# FeaturedA Concordance cosmology paper

1. Mar 30, 2017

### phyzguy

What do people think of this paper? If I understand correctly, they are saying that the need for Λ disappears if we do N-body simulations that properly take account of GR. I think they are saying that past simulations have used Newtonian approximations that don't properly account for density fluctuations. Can this be? If true, it seems like a major advance. Comments?

2. Mar 30, 2017

### Orodruin

Staff Emeritus
This does not seem to be the case to me. The difference between their simulations and the ΛCDM model is that their simulations treat a universe that is not homogeneous, i.e., it does away with the assumptions that go into the FLRW universe and therefore the ΛCDM model. This does not have to do with Newtonian approximations, but is an assumption that the universe can be treated as essentially homogenous on large scales.

3. Mar 30, 2017

### phyzguy

Not as I read it. I don't see that they are challenging the hypothesis that the universe is homogeneous on a large scale. They are saying that the local inhomogeneities, which are certainly present, have not been treated properly in the context of GR.

4. Mar 30, 2017

### Chalnoth

It sounds like what they're saying is that the formation of structure itself produces the appearance of an accelerated expansion, which is not caught in typical perturbative theory due to the nonlinear nature of the process.

I remember a lot of theorists looking into ideas like this back in the early-mid 2000's (Rocky Kolb was a major proponent at the time), and my understanding was that the idea had been largely debunked. I'm not sure if this paper throws a wrench into that previous conclusion.

5. Mar 31, 2017

### phyzguy

Thanks for pointing this out. Here's a paper by Kolb, et.al. that supports the basic idea, and another paper that disputes the "no-go" theorem, which I assume is what you mean by the debunking. It will be interesting to see where this leads.

6. Mar 31, 2017

### windy miller

Ive seen a few papers recently ( I think one even made into Nature) casting doubt in the existence of accelerated expansion. what I would like to know from anyone who thinks this is plausible is how to do you explain then the measured flatness of the cosmos? As I understood without lambda there isn't enough stuff in the universe to make it flat.

7. Mar 31, 2017

### Chalnoth

My recollection is that the "no-go" theorem was just the first round of argument against the idea, that further analysis showed that you can't really reproduce the observed expansion without dark energy with the growth of structure alone, but also have to have large-scale inhomogeneity as well. But to be fair, it's been a while.

8. Mar 31, 2017

### phyzguy

I wondered this too, but the paper says that the simulations they did are with $\Omega_m = 1$, so it is still flat.

9. Apr 7, 2017

### phyzguy

Here is another paper just posted on the arXiv today on this topic. Again, the authors claim that "backreaction", which I understand to be properly accounting for the effect of inhomogeneities, can produce the observed accelerated expansion without the need for the Λ term.

10. Apr 11, 2017

### phyzguy

Yet another paper on this topic posted on the arXiv today. Clearly the issue is far from settled. I found this one very interesting, as the authors studied the impact of inhomogeneities on an exact solution to the Einstein equations, and found that the effects can be significant. However, they don't claim to have answered whether the large-scale accelerated expansion can be explained by the non-linear effects of the growth of structure.

11. Apr 11, 2017

### Chalnoth

It can be hard to tell from just looking at a couple of papers. Communicating with a cosmologist actively working in the general area would probably give you a pretty clear picture of the overall status. Sadly, it's been too many years for me.

12. Apr 17, 2017 at 10:40 AM

### phyzguy

This has been up as a featured post for a while and nobody else has responded, so let me add something else. The following quote is from the paper I posted in Post #1,
"Thus the physical meaning of these calculations is simple: according to our approximation, it is not the average but the typical energy density that governs the expansion rate of the Universe. At high redshifts, where the distribution is fairly symmetric, the typical value of (mode of the PDF) is close to the average and the Universe evolves without backreaction. At late times skewness increases, the volume of the Universe is dominated by voids, and the typical value of is negative, thus effectively M < 1. High density regions, where metric perturbations are perhaps the largest, are inconsequential to this effect: what matters is the non-Gaussianity of the density distribution, in particular, the large volume fraction of low density regions, as advertised earlier."

This makes perfect sense to me. Early on, when the universe is of uniform density, the Friedmann equation works fine, but at late times the density becomes highly non-uniform. In today's universe, most of the volume is in regions of below average density. These regions expand faster because of their lower density, and so the apparent expansion of the universe accelerates. Can anyone offer a rebuttal and explain to me why this picture doesn't make sense?

