AAS Meeting — Dark Matter
Published on Jan 7, 2007 at 11:40 pm.
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Filed under conference blogging, dark matter, physics.
Well, we are getting some interesting information here in Seattle. One thing that has apparently hit the news elsewhere is the new 3-D map of dark matter in the universe. Dark matter is invisible, so how do we map it?
Well, quite simply, we look for its gravitational effects. Dark matter, like all matter, interacts via the gravitational force. Einstein told us (and it was later verified experimentally) that gravity can bend light. Massive galaxies can have so much gravity that they bend light in a phenominon called gravitational lensing. This, in fact, is how we know that there is dark matter out there. We see far too much gravitational lensing going on. There is more gravitational lensing than can be accounted for with normal matter. So, “dark matter” was proposed as solution to that problem. I blogged about dark matter before. By measuring the amount of excess gravitational lensing, you can figure out how much dark matter is helping to bend light. When you do that, you get a plot like this:
It had been assumed that the correlation between visual matter (on the left) and dark matter (on the right) meant that dark matter and normal matter both congregated in the same area. But, we couldn’t be sure. After all, this simply said that there was dark matter in the same direction as normal matter. But, is the dark matter in front or in back of the normal matter? That isn’t obvious.
So, astronomers using the Subaru Telescope, the Hubble Space Telescope, and the Very Large Array spent yeats carefully studying small distortions in galaxy shapes, called weak gravitational lensing, in a small part of the sky. The NASA press release says it was a huge area of the sky, nearly nine times the area of the Full Moon in the sky. But, the Moon occupies really a very tiny part of the sky, so this is by no means a major part of the sky, but it is still very significant. Different galaxies are different distances. So, by measuring the dark matter between here and galaxies of different distances, we can figure out how far away the dark matter is. Adding that information to the plot above, you get this picture:
This is a three dimensional map of dark matter in that small region of the sky. This supports the idea that dark matter clumps just like normal matter. In fact, we think that it is the clumping of the dark matter that makes normal matter clump in those places in the first place.
So, that was released today. More info to follow.
-Astroprof
