Models without dark energy

[phyzguy]

Several years ago, I posted about this paper, which claimed to be able to eliminate the need for dark energy by performing cosmological simulations differently, basically intercnaging the order of calculating the expansion rate and the average density. One quote from that paper is, "The algorithm exchanges the order of averaging and calculating the expansion rate and, due to the non-linearity of the equations, the two operations do not commute." They also claimed that this reduced the observed Hubble tension.

Now there are several reports in the popular science channels, like this one from phys.org, again saying there is no need for dark energy. This is based on this paper in MNRAS, which was based on this earlier work from Wiltshire.

What I think all of these papers are saying is that as the universe evolves, a larger and larger fraction of the universe volume is taken up by voids which are nearly devoid of matter. These voids expand faster (because of the lack of matter), and this is what is driving the accelerated expansion rate.

To me these arguments are very persuasive, and I would like to hear others' opinions and thoughts.

 

[PeroK]

Now there are several reports in the popular science channels, like this one from phys.org,

You should know this is not a valid reference.

To me these arguments are very persuasive, and I would like to hear others' opinions and thoughts.

What makes them very persuasive?

 

[Ibix]

The idea of trying to explain new phenomena without introducing new entities is always a good one, and whether small(ish) scale inhomogeneity changes things significantly from an FLRW solution is a question that had occurred to me (although not in this context). So on a personal level I like this approach and think it's worth investigating - but the question is if it really works. Apparently the newer paper linked in the OP says yes, but I guess time will tell.

I'll see if I can make sense of the papers.

 

[javisot20]

Cosmologists have done a lot of work according to the cosmological principle, it is reasonable to explore its negation. Carrying out this exploration and discovering that denying the principle can eliminate dark energy does not imply that this work is physically correct, but theoretically it is interesting.

 

[phyzguy]

You should know this is not a valid reference.

Of course it is not a valid reference, which is why I linked the actual paper the popular link was based on. I suspect that I am not alone in seeing many links saying that "scientists" are saying there is no need for dark energy, and I thought people might appreciate seeing some of the science behind it.

 

[phyzguy]

What makes them very persuasive?

If dark energy is really not needed, that would be a major step forward. The idea is simple, as I said. The voids expand faster than regions with matter, and by calculating the expansion rate from the average matter density, we are making an error. The first paper did a detailed simulation assuming different regions expand at different rates, and came up with results that match Lambda-CDM without the need for Lambda.

 

[phyzguy]

...whether small(ish) scale inhomogeneity changes things significantly from an FLRW solution...

"smallish scale inhomogeneities"? The cosmic voids are 100's of Mpc across, and the mass density in the voids is 10% of the average mass density or less. How is this smallish scale?

 

[PeroK]

If dark energy is really not needed, that would be a major step forward. The idea is simple, as I said. The voids expand faster than regions with matter, and by calculating the expansion rate from the average matter density, we are making an error. The first paper did a detailed simulation assuming different regions expand at different rates, and came up with results that match Lambda-CDM without the need for Lambda.

It would be good if you were able to summarise Wiltshire's approach and how it differs from the standard Friedmann equation. In standard cosmology, you cannot consider the regions separately, because the model applies to the universe as a whole.

"smallish scale inhomogeneities"? The cosmic voids are 100's of Mpc across, and the mass density in the voids is 10% of the average mass density or less. How is this smallish scale?

The standard model assumes that, on the largest scale, the galaxy clusters are evenly distributed. With that assumption, the Friedmann equation should be valid. That's why it would be interesting to understand why the Friedmann equation fails.

Without dark energy, as I'm sure you know, the expansion rate of a vacuum-dominated universe should be slowing down. If you take dark energy out of the equation, then the Friedmann equation would have to be very wrong.

That said, the key is how Wiltshire is able to calculate the expansion rate for voids in isolation.

 

[javisot20]

The standard model assumes that, on the largest scale, the galaxy clusters are evenly distributed. With that assumption, the Friedmann equation should be valid. That's why it would be interesting to understand why the Friedmann equation fails.

Without dark energy, as I'm sure you know, the expansion rate of a vacuum-dominated universe should be slowing down. If you take dark energy out of the equation, then the Friedmann equation would have to be very wrong.

What I was understanding (and maybe I'm wrong) is that standard cosmology is not invalidated. Standard cosmology would continue to be a good approximation in part of the life of the universe, but according to this work a poor approximation for all life of the universe. Dark energy is not "eliminated", it is "replaced", we continue to observe galaxies that seem governed by dark energy or this new analytical point of view.

From Wiltshire's earlier work,
"In general, the question of how to synchronize clocks in the absence of the exact symmetry described by a timelike Killing vector in general relativity does not have a solution, and the definition of quasilocal gravitational energy depends on choices of the splitting of spacetime into spatial hypersurfaces, the threading of
those hypersurfaces by observers, and the associated choice of surfaces of integration. Such choices are in general non-covariant and non-unique. One is essentially reduced to asking which choices of frame have the greatest physical utility. Since the ambiguities have their origin in the equivalence principle, my view is that the equivalence principle should be properly formulated and respected in the relative calibration of average frames in cosmology. I have therefore extended the strong equivalence principle as a cosmological equivalence principle (Wiltshire,2008) to apply to average spatially flat regions - cosmological inertial frames –undergoing a regionally homogeneous isotropic volume expansion with deceleration over arbitrarily long time intervals."

 

[ChemAir]

Wiltshire's (et. al.) more current work. I apologize if this has been posted somewhere else

December 2024 popsci article

Reference from that article
https://academic.oup.com/mnrasl/article/537/1/L55/7926647?login=false

 

[PeterDonis]

The standard model assumes that, on the largest scale, the galaxy clusters are evenly distributed.

