ON THE DARK SIDE-cont.

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Edwin Hubble

Dark Matter

Vera Rubin

Black Holes
Related Topics


The Hubble Constant

MACHOS

Carlos Frenk: Where Is the Missing Matter?

 

Before most cosmologists were even thinking about the question, the Swiss astronomer Fritz Zwicky was studying the motions of individual galaxies in the large Coma Cluster. Using the same technique as Edwin Hubble did to measure the galaxies’ velocities, he found that they appeared to be moving too fast. At the speeds they were traveling, the galaxies should have long ago flown apart, but there they were, more than 10 billion years after the Big Bang, still clustered tightly together. Zwicky concluded that there must be ten times more matter in the cluster than we can see. And because most of the material in the universe seems to be collected into groups and clusters of galaxies, his finding implied that the universe might contain ten times as much matter as well. However, Zwicky’s unexpected discovery remained underappreciated for many years.

      Beginning in the 1950s, astronomers started to learn that this so-called dark matter doesn’t lurk just in clusters. American astronomer Vera Rubin looked at the motions of stars in hundreds of spiral galaxies and found that they, too, behaved in an unexpected way. In the Milky Way, for example, the Sun circles the galaxy’s center at a speed of about 150 miles per second. Most astronomers suspected that the stars would move in a similar relationship to the center of a galaxy as the planets move in relation to the Sun: faster the closer you got to the center and slower the farther away you were.
Milky Way Galaxy

A portion of the  Milky Way Galaxy showing the bulge at center,  as seen at  near-infrared wavelengths.

      In the Milky Way, however, the speeds of stars stay pretty much constant out as far as the telescope can see. One way this can happen is if a vast halo of unseen matter lies beyond the edge of the visible galaxy; otherwise, the outermost stars would go flying off into intergalactic space. The conclusion: the Milky Way and other spiral galaxies contain about ten times as much dark matter as they do light-emitting matter. (Keep in mind that although it's invisible to us, dark matter doesn’t have to be pitch-black—it could just be so faint that it doesn't show up at an appreciable distance.)

       What can this stuff be? In many ways, it’s easier to say what it isn’t. It can’t be hydrogen gas in any form because that would give off radiation in some part of the spectrum. It can’t be dust because large amounts of dust in the Milky Way’s halo would block the light of distant galaxies to an extent greater than currently observed. Star-sized black holes and neutron stars can also be ruled out because gas falling into these objects would produce more X-rays than we see.
 

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