Dark energy 'doesn't exist' so can't be pushing 'lumpy' universe apart, physicists say

[KJW]

https://phys.org/news/2024-12-dark-energy-doesnt-lumpy-universe.html

"One of the biggest mysteries in science—dark energy—doesn't actually exist, according to researchers looking to solve the riddle of how the universe is expanding.

Their analysis has been published in the journal Monthly Notices of the Royal Astronomical Society Letters."

 

Basically, they claim that there is no need for dark energy because the apparent accelerated expansion of the universe is really due to how we calibrate time and distance.

 

[TheVat]

This timescape model is fascinating.  When you read their explanation of different measures of time in the filaments, superclusters and voids, it makes you wonder what took so long to arrive at such a theory.  It just makes sense.  Large-scale structure, "lumpiness," has been known since what, the 80s?  

Could this be farewell to lambda?  (tempted to call it the silence of the lambda)  

[DeepBlueSouth]

4 hours ago, TheVat said:

Could this be farewell to lambda?  (tempted to call it the silence of the lambda)  

loving this +1

according to wikipedia:
 

Quote

Alan Guth and Alexei Starobinsky proposed in 1980 that a negative pressure field, similar in concept to dark energy, could drive cosmic inflation in the very early universe. Inflation postulates that some repulsive force, qualitatively similar to dark energy, resulted in an enormous and exponential expansion of the universe slightly after the Big Bang. Such expansion is an essential feature of most current models of the Big Bang. However, inflation must have occurred at a much higher (negative) energy density than the dark energy we observe today, and inflation is thought to have completely ended when the universe was just a fraction of a second old. It is unclear what relation, if any, exists between dark energy and inflation. Even after inflationary models became accepted, the cosmological constant was thought to be irrelevant to the current universe.

Nearly all inflation models predict that the total (matter+energy) density of the universe should be very close to the critical density. During the 1980s, most cosmological research focused on models with critical density in matter only, usually 95% cold dark matter (CDM) and 5% ordinary matter (baryons). These models were found to be successful at forming realistic galaxies and clusters, but some problems appeared in the late 1980s: in particular, the model required a value for the Hubble constant lower than preferred by observations, and the model under-predicted observations of large-scale galaxy clustering. These difficulties became stronger after the discovery of anisotropy in the cosmic microwave background by the COBE spacecraft in 1992, and several modified CDM models came under active study through the mid-1990s: these included the Lambda-CDM model and a mixed cold/hot dark matter model.

so yes, as you suggested the 1980's seemed to bring us those concepts of anisotropy in the models. as little as I understand about physics, it boggles my mind that the universal share of low density "dark energy" share was off by that much under the assumption of this concept. so does that mean those accounted for energies are just shared among those low gravity regions? [woefully ignorant here on these concepts, sorry]

[geordief]

5 hours ago, TheVat said:

This timescape model is fascinating.  When you read their explanation of different measures of time in the filaments, superclusters and voids, it makes you wonder what took so long to arrive at such a theory.  It just makes sense.  Large-scale structure, "lumpiness," has been known since what, the 80s?  

Could this be farewell to lambda?  (tempted to call it the silence of the lambda)  

If this was verified (I just have a faint idea of this) would it provide a means to tweak gravitational lensing?

[Markus Hanke]

I also agree that these are interesting proposals. However, the devil is in the details, because it seems these models need to make a number of their own assumptions to actually work, and it also seems that it is not at all clear that they actually really do produce the correct effective dynamics. Here are some more technical details about inhomogenous cosmologies in general:

https://ncatlab.org/nlab/show/inhomogeneous+cosmology

Maybe it can work, but I just think this requires more study before jumping to any conclusions.

[exchemist]

4 hours ago, Markus Hanke said:

I also agree that these are interesting proposals. However, the devil is in the details, because it seems these models need to make a number of their own assumptions to actually work, and it also seems that it is not at all clear that they actually really do produce the correct effective dynamics. Here are some more technical details about inhomogenous cosmologies in general:

https://ncatlab.org/nlab/show/inhomogeneous+cosmology

Maybe it can work, but I just think this requires more study before jumping to any conclusions.

