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Black Holes & Dark Matter
 Artist's concept of a binary black hole pair. 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 Lite 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 Lite will 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.
 Example of a spiral galaxy, NGC 4414. |
 Example of an elliptical galaxy, M87. |
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