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Do the largest structures in the Universe actually exist?

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November 20, 2013

The Universe is big, and so is the challenge of understanding its large-scale structure (P...

The Universe is big, and so is the challenge of understanding its large-scale structure (Photo: Hubble Ultra Deep Field / NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team)

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Our knowledge of the large-scale structure of the Universe is gradually taking shape. However, our improved vision is mostly being statistically squeezed from huge data sets. Working backward from a statistical analysis to a putative fact about the (singular) Universe, to which statistics do not apply on a cosmological scale, is a dicey business. A case in point is a recent look at the biggest known structures in the Universe – large quasar groups.

A quasar is a very energetic active region at the center of a galaxy , typically about the size of the Solar System, and with an energy output roughly 1000 times the energy produced by the entire Milky Way.

The X-ray image of the quasar PKS 1127-145, a highly luminous source of X-rays and visible...

At present, the largest structures in the Universe appear to be collections of quasars that form large quasar groups (LQGs). LQGs are measured in hundreds of megaparsecs (a parsec is about 3.26 light years), and have been proposed as the precursors for the sheets, walls, and filaments of galaxies and dark matter that fill the present-day Universe.

Early this year, Professor R.G. Clowes and his research group at the University of Central Lancashire in the UK reported that their analysis of quasar locations and distances from the Sloan Digital Sky Survey had identified a candidate for a very large quasar group (now referred to as the Huge-LQG) at a distance that required 9 billion years for its light to reach Earth.

The Huge-LQG contains 73 quasars spread over a region measuring some 1.24 x 0.64 x 0.37 Gigaparsecs, which would make it the largest structure known in the Universe. Their analysis suggested that the quasars in the cluster appear closer together than expected for random positions.

Let's take a quick look at how clusters of quasars were identified. A sphere of a given size (100 Mpc diameter spheres were used) is drawn about the location of a quasar. If any other quasars appear in that sphere, they are considered to be part of a cluster with the first quasar, and similar spheres are drawn about them. Eventually, when no more quasars are caught within the latest generation of spheres, the extent of the cluster is defined by the volumes of the spheres. This is sometimes called a friends-of-friends algorithm.

Map of the Huge-LQG showing only the group members expanded to spheres 66 Mpc in diameter ...

This procedure is terribly sensitive to the sphere size chosen to examine a region of space for the presence of a cluster. The map of the Huge-LQG above (also including a second LQG at the upper left) is described in spheres that are two-thirds the size of the spheres used to define the cluster; a difference large enough to split the single cluster into a number of smaller groupings. On the other end, if the sphere size chosen for the study were increased by only 20 percent, the algorithm will identify a LQG spanning the entire data set. It is difficult to know which set of clusters are real, if any.

The Huge-LQG, if validly identified, was claimed to represent a violation of the Cosmological Principle (CP). The CP roughly says that the Universe appears about the same from any position within itself, that there are no special locations. At first glance, the existence of such a large structure appears to violate that concept.

If so, nothing much would actually change in theoretical cosmology, as the CP only provides a starting point – the structures of more complex cosmologies are treated as perturbations around the homogeneous and isotropic universe described by the CP. In particular, this has no bearing on the occurrence (or not) of a Big Bang at the beginning of our Universe, despite the delighted claims of many online skeptics and "creation theory" sites.

A key issue, however, presents itself. Is the perceived clustering of the Huge-LQG of sufficiently low probability that it reflects physics, rather than randomness? What does that even mean when we have only the one Universe, making odds of multiple outcomes meaningless?

The same region of space shown in Clowes' map of the Huge-LQG, with all the quasars appear...

A post-doctoral researcher named Seshadri Nadathur, then at the University of Bielefeld, decided to take a closer look at the Huge-LQG. His map above duplicates Clowes' map of Huge-LQG, but includes all the quasars in the immediate region, and shows no obvious sign of what is very clear clustering in Clowes' map.

After performing a number of statistical studies on the quasar data, and finding extreme changes in the Huge-LQG membership and shape with small changes in the cluster finding parameters, he decided to determine the probability that apparent clusters the size of the Huge-LQG would appear in a random assortment of quasars. He set up 10,000 regions identical in size to that studied by Prof. Clowes, and filled them with randomly distributed quasars with the same position statistics as did the actual quasars in the sky.

