CosmoCoffee

Normally, I would just write an e-mail to Jochen, but this could be a good topic for cosmocoffee.

I am sure whoever reads this nice paper is likely to have the same question. When dealing with models with w<-1, and especially those which cross from w>-1 to w<-1, one doesn't really know what to do with the fluctuations. One could flip the sign of the kinetic term when w<-1, which is probably what the authors have done (or maybe not?), but does this solve the problem? How do these constraint change if the fluctuations in dark energy are turned off? If the changes are large then wouldn't that imply that the results are only valid for a class of models of measure zero?

Maybe one of the authors, or anybody, could comment on why we should believe their constraints.

Levon

Well, they say they use the Weller & Lewis for DE pert. in CMB ( which uses the sign flip trick), but in my understanding you don't need to take into account perturbations for SN or X-ray data...

Hi guys

nice that you discuss our paper here. If you read Weller and Lewis careful, there is
a method where you do not need to resort to a scalar field to do perturbations in
the dark energy, so no sign flip trick ! We just look at a fluid with sound speed c_s^2 = 1.
This allows one to go over the transition. There will be a paper by Rapetti and Weller
which will discuss this in detail :)

Yours
Jochen

Hi guys, nice to see the cosmocoffee brewing.

Jochen, I will look forward to your next paper. But still, what about my original question -- does your way of doing the pertrubations make a big difference when it comes to the final DE constraints. You must have tried to do it with fluctuations turned off and/or with the flip of sign. I would think they would be compatible to those due to different sound speeds, something you have discussed in that same paper with Antony.
I like your parametrization of w(z) and just want to get a feel for what your constraints on it would become if you ``marginalizes'' over different
ways of doing the DE perturbations.

Levon

Hi Levon

in Weller and Lewis (2003) we showed that our way of doing perturbations is equivalent
in the case of standard scalar field dark energy models where you can show the sound speed is one. Of course when it comes to models with w<-1 you open up a 'can of worms'.
In the present analysis we assume that the sound speed is constant one also in this case.
In the case of constant w<-1 this corresponds to 'sticking in a "-" sign in front of the kinetic
term'. However as pointed out above there is no, as far as I am aware, scalar field dark energy model with a transition from w<-1 to w>-1.

No in Weller and Lewis we showed that it is important to include perturbations, but I suppose it is not important how you include them. Of course there is the issue that 'k-essence' models can have different sound speeds which also vary with time

Yours
Jochen

Thanks, Jochen.
So you're saying that the way in which you include the perturbation for w<-1 in your type of set up is not that important, as long as you include it. That's probably because when w approaches -1 the perturbation goes to zero (if you prevent it from diverging by fixing c_s^2=1). Fair enough. This is probably as good as one can do.
Levon

sort of, yes