4 ms·

> it all really looks awfully close to "okay, fine, the dark energy/matter apparently also can do/be this stuff as well, just so we can write in whatever corre

by pdonis 2mo ago

> it all really looks awfully close to "okay, fine, the dark energy/matter apparently also can do/be this stuff as well, just so we can write in whatever correction factors we need to make the theory fit the observations". The dark energy density is already a parameter in the model. The simplest case for such a parameter is that it's just a constant, so in the absence of evidence to the contrary, Occam's Razor led cosmologists to adopt it. But now we have evidence that suggests that it's not a constant, so we're looking at the next simplest case, a function of time (but still constant everywhere in space at each instant of time). The article describes how the DESI data suggest that it's a slowly decreasing function of time. What has not happened is people making up models and continuing to insist on them even after the data says otherwise. That's what the GP was saying religion does.

suuuuuuuu 2mo ago
Pedantic point, but in the time-evolving model ("model"), dark energy is not uniform across space - but the non-uniformity turns out to be negligible.
- pdonis 2mo ago
  > in the time-evolving model ("model"), dark energy is not uniform across space What are you basing this on?
  - suuuuuuuu 2mo ago
    Domain expertise. (I would ask the same of you!) Fluids always have perturbations, except in the special case of w = -1 (cosmological constant); otherwise, dropping them violates energy-momentum conservation and gauge invariance. Here are a few excerpts from DESI: * https://arxiv.org/html/2404.03002v3#:~:text=Although%20a%20cosmological,i.e.%20constant). https://arxiv.org/html/2404.03002v3#:~:text=Although%20a%20c... * https://arxiv.org/html/2404.03002v3#:~:text=Since%20the%20parameter,angular%20power%20spectrum. https://arxiv.org/html/2404.03002v3#:~:text=Since%20the%20pa...
    - pdonis 2mo ago
      > 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?
      - suuuuuuuu 2mo ago
        > 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".
      - raattgift 2mo ago
        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 https://doi.org/10.1103/PhysRevD.78.087303 aka https://arxiv.org/abs/0808.3125 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.

by pdonis 2mo ago

suuuuuuuu 2mo ago

Pedantic point, but in the time-evolving model ("model"), dark energy is not uniform across space - but the non-uniformity turns out to be negligible.

pdonis 2mo ago
> in the time-evolving model ("model"), dark energy is not uniform across space What are you basing this on?
- suuuuuuuu 2mo ago
  Domain expertise. (I would ask the same of you!) Fluids always have perturbations, except in the special case of w = -1 (cosmological constant); otherwise, dropping them violates energy-momentum conservation and gauge invariance. Here are a few excerpts from DESI: * https://arxiv.org/html/2404.03002v3#:~:text=Although%20a%20cosmological,i.e.%20constant). https://arxiv.org/html/2404.03002v3#:~:text=Although%20a%20c... * https://arxiv.org/html/2404.03002v3#:~:text=Since%20the%20parameter,angular%20power%20spectrum. https://arxiv.org/html/2404.03002v3#:~:text=Since%20the%20pa...
  - pdonis 2mo ago
    > 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?
    - suuuuuuuu 2mo ago
      > 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".
    - raattgift 2mo ago
      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 https://doi.org/10.1103/PhysRevD.78.087303 aka https://arxiv.org/abs/0808.3125 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.