Here are a few excerpts from DESI:
* https://arxiv.org/html/2404.03002v3#:~:text=Although%20a%20c...
* https://arxiv.org/html/2404.03002v3#:~:text=Since%20the%20pa...
Sorry, not buying the argument from authority here.
> Fluids always have perturbations
Not sure I agree with this as a sweeping general claim; but in any case, my question was about what in the particular models under discussion you were basing your statement on.
> except in the special case of w = -1 (cosmological constant)
Yes, this part I agree with, a cosmological constant has to be, well, constant.
> otherwise, dropping them violates energy-momentum conservation and gauge invariance
I don't understand the argument here.
> Here are a few excerpts from DESI
Unfortunately these links don't seem to be showing me specific excerpts, just the whole paper. Can you give page/section references or equation numbers?
You asked what I based my answer on, and domain expertise is the answer. The rest was an actual argument.
> Yes, this part I agree with, a cosmological constant has to be, well, constant.
This is a nominal fallacy, since the reason it must be homogeneous (rather than just time independent) is actually the same reason all other (w != -1) fluids must not be homogeneous.
> Not sure I agree with this as a sweeping general claim > I don't understand the argument here.
The argument is general because it rests on energy-momentum conservation and gauge invariance. The perturbed energy-momentum equations for a fluid have source terms \propto (1 + w) * <metric perturbations>, and therefore cannot be solved by fluid perturbations that are zero at all time and locations unless w = -1 or the metric is also homogeneous. The same guarantee of dynamics underlies the gauge invariance argument: while one can choose a frame in which a single fluid is homogeneous ~~at any instant, that gauge choice is only valid at all times if the fluid's energy density is time-independent~~ EDIT: that property is only gauge invariant when w = - 1.
> Unfortunately these links don't seem to be showing me specific excerpts, just the whole paper. Can you give page/section references or equation numbers?
Open in a chromium based browser or search the article for "perturbations".