Ever since the first astronomers turned their eyes to the heavens, mankind has been preoccupied with the dazzling points of light that stand out from a black and seemingly endless void. But now with the Space Telescope and the theory of Dark Matter, astronomers now fix their lenses on what they are unable to see.
To grasp the idea of Dark Matter, it is important to understand it is material that is invisible to all forms of electromagnetic radiation. Dark Matter is sometimes defined as mass that does not shine. Unable to observe Dark Matter by conventional means, astronomers have had to turn to other methods to detect its presence.
The case for Dark Matter begins here. There is not enough known mass -- and hence not enough gravitational force -- in the universe to stop the expanding universe. Known mass is everything we see: the earth, the planets, the stars. If the universe were to contract, known mass would have to account for only 1% of all matter. Mass of an unknown nature must therefore exist to stop expansion eventually.
The most convincing argument for the existence of Dark Matter has been revealed by Dr. Vera Rubin of the Carnegie Institute. It is an argument that begins centuries ago, on a summer day in 1666 when, according to Voltaire, Isaac Newton fell victim to an apple's fall at his home near Grantham, England. This event led him to embark on a journey which would end in his most significant conception: the force of gravity varies inversely with the square of the distance between the two objects. From this simple relationship, objects closer to a center of mass should orbit faster. This is true in our solar system. Each successive planet outward from the Sun orbits slower and slower.
Dr. Rubin observed several galaxies, including the Milky Way, and found, to scientists' surprise, that as stars increased in distance from the galaxy's center, their linear speeds remained constant. From this, she deduced that if Newton's laws still applied to galactic motion, the apparent center of the galaxy must not be the galaxy's center of mass. Dr. Rubin reasoned that unseen matter -- Dark Matter -- must increase with distance from the galaxy's center.
After Dr. Rubin's deduction, scientists are now able to predict how much Dark Matter must be present in a galaxy to produce its visible motion. Hubble Space Telescope has entered the picture recently. Astronomers previously theorized Dark Matter was comprised of faint red stars too dim to be seen from Earth.
Two areas of the sky are likely locations of these stars: a globular cluster in Ara and part of the galactic halo in Eridanus. Hubble images should contain at least 500 dim red stars, but the regions had far fewer stars than expected. Astronomers can now rule out these stars as possible forms of Dark Matter.
Photo. The Hubble Space Telescope's Dark Matter searches turned up results that disproved a theory that dim red stars are a possible form of Dark Matter. In this image, astronomers had hoped to find at least 500 of the stars to support the theory; the yellow spots represent these "simulated stars." The actual HST image clearly does not match the theoretical information, though. After several other similar tests, astronomers were forced to discount this possibility in their search for Dark Matter. Courtesy Space Telescope Science Institute, NASA.
We know dark matter exists in globular clusters because we see its effect on light from galaxies behind the clusters. Dark matter in a cluster bends light. It produces a gravitational lens effect. This effect multiplies the number of galaxies we see and makes distant galaxies appear closer and brighter than they should.
Photo. The Gravitational Lens Effect. Courtesy Space Telescope Science Institute, W. Couch/UNSW, NASA.
The Space Telescope has told astronomers what Dark Matter is not. In the future, Hubble has the potential to supply the answers that our curiosity seeks.