The Sciences

AAS Report #2: Dark matter and large scale structure

Bad AstronomyBy Phil PlaitJan 8, 2007 4:37 AM


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A note: I am attending a meeting of the American Astronomical Society in Seattle. I will blog as much as I can from this meeting, as there is a LOT of news coming out, as well as lots of fun and wonderful scientific geekiness. This particular blog entry is a bit long as I have to explain some relatively complex things to get to the point. I think most of my reporting will be somewhat less wordy. But no promises! I love this stuff, and I love to talk about it. Today, astronomers released news about the largest and most detailed survey of the deep Universe ever made. It used telescopes in space and on the ground to make a huge census of matter across the Universe, from local regions out to a distance of about 6 billion light years. It's called COSMOS, for the Cosmic Evolution Survey, the telescopes involved were Hubble, XMM-Newton, Spitzer, Keck, the Very Large Telescope in Chile, the Very Large (radio telescope) Array in New Mexico, and the Subaru observatory. It involved over 100 scientists from more than a dozen countries. This survey is incredible: it mapped the location of more than two million galaxies over an area two degrees on a side on the sky-- bigger than 16 full Moons. In the end, astronomers made what is essentially a three-dimensional large-scale map of normal matter (the stuff that makes up you, me, doorknobs, snakes, planes, everything we can see and touch). Here's a (very) small piece of the survey image from Hubble:

Click on it for a much higher-resolution version. It's very pretty. Amazingly, the map of galaxies made is not the most interesting part of this survey. What makes COSMOS unique is that using sophisticated techniques, the astronomers were also able to map out the location of dark matter as well! This is the stuff that makes up the vast majority of matter in the Universe, but which we cannot detect directly. It's been known to exist for decades -- it was first postulated in 1933 -- but the evidence has been indirect. Dark matter still has gravity, and so we see it through its effects -- galaxies surrounded by dark matter halos rotate differently than they would without them, and dark matter in clusters of galaxies reveals itself through the motion of those cluster members. But a trick of relativity winds up betraying the presence of dark matter, too. As Einstein postulated, matter bends space, the way a heavy weight in a bed will warp the mattress. Light traveling though empty space will move in a straight line, but if matter is warping space, light will travel along the bent space as well. Imagine light from a distant galaxy is on its way to us. But between this galaxy and us is some large mass, like a clump of dark matter. The light from the galaxy will bend around the matter, and when it gets to us it will be slightly distorted, just as if the light has passed through a lens. This process, in fact, is called gravitational lensing (for more about this, see my writeup about the Bullet Cluster, image #4 in my Top 10 Images of 2006). So astronomers can map out the location of dark matter by very carefully taking observations of large areas of the sky and painstakingly teasing out the distortions in the shapes of background galaxies. The observations need to be done from space because the background objects are faint, small, and very close together. Space-based telescopes can more easily see fainter objects than ground-based 'scopes, and have better resolution-- they can separate closely spaced objects better. That's why Hubble was used for this survey. In fact, the survey was massive: it took 10% of Hubble's time over two years. It's the biggest single project ever done by Hubble. But Hubble has limitations. To get the dark matter location, the distances to many thousands of galaxies had to be found. This was done from the ground, using the Very Large Telescope and the Subaru Observatory, using a technique called photometric redshift. Basically, the galaxies were observed through many different color filters, and the brightness of a given galaxy in each filter can be used to determine its distance. Armed with the distortion to the galaxies' shapes and their distances, astronomers began to map out the location the dark matter. Let me pause for just a second here-- we've known about dark matter for a long time, but the problem is, the dang stuff is dark. It's hard to see. People have been arguing over whether it even exists, yet now, not only do we know it's there, we know where it is! This is an amazing advance. There is one very big result you can see just by glancing at the observations:

The left image, in red, shows the location of normal matter on the sky. The right image, in blue, is where the dark matter sits. They match! It's been theorized for some time that as the Universe cooled after the Big Bang, dark matter formed long filaments many millions of light years across. As that happened, normal matter would be attracted to it gravitationally. Today, the normal matter should trace the location of the dark matter. Now we can see this is true! But it gets better. Because astronomers were able to see dark and normal matter at very large distances, they were able to track how the stuff changes over time. That's because the farther away we look, the farther back in time we see -- we see an object a million light years away as it was a million years ago. So by looking back far enough, you can actually see how stuff behaved in the distant past, billions of years ago. And things have changed! Perhaps the most important thing astronomers saw in their data is that dark matter was smoothly distributed at times early in the Universe, and became clumpier as time wore on. They created an image to show this. It's a 3D map of the dark matter:

You can think of the left hand side of the image being dark matter that is nearby, and the right side being farther away. But remember, distance=time. You can see that dark matter now (on the left) is clumpy, whereas dark matter was smoother a long time ago (on the right). Again, that is just what models predict. Over time, the smooth distribution of the matter in the Universe became lumpy as small ripples in the matter were amplified by gravity. In other words, any place where there was a little more matter had more gravity, and attracted even more matter. This process continued, until after billions of years we got our current Universe: clumpy, lumpy, and full of wonderful galaxies, stars, and planets. The COSMOS survey data is incredibly rich, and will be mined for information for years to come. I have only touched on what was learned from it so far, and there is much more scientific knowledge lurking within it. And even now, astronomers are planning even more ambitious surveys of much larger areas of the sky. These surveys will refine the data taken already, and will also start to tackle dark energy, the mysterious force that appears to be accelerating the expansion of the Universe. We know even less about that than we do about dark matter. As usual, the more we know, the more there is to know. There's still so much to learn!

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