Largest structure in the universe discovered so far

[PhysicsDad]

Just saw this http://www.space.com/19220-universe-largest-structure-discovered.htmlarticle about a quasar group that's 4 Gly long. Not to speculate too much, but any thoughts on how much of an impact this could have on our current models and understanding of the formation of the universe? The article says it "challenges modern theory" but I am curious, to what extent could it change how we look at the universe?

Moderators: if this becomes too speculative, I apologize, and please shut it down if that is the case.

Thank you all ahead of time.

 

[Mordred]

This has come up before a while back on this thread here
https://www.physicsforums.com/showthread.php?t=669837.

 

[PhysicsDad]

Ahh, I must have missed that... Thank you

 

[Mordred]

no problem. Some good info on the other thread should answer your earrlier question.
The size is at the near limit of homogeneous, which is usually described as above 100 Mpc.
Below 100 Mpc we already know the universe in inhomogeneous. So I don't believe this will have any significant affect on current Homogeneous/Inhomogeneous related issues.

 

[PhysicsDad]

This is a quote from another article on the same site, describing the size of this formation.

http://www.space.com/19227-biggest-structure-universe-explained-infographic.html

Quote:
The currently accepted Cosmological Principle, based on the work of Albert Einstein, suggests that the largest structures we should be able to find would be about 370 megaparsecs across (more than 1.2 billion light-years). The newly found quasar group is 1,200 megaparsecs across, a distance that would take four billion years to cross at the speed of light.
End quote

Unless I am misunderstanding what you mean, this seems to be A LOT bigger than 100 Mpc

Am I just not understanding something correctly?

 

[Mordred]

No you didn't miss anything I did, lol my turn. I found a full paper linked to one of the links in the other thread I posted. They are being cautious understandably so.

If you look in the conclusions on this paper it best describes some of the concerns. Also as Pointed out we also have to be cautious as to what constitutes a structure.
http://mnras.oxfordjournals.org/content/early/2013/01/07/mnras.sts497.full.pdf+html

Chronos mentioned that last bit.

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.

Guess we will have to wait and see.

 

[Garth]

It may be that in the standard [itex]\Lambda[/itex]CDM model there simply has not been enough time for such a large structure to form from an initial homogeneous (+ CMB anisotropies) state.

It would then be another example of old structures in a 'young' universe such as discussed in my old (and now locked thread) Is There An Age Problem In The Early LCDM Model?
.

Garth

 

[Chalnoth]

Quote:
The currently accepted Cosmological Principle, based on the work of Albert Einstein, suggests that the largest structures we should be able to find would be about 370 megaparsecs across (more than 1.2 billion light-years). The newly found quasar group is 1,200 megaparsecs across, a distance that would take four billion years to cross at the speed of light.
End quote

Unless I am misunderstanding what you mean, this seems to be A LOT bigger than 100 Mpc

Am I just not understanding something correctly?

The original paragraph isn't really accurate. There are expected to be structures at all scales. In fact, the highest-amplitude perturbations produced by inflation are at around 250Mpc.

The main point here is that inflation set up initial density perturbations at every length scale. At smaller scales, those density perturbations become mixed-up by the effects of local gravity, but at large scales those initial perturbations hang around. Go much about 250Mpc, and structures should become increasingly rare, but should never entirely disappear.

Edit: Note that you don't expect to have gravitationally-collapsed structures, like galaxy clusters or superclusters, that are larger than about 80Mpc, because they won't have had time to collapse. But there's no problem with having a coherent string of such collapsed objects on much larger scales.