He found that 850 (8.5 percent) of the 10,000 simulated randomly populated regions had clusters larger than the Huge-LQG. His conclusion was that, in the absence of better data, the observation of the Huge-LQG is best explained as the action of a computer algorithm biased to find clusters looking at a spatially random scattering of quasars.

What, if anything, does this mean? Nothing, in a way, as the argument on both sides goes backward from a statistical analysis of possible universes to what an observation means in this solitary Universe we call home. This brings up the concept of naturalness in science.

Most people feel more comfortable with an explanation whose predictions don't change drastically when the parameters are changed. For example, in Newton's theory of gravity, physical behavior doesn't change qualitatively when the gravitational constant is changed a bit; orbits just swing a little closer in or further out. Perhaps the most famous reaction to arguments based on naturalness is Niels Bohr's irritated comment asking Einstein to stop telling God what to do.

On the other hand, when an explanation requires not only a physical mechanism, but also a statistically unlikely set of parameters, to believe in the explanation feels more difficult. Such explanations are often called "fine-tuned," usually with derision.

Whether one embraces or eschews naturalness in physical theory, the Universe doesn't care one way or the other – it embodies the natural. Naturalness is a statistical, philosophical, and psychological notion that provides no authority for understanding a single entity like our Universe. Despite this, many scientists do use naturalness as a guide to help make choices (usually when guidance provided by actual data fades): It may still lead to a path out of a foggy maze.

Good scientists are skeptics, especially of what they themselves believe. An important discovery or new concept is quickly studied and criticized by the herd, with the goal of refuting or refining. However, if the Universe says your description of its operations is correct, it doesn't matter if a committee of geniuses thinks you are wrong. The opposite is also true. What makes science work is that the final authority is the Universe, which cannot be swayed, influenced, or confused. That outbids intellectual consensus, sloth, error, deafness, and ignorance any day of the week.

Sources: University of Central Lancashire[PDF] and University of Bielefeld[PDF]

About the Author
Brian Dodson From an early age Brian wanted to become a scientist. He did, earning a Ph.D. in physics and embarking on an R&D career which has recently broken the 40th anniversary. What he didn't expect was that along the way he would become a patent agent, a rocket scientist, a gourmet cook, a biotech entrepreneur, an opera tenor and a science writer.   All articles by Brian Dodson
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12 Comments

Thanks Brian for another interesting read. I do struggle to comprehend some of the concepts you postulate. Could you please elaborate on "What makes science work is that the final authority is the Universe..".

To suggest the Universe is an authority on anything seems somewhat absurd to me. It makes no claims on anything, only rational beings that observe it in action can make any inferences. The very point it is called "universe" is only meaningful to us because we assign a vast array to that term.

Again, respectfully, can you please elaborate. I suspect I might have your thoughts confused in my interpretation.

In relation to your comment; "Such explanations are often called "fine-tuned," usually with derision."

The Australian Broadcasting Commission has a science show called Catalyst that investigated the fine tuning of the Universe (video may geoblocked but there is a transcript).

http://www.abc.net.au/catalyst/stories/3836881.htm

The program interviewed several preeminent Physicists. Not one treated the statistical odds of the Universe existing as it does with anything that could be regarded as derision.

Australian
21st November, 2013 @ 01:18 am PST

Another fine, accessible and well written piece. Thanks.

HighPockets
21st November, 2013 @ 09:29 am PST

The last paragraph is brilliantly written and exemplifies what differentiates science from mere philosophy.

quax
21st November, 2013 @ 09:34 am PST

It is hard to imagine structures larger than galaxy filaments, or supercluster complexes...

Jay Lloyd
21st November, 2013 @ 10:00 am PST

Philosophy precedes science. Without a sound understanding of metaphysics and epistemology science will be difficult if not impossible. But that is not how humankind developed philosophy. First, the natural world was studied, then later in life, a comprehensive world view was attempted. The more successful at the first, were better at the latter. Why? Because they were identifying (making explicit) the implicit assumptions that served them so well. When I read, "Naturalness ... provides no authority for understanding ...", followed by "... if the universe says..." I recognize philosophical ignorance in epistemology. For example, when Einstein made his comments about God, he was speaking poetically to convey a metaphysical/epistemological concept, not making a reference to the concept of God. (He was an atheist.) He was saying that randomness is a psychological concept not a metaphysical one, i.e., when we say an event is random we are saying nothing about the event. We are confessing our ignorance, our inability to find a connection. Some take their ignorance as proof that some things are not knowable. This is a profoundly anti-mind theory. It explains nothing except a lack of self confidence.

