Was 2024 the year we finally started to understand dark energy?

[Admin]

2024 was a big year for the study of the universe's most troubling and mysterious "ingredient," dark energy. Here's what scientists found.

Was 2024 the year we finally started to understand dark energy? : Read more

 

[Zdepthcharge]

Well... Maybe. The Timescape paper is certainly the largest advancement in "dark energy" research we've had since the whole idea was introduced. And for those that are living in the dark, the Timescape paper, simplified, says that the calculations that led people to dark energy were made in error. There is no dark energy.

 

[contrarian]

There is no dark energy.

It certainly seems questionable.

But then you read about changes : "That means it is dynamic and changing."

If the expansion rate is changing, there must be some dynamic explanation, it would seem. Could it be related to mass of the universe - perhaps something about its total mass, or something about gravity that is unknown to us?

Who else is contrarian on this one?

 

[Unclear Engineer]

"Changing dark energy" is just one more leap down the rabbit hole of adding unexplained forces and materials to make the theory calculations match the observations where the known physics do not provide any way to make a match.

Science does have a history of making advances in knowledge by observing such mismatches and using them to look into something more closely, leading to the discovery of "new" physical laws. Special Relativity is a prime example of a successful theory that resulted that way.

On the other hand, when the same theory has come to the point that it is dependent on 20 times more mass and energy than we can detect or explain, and we still need to add the assumption that the unknown energy behaves in different manners at different times, science also has a history of that being a serious red flag that the theory is not correct.

So, I do applaud the theorists who are trying to think outside of "the box" that the Lambda-Cold-Dark-Matter model of cosmology has become.

 

[Torbjorn Larsson]

The article does not describe new research that would reject LCDM. On the contrary, the latest DESI results confirms LCDM (so general relativity). None of the deviations (apart from the usual cosmic ladder methods giving Hubble tension) reaches 3σ significance. “The statistical significance of the differences compared to DESI, calculated as described above, stands at 1.6σ for PantheonPlus, 2.0σ for Union3, and 2.6σ for DESY5.” [DESI 2024 VI: Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations]

Evolving dark energy is more of the same: “The modified gravity model gives results discrepant with ΛCDM at the 2.4σ level, while for w0waCDM it is at 2.5σ, based on the best-fit χ^2 values." [Modified gravity interpretation of the evolving dark energy in light of DESI data, Anton Chudaykin and Martin Kunz1, Phys. Rev. D 110, 123524 – Published 17 December, 2024]

Well... Maybe. The Timescape paper is certainly the largest advancement in "dark energy" research we've had since the whole idea was introduced. And for those that are living in the dark, the Timescape paper, simplified, says that the calculations that led people to dark energy were made in error. There is no dark energy.

For those who are living in the dark, the idea that Timescape rests on was rejected 20 years ago and is made in error. There is no viable 'Timescape', as the lead cosmologist on its rejection explains:

After performing those calculations — not just with a first-order or second-order approximation, but taking into account fully nonlinear inhomogeneities — a number of lessons emerge.

• It turns out that inhomogeneities, as a function of energy density, always remain small: no greater than about ~0.1% (or 1-part-in-1000) of the total energy density at any time, even many billions of years into the future.
• It also turns out that there’s a “key scale” where the greatest contributions arise: on scales of between about a few hundred thousand and around ten million light-years. Both larger and smaller cosmic scales, even including super-horizon scales, contribute less.
• And finally, it turns out that the inhomogeneities never behave as dark energy behaves, and in fact has an equation of state that always contributes further to a decelerating universe, not an accelerating one.

“The possibility that a known component of the universe may be responsible for the accelerated expansion remains intriguing. However, we conclude that sub-horizon perturbations are not a viable candidate for explaining the accelerated expansion of the universe.”

https://bigthink.com/starts-with-a-bang/lumpy-explain-dark-energy/

The expert on this also points out that Wiltshire is a known fringe scientist that has taken a stand on this for decades.

But that expert also takes a more lenient stance on the purported testing in the latest "Timescape" paper than I did when I read it naively. In effect the model introduces a new parameter - backreaction, which we have yet to observe - and we can expect it to fit better. The paper test verifies that it does so but doesn't make a model comparison of both quality (minimal set of parameters) and fit, for example using the Akaike information criterion (AIC). https://en.wikipedia.org/wiki/Akaike_information_criterion

On the other hand, when the same theory has come to the point that it is dependent on 20 times more mass and energy than we can detect or explain,

I agree with the part on the proposal of dynamic dark energy as it has a lot to prove (see above on the same point on model comparison for backreaction), But not this part since LCDM is so far the simplest model - and for comparison, with 5 parameters the LCDM era is simpler than the 6 parameter inflation era preceding it - and we have both detected dark matter and vacuum "dark" energy by many different means which has enabled description.

Specifically for vacuum energy the list of detections goes from spontaneous emission over Lamb shift to late era cosmological expansion. https://en.wikipedia.org/wiki/Vacuum_energy Its description comes from effective quantum field theory (existence) in combination with inflation theory (magnitude, from Weinberg's prediction of its value in anthropic inflationary multiverses).

 

[TallDave]

The hit piece on Wiltshire is a bit silly. First off, the "expert" cites obsolete data on density variations from 2005, before most of the very large cosmic structures had been detected. Then he goes off on a weird tangent about a study he co-authored 20 years ago with no relevance to timescape theory. Finally, he implies Wiltshire is fringe even though everything he's done fits neatly within relativity without requiring any "dark" forces or modifications to gravity, and is based on theoretically well-established equations published by Buchert decades ago.

The criticism that it "adds another parameter" is also nonsense. FLRW is an approximation of GR, used because flat space is much easier to calculate. The bits they're ignoring either matter, or they don't. LCDM says they're small enough to ignore. Timescape says they aren't. But no one argues they don't exist. Nor does data contradicting a constant dark energy "confirm" LCDM just because it hasn't yet disproved it to 5 sigmas.

The Euclid 1A/BAO results will be detailed enough to either confirm or disprove timescape (maybe by this summer), but LCDM's days are numbered regardless. It isn't just dark energy fluctuations and Hubble tension, it's too-early galaxies, too-large cosmic structures, and local (non-Copernican) Keplerian decline in the Milky Way (all of which may be explainable in timescape). At some point, some better explanation needs to be found.