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Significance of the Huge-LQG for homogeneous universe

by LastOneStanding
Tags: homogeneous, hugelqg, significance, universe
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LastOneStanding
#1
Feb6-13, 12:25 PM
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I've poked back through the past few weeks of threads, and I've only seen two posts (here and here) with very little discussion commenting about the recent Huge-LQG discovery (http://mnras.oxfordjournals.org/cont...as.sts497.full). I was hoping some cosmologists could comment on its potential significance.

In the first post, Chronos writes:
Quote Quote by Chronos View Post
In a universe of this size there are bound to be anomalies. How large is large enough to be improbable is a statistical exercise.
Is this overly dismissive? That "statistical exercise" has been done by Yadav et al., and it is precisely on the basis of that calculation that Clowes has suggested the discovery is inconsistent with the cosmological principle. Granted, Yadav's paper doesn't itself seem to have gotten much attention (Google Scholar gives nine citations) so is Clowes wrong to suggest that the Huge-LQG is a problem for homogeneous models of the universe? If it is a problem for homogeneity, how much of concordance cosmology would need to be rethought (i.e. can Lambda-CDM and various results about inflation, etc., be easily retooled for an inhomogeneous metric)? Or can this be accounted for within the FLRW model, contrary to my reading of Yadav et al.?

I'd be very grateful if some cosmologists could weigh in on these questions.
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Mordred
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Feb6-13, 12:41 PM
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I can't answer for sure but homogeneous is usually described at 100 Mpc..lower sizes are usually inhomogeneous as well as around 200 Mpc and above. Keep in mind I'm only going off what I recently studied so could be wrong on those numbers.
marcus
#3
Feb6-13, 01:30 PM
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I'm glad to see the publishing journal MNRAS provides free access. Another online source for the same article is:
http://arxiv.org/abs/1211.6256

The INSPIRE record of the article for future reference to see who eventually cites it is
http://inspirehep.net/record/1204404?ln=en

I just glanced briefly at it---they use cautious language and speak of a "potential" challenge. They claim only that the large concentration they found is on the "borderline" of what is compatible with usual idea of homogeneity.

They refer to 2012 work by Park, Choi, Gott and others simulating the random gathering together of structure in a basically homogeneous universe, which apparently showed that quite large clumps could occur. They mention past scandals that blew over. Clumps that seemed too big but were later accepted as not violating the overall uniformity idea.

It is certainly interesting, and the whole issue of the cosmological principle (or working assumption ) bears watching!

I'll get Park Choi Gott et al and see what they say. http://arxiv.org/abs/1209.5659

==quote Park Choi Gott paper==
The Sloan Great Wall (SGW) recently found in the Sloan Digital Sky Survey (SDSS) region casts doubt on the concordance cosmological model with a cosmological constant (i.e. the flat LCDM model). Here we show that the existence of the SGW is perfectly consistent with the LCDM model, a result that only our very large cosmological N-body simulation (the Horizon Run 2, HR2) could supply. In addition, we report on the discovery of a void complex in the SDSS much larger than the SGW, and show that such size of the largest void is also predicted in the LCDM paradigm. Our results demonstrate that an initially homogeneous isotropic universe with primordial Gaussian random phase density fluctuations growing in accordance with the General Relativity, can explain the richness and size of the observed large-scale structures in the SDSS. Using the...
==endquote==

If we take this paper seriously the message seems to be that you have to do very large N-body computer simulations of how clumping can emerge from initial near-uniformity.
They would suggest that we haven't gone far enough with the simulations yet to know how much clumping on what scales to actually expect.

So maybe that cluster of 73 quasars in a region only 500 Megaparsecs wide is to be expected just cosmic business-as-usual, or maybe not. Just have to stay tuned.

LastOneStanding
#4
Feb6-13, 01:45 PM
P: 718
Significance of the Huge-LQG for homogeneous universe

Awesome, marcus! That's exactly what I was hoping for. I look forward to hear what Park et al. have to say about it. Thanks for confirming the source has free access—I'm on a university network, so sometimes it's hard to tell.

Supposing it does turn out that this and similar finds make homogeneity an untenable assumption, can you speculate on the likely impact to cosmology? Would it be a "adjust the models, back to business as usual" scenario, or is something like this full-blown paradigm shift territory?
marcus
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Feb6-13, 02:06 PM
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Quote Quote by LastOneStanding View Post
Awesome, marcus! That's exactly what I was hoping for. I look forward to hear what Park et al. have to say about it. Thanks for confirming the source has free access—I'm on a university network, so sometimes it's hard to tell.

Supposing it does turn out that this and similar finds make homogeneity an untenable assumption, can you speculate on the likely impact to cosmology? Would it be a "adjust the models, back to business as usual" scenario, or is something like this full-blown paradigm shift territory?
I can't speak to that. I'm just a cosmology watcher (retired mathematician who loves the stuff, not an expert). You may have a better feel for it. I'll just wait and see.

Right now, I'm excited by the recently revised WMAP9 "cosmological parameters" paper which seems to show:
(1) a leaning towards positive mean curvature, suggesting spatial finiteness
(2) a leaning towards Neff ≈ 4, the effective number of neutrino species in the radiation dominated era---or anyway the number of weakly interacting mass-ful species.
The two papers at the top of my list of ones to think about are:
http://arxiv.org/abs/1302.0254
The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 1 Feb 2013)

http://arxiv.org/abs/1212.5226
Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Parameter Results
G. Hinshaw, D. Larson, E. Komatsu, D. N. Spergel, C. L. Bennett, J. Dunkley, M. R. Nolta, M. Halpern, R. S. Hill, N. Odegard, L. Page, K. M. Smith, J. L. Weiland, B. Gold, N. Jarosik, A. Kogut, M. Limon, S. S. Meyer, G. S. Tucker, E. Wollack, E. L. Wright
(Submitted on 20 Dec 2012 (v1), last revised 30 Jan 2013 (this version, v2))
Mordred
#6
Feb6-13, 02:25 PM
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Thanks Marcus for those additional papers you posted. As I just recently started studying
ΛCDM I can't really comment on how it will affect related equations. There were a couple of things I found intriquing in some of the papers you posted. One of them being dark matter density being higher or lower depending on what direction you look. Their has been a fair amount of papers showing alternate ΛCDM model variations.

http://www.physicsforums.com/showthread.php?t=669833

http://www.physicsforums.com/showthread.php?t=669330

Nice thing on the ΛCDM is that its continually being examined and I would imagine improvements are on the way.
Chronos
#7
Feb6-13, 04:10 PM
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Another problem you run into with large scale structures is what constitutes a structure? Is it the number of galaxies in a certain volume, the matter/energy density contained within that volume, or something else? It surely must be something you can objectively quantify. Another issue is the evolutionary history of the bodies involved - are they related? Is it a grouping of bodies with a common history, or a chance alignment between two unrelated overdense regions that happen to be wandering across our line of sight? I think these are among the reasons Clowe is guarded in his conclusions.


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