SIM PlanetQuest will help scientists tackle one the biggest mysteries of modern astronomy: How much of the universe is composed of dark matter? Simply put, dark matter is stuff that can't be seen by telescopes. It doesn't give off light; it doesn't block light; yet astronomers know it's there because of the gravitational effect it exerts on stars and galaxies. In fact, scientists today believe that most of the matter in the universe, approximately 80 percent, must be in the form of dark matter.
The strongest evidence for dark matter comes from the study of the motions of galaxies. Observations have revealed that galaxies rotate much faster than they should -- so fast that they can't be held together by the gravity from the ordinary matter they contain. Scientists believe that vast quantities of unseen dark matter must be present to prevent galaxies from flying apart.
SIM PlanetQuest will measure the motion in the sky of nearby galaxies, in most cases, for the very first time. These measurements, in conjunction with other available data, will provide the first total mass measurements of individual galaxies, and enable scientists to estimate the spatial distribution of dark matter in the universe. This, in turn, will give scientists a more accurate model for the formation of galaxies and galaxy clustering.
A search for dancing black holes
The term "black hole" describes a region of space where the pull of gravity is so strong that nothing, not even light, can escape. Scientists speculate that supermassive black holes millions or billions of times more massive than our sun dwell at the center of most galaxies.
Galaxy types that may harbor central black holes include spiral galaxies, like our own Milky Way, and elliptical galaxies. Spiral galaxies have a round, flattened shape, with a spiral arm structure. Ellipticals are oblong or spherical in shape and contain no arm structure. Many scientists believe elliptical galaxies are the product of violent past collisions between two or more spiral galaxies.
If two spiral galaxies were to merge into a single galaxy, their respective black holes would tend to sink toward the center and either merge or form a rotating pair, known as a binary system. SIM PlanetQuest will peer track the bright "photocenter" of candidate galaxies. Just as a star's motion will help the mission detect planets, a moving photocenter will allow it to detect pairs of black holes circling each other - the "smoking gun" evidence of a past collision.
