The Missing Mass

Most astronomers believe that the universe was born billions of years ago in a so-called "Big Bang"--a violent explosion of a primordial clump of matter. Ever since, fragments of that clump --consisting largely of the islands of stars called galaxies--have been flying apart. But the universe is expanding at a steadily decreasing speed; the outward flight of each galaxy is being slowed by the pull of gravity from the others. If that pull is strong enough, the galaxies will eventually be braked to a halt. Then they will begin falling back to crush together in a final cataclysm. If the tug of gravity is too weak, the universe will continue to expand forever.

An infinitely expanding universe is a concept that does not sit well with many scientists, but in their search of the heavens, astronomers have so far found only a fraction of the mass needed to produce enough gravitational force to halt and reverse the outward flight. Now, Astrophysicist Jeremiah P. Ostriker of Princeton University thinks that he may have found what his colleagues have been looking for. At a recent meeting of the American Physical Society in Chicago, he suggested that enough mass to "close" the universe may be hidden in great halos of matter around the galaxies. His evidence comes not from direct observation of those halos, but from some scientific deduction based on Newtonian physics.

Newton's laws dictate that the farther a planet is from the sun, the slower will be its speed in orbit. The planet Jupiter (fifth from the sun), for example, moves much more slowly than the earth (third from the sun). The same rules should apply to the billions of stars revolving around the central cores of distant galaxies, or to one galaxy revolving around another. In measuring the rotational velocity of the stars or galaxies, however, astronomers have discovered a perplexing fact: when the total observable mass of the galaxies is taken into account, the stars do not seem to slow their rotation sufficiently.

In an effort to account for this phenomenon, Ostriker worked out a neat mathematical explanation. He calculated that if the galaxies actually have ten times more mass than has been seen through the telescopes of earthbound observers, the velocities of the stars would satisfy Newton's laws. Ostriker theorizes that the mass exists in invisible halos of small, dim stars, interstellar dust and gases, and perhaps even "black holes" --cadavers of huge, ancient stars, so completely collapsed under their own powerful gravity that not even light can escape from them.

If such hidden mass really exists, astronomers may soon know. Several of the largest telescopes are being trained on far galaxies in a search for hard, observational evidence of Ostriker's halos.

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