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Hubble Space Telescope Detects Ring of Dark Matter
mknewman wrote with a link to a story on the NASA site indicating that they may have finally found dark matter using the Hubble telescope. We've discussed the stuff a few times in the last year, with the Hubble actually mapping out the dark matter in the universe in January. This, though, may be our first 'sighting' of the elusive substance. "NASA will hold a media teleconference at 1 p.m. EDT on May 15 to discuss the strongest evidence to date that dark matter exists. This evidence was found in a ghostly ring of dark matter in the cluster CL0024+17, discovered using NASA's Hubble Space Telescope. The ring is the first detection of dark matter with a unique structure different from the distribution of both the galaxies and the hot gas in the cluster. The discovery will be featured in the June 20 issue of the Astrophysical Journal."
I'd assume you'd "see" it by observing how it interacts with massive bodies around it, like planets, stars, gas clouds, etc.
I was about to write a comment panning this submission, because apparantly a one-paragraph press release - that doesn't give much room for an intelligent discussion - was the only information on this discovery. But I did find an abstract for a talk given at the American Astronomical Society Meeting 209, which was held in January this year.
Unfortunately I can't find the paper itself. So there is slightly more info, but not much :-(
Given that the press conference isn't until May 15, I can't say for sure, but based on the brief blurb on the NASA website, it's almost certainly a gravitational lensing measurement.
It's true that dark matter doesn't interact directly with light, but it does curve space (ie. generate gravity), which light travels through. So light feels the gravitational effect of dark matter, a phenomenon known as "gravitational lensing". Essentially, the images of background galaxies going through a concentration of dark matter become magnified and distorted.
I don't know whether this is a strong lensing or weak lensing measurement. In strong lensing, the distortion is extreme and the images of the background get stretched into long tangential (and radial, though they're rarer) arcs like this. In the case of weak lensing, the distortion in any one image is small, but all images in a certain area are distorted coherently so you can statistically disentangle the signal.
Given the distorted images of the background galaxies, you can determine what mass distribution was responsible for those distortions, thereby producing a "mass map". It appears that in this case (again, based on the brief blurb), the mass map shows some sort of ring-like structure that isn't seen at any other wavelength (which non-dark matter would produce).
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There's an incredibly boring book, "Guns, Germs, and Steel", which I've mostly read (it's easier than reading the Old Testament!). The basic question posed in the first paragraph is "Why did Europe dominate the world?" He goes into fairly convincing arguments for why we are advancing faster and faster... technology feeds on itself in a positive feedback loop. He discounts the importance of the giants, like Newton, and focuses on the size of populations, the ease of communication of ideas and domesticated plants and animals between them. Technology is advancing at an unstoppable pace. The way it's going, it seems likely we'll either use it to kill ourselves, or birth a new race that we design... either biological through genetic manipulation, or electronic, or perhaps a combination of both.
Beer is proof that God loves us, and wants us to be happy.
...last year: astronomers could see in the aftermath of two colliding galactic clusters.
The visible matter's momentum through space was impeded at quite a different rate than dark matter. This left four distinct zones of gravitational lensing, but only TWO were associated with visible matter. The other two were dark matter halos that had been separated from each galactic cluster.
Part of the confusion is that there are 2 separate concepts that both go by the name "dark matter".
Dark matter in the broad sense is matter that we detect gravitationally but can't observe directly through any interaction with light (and if this measurement is from gravitational lensing, which I suspect, then it certainly falls into this catagory). We infer that it exists because the motions of stars and gas in galaxies, galaxies and hot gas in galaxy clusters, and the universe as a whole all act as though they are acting in the gravitational field produced by much more mass than what we can directly detect.
Some fraction of this dark matter is normal ("baryonic") matter that just happens to be very difficult to detect due to its temperature and density... for example, a lot of it is diffuse gas at ~100000K, which is too cool to emit X-rays but too hot to emit much line radiation.
However, from Big Bang nucleosynthesis calculations, we can estimate how much baryonic matter there is in the universe because the relative fractions of different isotopes of H, He, Li and Be are quite sensitive to the total amount of baryonic matter. And the total amount of matter required to account for the dynamics of the universe is about 6 times higher than the amount of baryons that Big Bang nucleosynthesis measurements indicate.
Therefore, there must be non-baryonic dark matter too, made of exotic particles (or neutrinos, but there most likely aren't enough of those either). This is also sometimes just called "dark matter", which is confusing.
Interestingly, galaxy clusters, like the one studied here, have most of their baryonic matter in the form of hot X-ray gas that is detectable... the density of baryonic matter we can detect within a galaxy cluster is about what you'd expect given the BBN calculations. So any dark matter in a galaxy cluster should be non-baryonic dark matter, which is why measurements like this are exciting.
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Last August, an announcement was made about the discovery of dark matter using gravitational lensing, based on CHANDRA observations.
This past January, a map of dark matter was created using millions objects in the night sky as a result of a massive collaboration effort known as COSMOS. The 3d dark matter map of a 2 degree section of the sky matches predictions of the evolution of dark matter.
So, what exactly does mknewman mean by, "may have finally found dark matter"? What rock have you been hiding under for the past year? (Not that the rest of slashdot is caught up, given comments random people make whenever dark matter shows up in an article)