Giant Sheets Of Dark Matter Detected 231
Wandering Wombat writes "The largest structures in the universe have been, if not directly found, then at least detected and pounced upon by scientists. 'The most colossal structures in the universe have been detected by astronomers who tuned into how the structures subtly bend galactic light. The newfound filaments and sheets of dark matter form gigantic features stretching across more than 270 million light-years of space — three times larger than any other known structure and 2,000 times the size of our own galaxy. Because the dark matter, by definition, is invisible to telescopes, the only way to detect it on such grand scales is by surveying huge numbers of distant galaxies and working out how their images, as seen from telescopes, are being weakly tweaked and distorted by any dark matter structures in intervening space.' By figuring how to spot the gigantic masses of dark matter, hopefully we can get a better understanding of it and find smaller and smaller structures."
Re:Sheets and Filaments (Score:5, Informative)
Second, my understanding is that dark matter (whatever it is) must be fairly weakly-interacting. The normal matter that we see aggregating into stars and galaxies interacts with itself (the particles bounce off each other, exchanging momentum, and also they repel each other at very short distances). This interaction, in addition to gravity, dictates the shapes we see for ordinary matter.
Dark matter doesn't interact strongly (with matter, and presumably with itself), so it aggregates differently. Imagine a cluster of dark matter that is being gravitationally collapsed: as the particles get closer to each other, instead of bouncing off each other (and thereby e.g. transforming their large-scale kinetic energy into heat), they 'pass through' each other (actually just pass by each other without scattering). This means that the matter will aggregate differently (the dark matter particles will mutually gravitate and orbit, but can't coalesce).
I'm painting a simplistic picture, but the point is that there are some fundamental differences about how dark matter interacts, versus ordinary matter. I believe the filamentary structure itself is an artifact of the universe's inflationary epoch, where massive expansion has amplified small-scale quantum fluctuations into the very large-scale distribution we now see.
Re:The Rubber sheet analogy is WRONG!!! (Score:5, Informative)
If dark energy sounds counter-intuitive: it should! Of course we don't really know what it is (yet), but the experimental evidence available thus far does not suggest that matter is repulsive at large distances, but rather that "something" fills spacetime and exerts an expansion force that is inversely proportional to its density.
Journal Reference (Score:4, Informative)
Re:How can they tell this is caused by Dark Matter (Score:2, Informative)
Also, many of the measurements come from Hubble images, for which there is no atmospheric turbulence to deal with (atmospheric effects also average-out over a fairly short period of time, and though they decrease resolution they are easy to differentiate from astronomical sources of distortion).
The error bars are small enough that we know the light from distant sources is being deflected. The simplest explanation is that there is a cluster of mass between us and the source, whose gravity is deflecting the light.
Re:What if.... (Score:5, Informative)
But that's exactly how it's being treated by physicists. Here [wikipedia.org] are the very equations that physicists use to described the bending of spacetime by matter, dark or not.
Re:Sheets and Filaments (Score:5, Informative)
No. When we try to predict the large scale distribution of matter using simulations [physorg.com] we get filaments.
Re:Heh (Score:3, Informative)
Re:Sheets and Filaments (Score:4, Informative)
Re:Wait a second (Score:3, Informative)
Re:How can they tell this is caused by Dark Matter (Score:3, Informative)
I agree that "dark matter" is not necessarily the cause. It could be a gravity wave or some other mechanism not associated with mass.