That link is a beautifully readable summery of the concept. I actually think I understand it now. 🙂

Perhaps we are moving into an era in which, for the first time, enough observations can be accumulated to validate the choice of assumptions and see if it can really work.  

I wonder where this would leave the proponents of a connection between dark energy and zero point energy of the vacuum, though. 

 

Edited December 26, 2024 by exchemist

[Markus Hanke]

2 hours ago, exchemist said:

Perhaps we are moving into an era in which, for the first time, enough observations can be accumulated to validate the choice of assumptions and see if it can really work.

Indeed. 
We should also remember that the finding referenced in the OP regarding the ‘timescape’ model only fits the 1a supernova data to a confidence level of 3 sigma, which is below the necessary threshold. But yes, I do think it’s worth further study.

Edited December 26, 2024 by Markus Hanke

[swansont]

I saw a comment from Sean Carroll that basically says to remember there’s a difference from some speculative idea being floated, and actual discovery. This is the former. To be considered the latter requires evidence.

[exchemist]

2 hours ago, swansont said:

I saw a comment from Sean Carroll that basically says to remember there’s a difference from some speculative idea being floated, and actual discovery. This is the former. To be considered the latter requires evidence.

Yes but this is more than mere speculation, surely? The paper is about fitting  data from something they refer to as the SNe Ia dataset to their model.  Here is the paper: https://academic.oup.com/mnrasl/article/537/1/L55/7926647?login=false

Not being an astronomer I don’t know what this dataset is, but it seems to be something used to estimate cosmological distances. So it would seem to be based on evidence, not just theory.

Edited December 27, 2024 by exchemist

[swansont]

2 hours ago, exchemist said:

Yes but this is more than mere speculation, surely? The paper is about fitting  data from something they refer to as the SNe Ia dataset to their model.  Here is the paper: https://academic.oup.com/mnrasl/article/537/1/L55/7926647?login=false

Not being an astronomer I don’t know what this dataset is, but it seems to be something used to estimate cosmological distances. So it would seem to be based on evidence, not just theory.

The model has been around for a while - it did not arise from current data. As Markus pointed out, it’s only a three-sigma fit, and there may be other phenomena that it needs to match. This could be like MOND, where it matches one anomaly but fails to line up with other data. I don’t know. 

We get pop-sci summaries (or press releases) much faster and widely distributed than we used to, but the actual science takes a while. It’s easy to get out over your skis.

[exchemist]

40 minutes ago, swansont said:

The model has been around for a while - it did not arise from current data. As Markus pointed out, it’s only a three-sigma fit, and there may be other phenomena that it needs to match. This could be like MOND, where it matches one anomaly but fails to line up with other data. I don’t know. 

We get pop-sci summaries (or press releases) much faster and widely distributed than we used to, but the actual science takes a while. It’s easy to get out over your skis.

Sure but my point was that a "3σ fit" means it's not just a "speculative idea", as Carroll dismissively calls it, but supported by evidence, even if that evidence is far from conclusive at this stage.  As Markus's post makes clear, the idea itself is not new. What's new is a fitting of actual data to a model of this type. At least, that is my reading of it. 

Edited December 27, 2024 by exchemist

[swansont]

3 hours ago, exchemist said:

Sure but my point was that a "3σ fit" means it's not just a "speculative idea",

But there wasn’t a 3σ fit when the idea was put forward. That’s why it was speculative. And a 3σ fit is still short of a discovery. 

CERN has had intriguing bumps in data at 3σ which disappeared when more data was gathered. 

Carroll notes that supernovae, CMB, lensing, baryon acoustic oscillations, and more, are all effects that must fit with the theory. You also want a different research group to confirm the model’s predictions.

[Markus Hanke]

3 hours ago, swansont said:

Carroll notes that supernovae, CMB, lensing, baryon acoustic oscillations, and more, are all effects that must fit with the theory. You also want a different research group to confirm the model’s predictions.

Indeed. Which brings up another issue, in that it is actually very difficult to extract specific predictions from this model, since it lacks much of the symmetry of FLRW (and is thus not analytically solvable), so you’re stuck with numerical simulations that  are computationally complex, and rely on precise knowledge of the exact distribution of matter and radiation in the cosmos.