13. Apr 17, 2017 at 11:08 AM

### Buzz Bloom

HI phyzguy:

I am able to visualize the universe in a state in which most of its volume has a density appreciably lower than the average density. However, I have difficulty visualizing this state evolving as a result of further expansion with respect to co-moving coordinates with the lower density areas expanding faster than the higher density areas. The best I can do is with the balloon analogy that marcus wrote about so well. The whole balloon expands uniformly as its radius gets bigger, but there are local areas of denser mass that are more-or-less stable. That is, these higher mass areas have sufficient mass that the volume containing this mass does not grow. That is, within these small separated volumes, smaller clumps of matter do not move away from each other with the expansion of the balloon. Also, the overall rate of balloon expansion depends on the average density, and not on he lower density of the larger volume areas. What would be observable, if there are observers, is the red-shift of photons from many distant high density volumes as observed from a location within one high density volume. This would be the Hubble red-shift.

Does this make sense to you?

The statement you quoted
"it is not the average but the typical energy density that governs the expansion rate of the Universe"​
seems to be distinctly different the behavior GR predicts. I gather this is intentional, and that is the source of the controversial nature of the article's conclusions.

Regards,
Buzz

14. Apr 17, 2017 at 1:08 PM

### phyzguy

Why is this difficult to visualize? Why must the balloon expand uniformly? Why can't the surface of the balloon be "lumpy", with some regions expanding faster than others?

The point is that you don't know this. Again quoting from the Racz, et.al. paper, they say,
"As fluctuations grow due to non-linear gravitational amplification, space-time itself becomes complex, and even the concept of averaging becomes non-trivial.
and,
"The algorithm (that they use) 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,"

The point is that it is not clear what GR predicts. The Friedmann equation assumes the density is spatially uniform, and today's universe is nowhere near uniform in density. Nobody has solved the Einstein Field Equations for the non-uniform density present in the universe today. The paper I linked in Post #10 is an attempt to do this in a small region, and they conclude that the impact of the density fluctuations cannot be ignored.

15. Apr 17, 2017 at 3:31 PM

### Buzz Bloom

Hi phzguy:
I am not sure I understand what you are saying here. As I see it, the issue is whether a calculation regarding a small area with density fluctuation demonstrates that this influence effects expansion as a whole. Does the paper say that this has been demonstrated? Or is it just a supposition that it might?

I am unable to do the math, but I have been thinking about a hypothetical universe which is mostly a vacuum, but there is included a distribution of black holes throughout this universe such that the average mass density equals the estimated total mass density of our current universe. Do you think the paper supports an argument that in this hypothetical universe the mass of all the black holes can be ignored when calculating the rate of expansion? If so, would the argument be valid no matter how large the mass density of the black holes happens to be?

Regards,
Buzz

16. Apr 17, 2017 at 3:53 PM

### Buzz Bloom

Hi phzguy:

Can you describe an expansion visualization using the balloon analogy applied to the vacuum universe with spherically shaped galaxy sized clumps of stable matter? Does the paper say whether the vacuum will expand faster or slower than the clumps? My understanding is the volume occupied by the clumps do not expand. I am not saying that the space S in which the volume occupied by a clump exists does not expand, but when that happens, the clump then still occupies the original volume, not the expanded volume of S.

What I cannot visualize is how the paper's authors might describe the shape of the balloon surface when most of it is expanding at a larger rate than the rate small isolated regions expand.

Regards,
Buzz

17. Apr 18, 2017 at 4:20 AM

### timmdeeg

Interesting question. Ignoring the masses of the black holes would mean that according to the Friedmann equations this universe should expand exponentially.This however is what cosmologists expect for the far future when the average matter density is ignorable.

So I can't think that this paper supports ignoring the black hole masses in your hypothetical universe. But yet I wonder whether the global expansion depends on the ratio of empty to non-empty volume.

18. Apr 18, 2017 at 10:58 AM

### Jorrie

I have read these papers, but frankly I do not understand enough of their model (back-reaction) to be able to see the mechanism that would a cause global appearance of accelerated expansion. I can understand that there may be a region (say on the far side of a void) from us that may have a kinematic recession rate that is larger than the Hubble flow, but how that makes the overall Hubble flow faster, I do not follow.

19. Apr 18, 2017 at 11:41 AM

### phyzguy

Well, my understanding is that what they are saying is that the low density regions expand faster because they contain less mass. Because most of the volume is lower than average density, this causes the global expansion to be faster than if the density were uniform. I guess another way of saying it is that most of the volume in the universe is "on the far side of a void" (or at least a lower density region) from us.

20. Apr 18, 2017 at 12:28 PM

### Chalnoth

I really don't see how this can work. It's certainly very much contrary to the way gravity works in most every other situation. With a spherically-symmetric mass distribution, for example, the gravitational attraction at some specific distance depends only upon the mass contained within a sphere of the radius of that distance. Whether that mass is distributed evenly or concentrated in the center, or in a shell, makes precisely zero difference as to the behavior of the test particle.

Similarly, I would tend to expect that once you look at the universe at large enough scales that it appears approximately uniform (roughly a few hundred million light years), the behavior would be completely independent of the specific distribution, and only dependent upon the mass density.

To put it another way, if the voids were to expand more quickly, why would they push the matter regions away from one another? Why wouldn't they just move through them?