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I have heard a bit about the Great Attractor (the gravitational anomaly that seems to be "sweeping" our universe in one direction). Someone (and forgive me, I do not recall the specifics) has hypothesized that this may be an indication of an other universe affecting us gravitationally.

I am interested in people's opinions on this. Would this be an experimentally provable phenomenon of string-theory (in all its different incarnations)? Or are there more viable hypotheses that would explain this?

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You might be interested in watching "Is Everything We Know About The Universe Wrong?" and google around for "dark flow". –  Alain Pannetier Jul 7 '11 at 7:52
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up vote 10 down vote accepted

Large-scale observations require a ton of processing and interpreting. So much so, in fact, that there was some debate as to whether the so-called "Great Attractor" even actually existed.

Interacting, independent universes are a hallmark of string theory. String theory has another characteristic for which it is notorious, though, and that's a lack of strong experimentally verifiable hypotheses. String theory is absolutely fantastic at ex post facto explanations of experimental results, but it spectacularly crashes and burns when it comes to predictions of results as-yet unknown.

To illustrate the problem, the so-called "Standard Model" of physics has 14 fundamental constants, with no apparent reason for each of them having the particular value that we measure. There are certain conjectures, but no standard explanation, so you might consider that number 14 to be the amount of information "missing" from the Standard Model. That is, we have to add those 14 ingredients to the equations of the Standard Model in order to describe everything else about the Universe in actual hard numbers instead of placeholders.

I read an article recently that described one point of view on String Theory in which that theory is missing not 14 but 10^(some really big number) of ingredients(!) in order to nail down all of the physics it describes into hard numbers. With that many degrees of freedom, of course you can massage the numbers hard enough to make any result you like pop out, but it doesn't mean you have created anything useful or even true. So, yes, some string theorist may have shown that string theory massaged a certain way can produce interacting universes that look like The Great Attractor, but it's very hard to commit to that flimsy linkage between theory and experiment.

Even just the experimental outlook by itself is not much rosier. Due to the very nature of strings and the excellent agreement with experiment that our existing theories already have, no experiment looking for evidence of the "stringularity" of the Universe is going to be a smoking gun. Rather, any such an experiment will be subtle, require mind-blowing precision, be subject to a lot of confounding factors, and will be susceptible to widely varying interpretations.

To answer your last question, just for an example, another hypothesis that requires muuuuuuch less theoretical "architecture" than string theory and is actually pretty difficult to rule out experimentally: chance. Our observable Universe just happens to contain a really big clump. Tada! Tada?

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Astronomer joke: If you see four astronomers arguing about the Malmquist bias, at least three won't know what they're talking about. –  Andrew Jul 3 '11 at 21:49
    
+1 for 'massaging the numbers to make any result you like pop out' –  Christopher Jul 9 '11 at 5:34
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Yes I believe that the Great Attractor, and all dark matter sources for that matter, are indications of gravity permeating from other universes. However I struggle to come up with a way that this hypothesis can make a prediction about the state of our universe since we can't directly observe other universes.

So how could the theory untimely be proven or falsified? We could assume that gravity from our universe is likewise having an effect on matter in other universes so could a large gravitational event, such as the collision of two black holes, leave its imprint on the 'map' of dark matter as it effects matter in other universes? And since we can only detect dark matter by observing visible matter how will we be able to make the differentiation? Perhaps in time as observation techniques and theories are refined will we be able to identify these grand cosmic experiments.

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The Great Attractor isn't really evidence for anything exotic - in fact, it is not really singular or notable in any way.

One aspect of cosmology is studying the evolution of large-scale structure in the universe. The (very abbreviated) standard story is that the universe had some overdense and underdense regions even back when it was so hot as to be an opaque plasma through and through. Eventually it cooled, light decoupled from matter, and the overdense regions began to collapse under their own gravity.

The details of this collapsing process are still debated, since they involve simulating all sorts of fascinating processes, from AGN feedback to ISM turbulence to non-equilibrium inflows of matter. Observationally, though, we know the end result - a rather "spongy" distribution of visible matter (stars), by and large believed to trace the distribution of dark matter. There is structure across a very large range of scales: Individual galaxies clearly have substructure (like spiral arms), galaxies tend to group into clusters, and these clusters in turn are grouped into superclusters. If you mapped out all the galaxies we've surveyed, you would find a filamentary distribution of them, and superclusters are simply the knots where filaments intersect.

The Great Attractor is just another supercluster. It is a region with higher-than-average density, just as there are lower-density regions too - the universe is not perfectly homogeneous, which is what makes astrophysics interesting. The people who survey this large-scale structure tend to come up with fanciful names for the patterns they see in the randomness, hence the existence of voids and supervoids, walls, and more walls. This object really says nothing one way or another about string theory.

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