Einstein was stating an axiom he used to understand the world, that nothing is random metaphysically speaking. Random is an epistemological concept. Another metaphysical concept he exposed metaphorically was that the universe is not malevolent, just hard to understand sometimes.

If we view this astrophysicist's theory of LQG in this light we can see an attempt to find a pattern which failed. His theory falls apart when the size of his sphere is increased 20% or decreased, i.e., it is a dead end.

Don Duncan
21st November, 2013 @ 10:49 am PST

@ Don duncan

"Some take their ignorance as proof that some things are not knowable. This is a profoundly anti-mind theory. It explains nothing except a lack of self confidence."

This is just an opinion. There is a proof in information theory that most problems are in fact unsolvable. That is if you are generous and don't accept Gödel's incompleteness theorom which proves that there is no set of axioms that is complete and consistent as a basis for solving all mathematics.

kwarks
21st November, 2013 @ 05:58 pm PST

I thought the Sloan Great Wall was the largest known object in the universe at 1.3 billion light years across. Of course I might be wrong.

Tom Haydon
21st November, 2013 @ 06:00 pm PST

A cosmic structure had been detected by pln plotting gamma rays, and with almost 100% certainty to be real, it is 12 billion light years in diameter, and about 12 billion light years away. Horvath says it could be a galaxy clusters, and states there is no idea how it evolved to be. If it's galaxy clusters organized into an enormous structure, I would like to say that Einstein's relativity isn't correct for a homogenous isotropic universe. It would take millions of galaxy clusters probably 100 billion years to form this new structure. There are fractal patterns in plasma and dust, that could extend on and on. Size is irrevalent to nature, and no smallest ultimate particle nor structure exists for comprehension of human intelligence, is my opinion.

Pat Henson
21st November, 2013 @ 06:03 pm PST

9B years ago, there was a grouping identified here. If the Universe is indeed expanding, that group interval is doubtless gone in our "real now", at that area. Are the closest intervals farther away? If a big bang expanding, this might indicate a direction for an origin.

rdlongview
21st November, 2013 @ 09:18 pm PST

Hello, all!

Some good questions have been raised on a difficult subject. Let's start with Australian's question about the Universe being the authority. I meant this as a metaphor, tho touchstone might have been a better word than authority. What I was trying to say there was that if your description of how the Universe works disagrees with what the Universe actually does, so much the worse for your theory. One properly calculated but incorrect prediction, and you know you've missed something. That's when science gets fun.

On the fine-tuning issue, there are two ways of looking at it. One is that, sure enough, the parameters describing the Universe are those that allow us to exist. How else could it be if we are here to ask questions? This is an example of the often misunderstood Anthropic Principle. This is the context for most of the comments in the reference you gave. But while many physicists are comfortable with that view, many are not.

The other viewpoint is that the fine-tuning has to mean something (often a designer, whether or not explicitly mentioned), and this is the direction of which I was making a bit of fun. This is a version of the inverted attempts to apply statistics to a unique object, the known Universe. (There may be more, but guesses can't replace data.)

For example, a common objection to string theory is that there are a huge number (10^500 is often quoted) of different string theories. Some people claim that those odds make it silly to believe that string theory describes our Universe, and in doing so they are applying inverted statistics. It is essentially the naturalness argument.

However, IF string theory is a good description for our Universe, then, as there is only one known Universe, it must be described by only one of the 10^500 possible parameter sets. Why? There is only one known Universe. Whether or not we can find which set a different issue. But again, fine-tuning is not a valid argument against something of which there is only one example.

Pat, the paper on the gamma-ray "structure" has just appeared, so there hasn't been a chance for anyone to vet it yet. I will say that Horvath is a pretty well known crank who feels cheated out of Nobel Prizes, but that doesn't necessarily mean he is wrong here - too soon to tell.

Best to all,

Brian

bdodson
22nd November, 2013 @ 12:20 am PST

What do you think about this?

http://news.discovery.com/space/galaxies/universes-largest-structure-is-a-cosmic-conundrum-131119.htm

Sim
23rd November, 2013 @ 05:18 am PST

The content of the post was really interesting. The editorial content was misleading. "What makes science work is that the final authority is the Universe." Ironically, the subtext of the story was how the way you ask the question influences the answer. Since all questions are human, all answers are too. The author seems to be saying some questions are less human than others. I think he's wrong.

Pi31416
28th November, 2013 @ 03:05 pm PST
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