.

So what they are saying (taking account of GJ's correction) is no evidence for a changeable dark energy.

Thanks for posting the latest news on this!

[Chalnoth]

TSo what these people are doing is reassuring us that so far the data is still consistent with a steady w = -1. That the number w does not somehow change with time or redshift. Personally I am glad they are checking this repeatedly and stringently, and I am also very glad that it keeps coming out to be w = -1.
I like calm.
I'm not. It'd be really interesting if we could discover some dynamics to dark energy that would give us some hints as to what's going on. All we can say so far is, "cosmological constant not yet ruled out."