Yes.

With that assumption, the Friedmann equation should be valid.

Not exactly, no. It is only an approximation, because constant density everywhere on a spacelike slice of constant FRW coordinate time is only an approximation. The question is how good an approximation.

The standard model assumes that the approximation is good enough. The alternate models being discussed here explore the possibility that it is not.

I haven't had a chance to read through any of the references in detail, but I don't think the possibility that the approximation is not good enough can just be dismissed out of hand.

 

[PeterDonis]

Dark energy is not "eliminated", it is "replaced"

I don't think this is a good description. Dark energy in the standard cosmological model is a component of the stress-energy of the universe that looks like a cosmological constant. In the alternate models under discussion in this thread, there is no component of the stress-energy of the universe that looks like a cosmological constant. That means dark energy is indeed eliminated. The observations that in the standard model are taken to show the presence of dark energy are accounted for in the alternate models by modeling the dynamics differently from the standard Friedmann equation.

 

[PeroK]

The standard model assumes that the approximation is good enough. The alternate models being discussed here explore the possibility that it is not.

I haven't had a chance to read through any of the references in detail, but I don't think the possibility that the approximation is not good enough can just be dismissed out of hand.

I was hoping to understand why the OP found the arguments "very persuasive". I looked at the last paper referenced. It was beyond my knowldege, but it looked like it was questioning the statistical basis of the expansion data.

 

[PeroK]

What I think all of these papers are saying is that as the universe evolves, a larger and larger fraction of the universe volume is taken up by voids which are nearly devoid of matter. These voids expand faster (because of the lack of matter), and this is what is driving the accelerated expansion rate.

This sounds like a description of the standard model, with dark energy. The papers, as far as I can tell, are concerned with a different statistical analysis of the data - although it doesn't say explicitly that the data then fits a non-accelerating expansion. If the conclusion is that there is no dark energy, then I'd assume we are left with a non-accelerating expansion?

That may be my misunderstanding.

 

[PeterDonis]

This sounds like a description of the standard model, with dark energy.

Not to me. A standard model with dark energy has the same expansion rate everywhere in space, and the expansion is accelerating due to dark energy. The alternate model being described does not have the same expansion rate everywhere in space, and the expansion is not accelerating--there is no dark energy (no component of the stress-energy that looks like a cosmological constant).

Again, I haven't yet had time to look through the papers in detail to see how they arrive at the claim that their alternate model can account for observations as well as standard Lambda CDM. But it seems clear to me that the alternate model is significantly different from standard Lambda CDM.

 

[PeterDonis]

a different statistical analysis of the data

To me, having only briefly skimmed the papers, looking at different ways of doing statistical analysis of the supernova data seems misplaced. What I would be looking for is a formulation of the alternate model from first principles, then using it to derive predictions for what the supernova data should look like, and comparing those predictions with the data--and then putting that comparison alongside the standard comparison of Lambda CDM with the data, to look at their respective fits. The Wiltshire paper might be more along these lines than the Seifert et al paper.

 

[Ibix]

The Wiltshire paper might be more along these lines than the Seifert et al paper.

I haven't had as much of a chance to read as I was hoping today, but I think so, yes. You may need to read earlier work of his, referenced in the last paper in the OP.

 

[PeroK]

To me, having only briefly skimmed the papers, looking at different ways of doing statistical analysis of the supernova data seems misplaced. What I would be looking for is a formulation of the alternate model from first principles, then using it to derive predictions for what the supernova data should look like, and comparing those predictions with the data--and then putting that comparison alongside the standard comparison of Lambda CDM with the data, to look at their respective fits. The Wiltshire paper might be more along these lines than the Seifert et al paper.

Yes, as far as I can follow the papers, that's my understanding.

 

[PeroK]

It might be worth noting that originally the case for an accelerating expansion was a surprise. A lot of time and effort must have been spent trying to get the emerging data to fit the model of non-accelerating expansion. Eventually, those efforts must have been abandoned and the case for an accelerating expansion accepted. And, the theoretically simplest solution was to add dark energy (aka Lambda) to the model.

It's not the case that dark energy was in the model from the start and data was massaged to fit an a priori expectation of accelerated expansion. Quite the reverse, in fact.

 

[phyzguy]

Not to me. A standard model with dark energy has the same expansion rate everywhere in space, and the expansion is accelerating due to dark energy. The alternate model being described does not have the same expansion rate everywhere in space, and the expansion is not accelerating--there is no dark energy (no component of the stress-energy that looks like a cosmological constant).

Exactly, that is my understanding as well. I think what is being claimed is that the approximation of the standard model that the acceleration is the same everywhere in space is incorrect. Regions of lower density expand faster and regions of higher density expand slower. The overall averaged expansion rate is accelerating because the regions of lower density are constituting a larger and larger fraction of the volume of the universe as time goes on.

 

[javisot20]

I don't think this is a good description. Dark energy in the standard cosmological model is a component of the stress-energy of the universe that looks like a cosmological constant. In the alternate models under discussion in this thread, there is no component of the stress-energy of the universe that looks like a cosmological constant. That means dark energy is indeed eliminated. The observations that in the standard model are taken to show the presence of dark energy are accounted for in the alternate models by modeling the dynamics differently from the standard Friedmann equation.

What differentiates Wiltshire's proposal from a proposal in which dark energy is not the cosmological constant?, I mean, can there be a model with variable dark energy that is the same as the Wiltshire model?