Dark Matter, in Three Dimensions

Image comparing distribution of normal and dark matter
Comparison of the large-scale structure of normal matter and dark matter. Credit: NASA, ESA, and R. Massey (California Institute of Technology)

The American Astronomical Society is meeting in Seattle, and researchers just announced the results from an intense, international, multi-telescope survey looking deep into the universe. Using data from the survey, called COSMOS, astronomers were able to map out the distribution of dark matter and compare it to the distribution of normal matter. The data confirmed several theories we have, though we’re still quite far from understanding even the fundamentals of dark matter.

It has been theorized that dark matter became arranged in enormous filaments as the universe cooled after the big bang. And since normal matter would be gravitationally attracted to the dark matter, we would expect that galaxies would be distributed along the dark matter filaments as well. As you can see in the accompanying image, they match up remarkably well. There are some discrepancies, though they may be related simply to the limits of our ability to detect all the matter.

Three-dimensional representation of dark matter distribution.
Three-dimensional representation of distribution of dark matter. Source: NASA, ESA, and R. Massey (California Institute of Technology)

What’s also quite remarkable, though, is that scientists were able to construct a three-dimensional map of the distribution of dark matter. Now, as you may recall, light travels at a finite speed, so images from further away are also from longer ago. A light-year is the distance light travels in one year: about 9.46 trillion kilometers (5.88 trillion miles). The sun is about 8.3 light-minutes from Earth, so the image of the sun we see in the sky is from 8 minutes ago. Similarly, the nearest star system to ours (the Centauri system) is about 4.2 light-years away, so we see the stars as they were 4 years ago. So looking deeper into space is equivalent to looking back in time. In the illustration, the direction away from Earth would be to the right, so those images are the oldest. You can see that the clusters have broke up into smaller pieces over time. Dr. Plait has an excellent post on this over at Bad Astronomy. See also news stories at Nature news, New Scientist, and Space.com.

The image represents views from 3.5 billion to 6.5 billion years ago. Below is an animation showing the three-dimensional structure rotating (ESA/Hubble [M. Kornmesser & L. L. Christensen], used with permission. Higher-resolution, downloadable versions are available.)

Chart of the approximate distribution of matter and energy in the universe. Source: NASA.

The objects all around us are made matter. On Earth, matter exists primarily in three main phases: solid, liquid, and gas. The books you read, the air you breathe, and even you yourself are all made of matter. So is the Earth itself. But this makes up only a small amount of matter in the universe. Stars add more, and free hydrogen and helium add even more. Add in the elusive particles known as neutrinos, and you have all the matter that was thought to exist.

But estimates of all this mass weren’t sufficient to explain the observed behavior of the universe; astronomers hypothesized that there was a very large quanity of “missing” matter that we couldn’t see. Since it doesn’t emit light or reflect any that we can detect, and since we know so little about it, it was named “dark matter” (somewhat analogous to the “X” in X-rays). Perhaps future generations will gently smile in amusement at our ignorance of the nature of dark matter. Most of the matter in the universe is believed to be of this dark variety. If energy is included in our survey (remember, matter and energy are interchangeable), then an even more mysterious entity called “dark energy” is believed to outnumber all the matter (both dark and conventional combined).

Though we can’t directly see this dark matter, it exerts gravitational effects on matter. In addition, as per Einstein’s theory of relativity, it distorts space as well. These are the sort of effects astronomers have been looking for.

Finally, please note that dark matter and black holes are separate entities. Black holes are made of normal matter; they are invisible because their gravity is so strong it prevents light from escaping.