Science Goals
Dark Matter
Is there any "matter" missing from the Universe?
"An odd question," you might say! If there is something out there that I cannot even see, how would I know it's missing? And why should I care?
The NGC 2300 group of galaxies contains a large reservoir of million-degree gas glowing in X-rays. A false-color X-ray image of the hot gas is superimposed here on an optical picture of the galaxy group. Gravity from the galaxies alone is not enough to keep the gas in its place. There must be large quantities of dark matter whose gravity is preventing the gas from escaping.
Believe it or not, how much "matter" exists in the Universe dictates our destiny. By now, scientists have figured out that the Universe is expanding. But will this expansion come to an end or will it go on forever? The answer to that question depends on how much matter there is in the Universe.
There are two forces that play against each other: the outward momentum of expansion and the inward pull of gravity. If there is a lot of "matter" in the Universe, or more quantitatively, if the density of the Universe exceeds the "critical density," then the gravity takes over and the Universe will collapse back on itself. On the other hand, if the density of the Universe is less than the "critical density," everything will fly apart and the Universe will expand forever. That is why it is so important to measure exactly how much "matter" is out there.
Here's the problem, though: We can only "see" matter that is shining or luminous. If the matter is dark, like a planet or a dead star that no longer shines, we cannot see it no matter how powerful our telescope is. Nor can we see matter made of exotic elementary particles, matter that differs from the protons, neutrons, and electrons that form our bodies.
Scientists believe that perhaps over 90% of all the Universe's matter is in a form we cannot see. This has been one of the most astonishing discoveries in the history of astronomy. Scientists call this unseen matter "dark matter," and its exact nature is not known. Some say this matter is made simply of chunks of planets and dead stars. Others point to neutrinos, newly discovered elementary particles recently shown to have mass. Some physicists talk of hypothesized, incomprehensible forms of matter that could be all around us. Still others think dark matter is a combination of all of this. Regardless of its nature, dark matter makes itself known to scientists through its undeniable gravitational force.
All matter exerts gravity. The greater the mass, the greater its gravitational pull. The Sun attracts the Earth, and the Earth attracts the Moon and the tiny creatures that call the Earth home. Galaxies and clusters of galaxies are also bounded by gravity. Interestingly, when scientists added up all of the mass from "visible" stars and gas in certain galaxy clusters, they found that this mass wasn't enough to keep the clusters so closely bound. There must be hidden mass in between the neighboring galaxies holding the clusters together. This is the dark matter.
True, scientists cannot see the dark matter, but all hope is not lost. Constellation-X will map the regions of suspected dark matter. The mission will accomplish this by mapping the hot, X-ray emitting gas that permeates the space between galaxies. The hot gas is a tracer for dark matter because gas, like all matter, is attracted by the dark matter's gravitational pull. The more dark matter there is, the greater the gravitational attraction. Stronger gravitational tugging leads to faster moving and thus hotter gas. So the temperature and density of X-ray emitting gas can reveal some secrets about that sneaky dark matter.
As if there weren't enough missing matter, there's the problem of the missing baryons. Baryons form the ordinary matter we encounter in our daily lives. The Big Bang theory predicts a certain amount of baryons, and scientists have so far fallen short in detecting it. Some scientists predict that baryons in the form of gaseous matter lie in the space between galaxies, a place called the intergalactic medium. But because that gas is not very hot, it doesn't radiate high-energy light, and current X-ray telescopes cannot detect it. Constellation-X's superior sensitivity may detect these baryons by looking for the fingerprints of the gas' chemical elements.
In this visible light image of the Virgo galaxy cluster (left), a bright central elliptical galaxy is seen surrounded by a cluster of similar galaxies. The X-ray image of the same region (right) shows a large ball of hot gas, the mass of which is at least three times greater than all of the Virgo galaxies. This gas is trapped by the gravitational pull of dark matter that comprises the bulk of the mass of the entire system.
