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Comment by pdonis | Hacker News Reader

> Domain expertise.

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?

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> Sorry, not buying the argument from authority here.

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".

The problem is in allowing perturbations around effective w_{DE}=-1. The "phantom divide crossing" is the evolution of dark energy's effEOS across w = -1, the boundary between a quintessence regime (w > -1) and a phantom dark energy (w < -1) regime. Phantom models generically violate the null energy condition. A local crossing thus causes all sorts of problems for minimally coupled single scalar field DE (see e.g. https://doi.org/10.1103/PhysRevD.78.087303 aka https://arxiv.org/abs/0808.3125) as fluctuations of the DE field into the phantom regime must be controlled or offset assuming one does not want the total energy density to be negative. That turns out to be hard.

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