Giant Sheets Of Dark Matter Detected

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

[JustinOpinion]

First, remember that the distribution of dark matter and ordinary matter are, actually, pretty similar (we find galaxies accumulated along the dark matter filaments, and at smaller scales see dark matter concentrated into galaxies).

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!!!

[JustinOpinion]

The "rubber sheet" analogy is imperfect, but I don't think your revised analogy is correct.

Draw a line running between the bowling ball and marble, and take that cross section, note that the bowling ball and marble behave the same way at close distance like they do above, but when they are a opposite sides of the pool there is a slight "repulsive" effect. We call that Dark Energy!

"Dark energy" doesn't mean that normal matter is repulsive at large distances. Ordinary matter is always gravitationally attractive towards other ordinary matter, at all scales. Same for dark matter (whatever it is). "Dark energy" is, in fact, a "negative pressure" that pushes on spacetime itself, causing the universe to expand (and moreover gets stronger and stronger the lower its density becomes).

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.

This effect will also affect light waves moving past it, hence gravitational lensing.

Just to be clear: gravitational lensing also has nothing to do with dark energy... and nothing to do with dark matter specifically. Any source of gravity (ordinary matter, dark matter, etc.) will deflect the path of light rays (the effect is small but measurable). Thus gravitational lensing is a great way to determine the "amount of mass" within a volume of space. When that mass is correlated with brightness, we say it's ordinary matter (stars, etc.) and when that mass is correlated with seemingly empty patches of space (dark), we call it dark matter.

Journal Reference

[phizix]

The journal article is available at http://dx.doi.org/10.1051/0004-6361:20078522 [doi.org]

Re:How can they tell this is caused by Dark Matter

[JustinOpinion]

The evidence for gravitational lensing is much stronger than that. In the most extreme images, we can actually see multiple images of a single source. In this image [wikimedia.org], there are four copies of the distant quasar because of the lensing of the closer galaxy (center of image). Even though gravitational lensing is a fairly small effect, given the massive distances we're talking about, the deviations are readily measurable.

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....

[exp(pi*sqrt(163))]

> What if it isn't dark matter at all? But the Universe actually bending?

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

[exp(pi*sqrt(163))]

> We know very well what shapes distributions of particles form over time with only gravity acting on them and they look a lot like galaxies and very little like sheets and filaments.

No. When we try to predict the large scale distribution of matter using simulations [physorg.com] we get filaments.

Re:Heh

[spun]

Dark matter isn't antimatter. Antimatter isn't dark. Dark matter only interacts with regular matter (including regular antimatter) through gravity.

Re:Sheets and Filaments

[LionKimbro]

More than just simulations -- if you look at the SDSS [sdss.org] data, you can clearly see the filaments. Mitaka [nao.ac.jp] is a good way to see a summary of the data on a PC; Switch to launch mode, and then zoom all the way out. You'll see the filament form as you get closer to the present (the center,) and see things more homogeneous at the edges (in the past.)

Re:Wait a second

[MenTaLguY]

Yes, exactly. We're seeing the gravitational effects of large concentrations of mass, but that mass is neither blocking nor emitting light. Hence (for lack of a better name) we call it "dark matter".

Re:How can they tell this is caused by Dark Matter

[Big_Breaker]

The point is that the phenomenon is occuring out in space on a huge scale at a huge distance. It is not something occuring on Earth or even in our solar system.

I agree that "dark matter" is not necessarily the cause. It could be a gravity wave or some other mechanism not associated with mass.