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Old Jan 11th 2013, 10:18 PM
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Default The largest thing the universe

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The discovery is an apparent cluster of quasars some 4 billion light-years across. If it holds up to further scrutiny, it could challenge a long-held assumption that at the cosmos's largest scales, the physical processes at work and the distribution of matter and energy are the same, regardless of an observer's location.

It lies some 8 billion to 9 billion light-years away and contains 73 quasars. These quasars represent the tip of the iceberg, with many more galaxies present in the group that are too faint for researchers to see. The team estimates the Huge-LQG's mass at more than 3,000 trillion times the mass of the sun or about 1,300 times the mass of the Coma Cluster, which hosts more than 1,000 galaxies.

...

Interest in this latest large quasar group stems from assumptions the cosmological principle that Albert Einstein made as he applied his equations of general relativity to the cosmos. The assumption greatly simplified his calculations, which pointed to a size threshold for individual structures beyond which the distribution of matter would appear uniform.

Over the years, as astronomers have detected ever-larger groups of galaxies, and later interwoven filaments of galaxies, the threshold has increased, Clowes explains. It started out at about 98 million light-years and now hovers at the 1.2 billion-light-year mark.

http://www.csmonitor.com/Science/201...upend-theories
That's really, really big.
I don't fully understand the theoretical implications of it all though.
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Old Jan 11th 2013, 11:34 PM
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Default Re: The largest thing the universe

So that's where Demond Wilson's career went.

Isn't the Milky Way only 150,000 light years in diameter? Gives you a good scale as to the size of this beast.
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Old Jan 12th 2013, 11:07 PM
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Default Re: The largest thing the universe

Interesting. To say the least.

For those thus inclined (reading just the abstracts and summaries is highly recommended for mathematicaphobes )

A paper on the previously largest LQG's:
http://arxiv.org/pdf/1108.6221.pdf

The actual paper on the new extra large LQG:
http://mnras.oxfordjournals.org/cont...97.full#ref-50

And the paper that suggested the upper limit for the size of LQG's which is now apparently breached:
http://mnras.oxfordjournals.org/content/405/3/2009.full

I think it's a bit premature to announce the passing of the cosmological principle though. One only has to read the above articles to get an idea of how extremely complex these quasar analyses are and how many other parameters and/or assumptions might be off.
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Old Jan 13th 2013, 09:42 AM
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Default Re: The largest thing the universe

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Originally Posted by dilettante View Post
I don't fully understand the theoretical implications of it all though.
On the theoretical level it wouldn't be such a problem if the cosmological principle were to 'fall'. It's not an axiom of any of the major theories (relativity, all flavours of quantum, etc.).
But it would be a huge problem at another level and I think I can explain it very well with an analogy. Everyone is probably familiar with the kind of question one gets in school about finding out where or when two cars will meet when one goes from point A to point B at an average speed X and the other one from point B to point A with an average speed Y. Only a very bad teacher would mention it at the level of education these questions are asked but the answer to that question which the teacher will mark as correct does actually assume something that's very similar to the cosmological principle. Notably it assumes homogeneity of the trajectory A-B.

That is to say the answer is only correct if one assumes perfectly straight and perfectly level roads with no obstacles and perfect performance of the cars. That's not a real world situation of course. If this were a real world problem many more parameters would have to be calculated to come up with a correct answer. For the sake of simplicity I will introduce just one, viz. the distribution of stop signs along the road A-B. Without changing the average speed of the cars, the actual real world distribution of stop signs would influence the place where the cars would meet. If there were for instance more stop signs or a cluster thereof (representing e.g. a village) on the half of the road closer to A than on the half of the road closer to B then the point of meeting would shift closer to city A.

And that is almost exactly like the role the cosmological principle plays in cosmology. If one has good grounds to assume homogeneity, i.e. uniformity, throughout the universe then one can ignore the possibility of clusters of 'stop signs' or at least assume a random distribution which is easy to calculate. But if one were to find out that this homogeneity isn't the actual case then one would have to incorporate an estimate of the non-random, non-linear distribution of 'stop signs'. Applied to cosmology the analogy of these 'stop signs' in fact represents a whole plethora of things. Each one of them would have to be quantified and put into an equation and these equations would substitute the hitherto simple and single parameter that represented homogeneity in the overall equations. Needless to say that would make the calculations far more complex.

Every calculation that has ever been made under the assumption of the cosmological principle would have to be revisited and recalculated with the far more complex equations. The direct result would of course be a huge effort in time and resources (not to mention money). And at least temporarily many results hitherto considered accomplished would be on shaky grounds until the results of the new calculations are available.

Then there are two possibilities. Either the new results would still fit in the grand scheme of things without fundamental changes or they would not. In the latter case that would be a major upheaval that could potentially -likely might be a better word- shake basic theoretical grounds.

But this is still very much a hypothetical scheme. The cosmological principle isn't assumed for no reason or out of laziness in order to have simpler equations or on philosophical grounds but for the simple reason that until now there has never been much of an indication that the universe isn't homogeneous and there were a very great deal of reasons to assume it is. To name just one, the intensity of the cosmic background radiation is the same to 1 in 100 000 in all directions of the sky.

On the bright side of things, if the universe really isn't homogeneous then the precise nature of the inhomogeneity is bound to tell us a lot more about the nature and origin of the universe. It may lead to an explanation of dark matter for instance.
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