This week I had a meeting with Wayne Hu at the University of Chicago. We discussed possible ways how to treat a dark energy model correctly which crosses the so called phantom divide. Wayne wrote a neat paper about this.

Now let me try and tell you what the problem is. Standard dark energy in itself is already a strange concept. In order for the Universe to expand accelerated you require that the ratio of the pressure and energy density in the dark energy fluid is less than -1/3. That means a fluid with negative pressure ... sounds nasty ? Well if the Universe is actually dominated by some non-vanishing vacuum energy this ratio is actually -1. A fluid like this corresponds exactly to the so called cosmological constant, which was introduced by Einstein in the first version of his general relativity equations in order to obtain a static Universe. Einstein firmly believed that the Universe should be static. After Hubble discovered that the Universe is expanding, Einstein supposedly said that the introduction of the cosmological constant was his biggest blunder. I actually only found this quote in George Gamow's book "My Worldline", but I never found any source which is pointing directly to Einstein.

Now usually the cosmological constant is the limiting case, in a sense that the ratio between pressure and energy can not be smaller than -1. A smaller ratio would violate an important energy condition that the pressure plus energy density should be positive. However from a data point of view one might ask what if we allow for a smaller ratio. Recent Supernovae observations definetly allow for a ratio smaller than -1 as you can see in Riess et al . Models with such a ratio where introduced by Caldwell and he called them phantom. Now there are a lot of physical problems with such models, but from an observational point of view one can not exclude them yet.

One of the best data sets at the moment to constrain cosmological parameters is the observation of the cosmic microwave background with the Wilkinson Microwave Anisotropy Probe . This observation probes fluctuations in the temperature of the radiation left over from the Big Bang. This fluctuation has imprints of the underlying fluctuations in the matter and dark energy as well. Now there are ways to calculate the dark energy fluctuations as long as the ratio of the pressure and energy density of the fluid stays on one side of the -1. If it crosses nobody knew how to handle this correctly. This is where Wayne's paper comes in. The trick is that one requires internal degrees of freedom in the fluid. He explained me how this works this week. Very clever guy, I have to say.

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