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No. 89: Sep-Oct 1993

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In The Dark About Dark Matter

During our last two-month collecting cycle, four "dark matter" items worth mentioning turned up. A wider search undoubtedly would have netted many more, because dark matter is worrying a lot of astronomers.

Observations of visible matter, the only kind we can see directly, suggest that most of the universe is, in fact, composed of dark matter. This conclusion comes mainly from the belief that something unseen (dark matter) is tugging on visible matter, making it do things the laws of motion say it should not do. All visible bodies, therefore, seem to be careening about in a dense cloud of unseen, unknown masses. These might be dark, Jupiter-sized objects, black holes, and/or some exotic forms of matter. We must choose between the reality of dark matter or admit that something is awry with our laws of gravitation and motion when they are applied on a cosmological scale

Now, let us examine those four darkmatter items from the recent literature:

D. Lin, a University of California astronomer, has shown that the Large Magellanic Cloud that orbits around our own galaxy (the Milky Way) is being torn apart ("cannibalized") by the powerful gravitational pull of a dense cloud of dark matter surrounding the Milky Way. This dismemberment of the Large Magellanic Cloud cannot be explained by the gravitational forces exerted by the stars in our galaxy that we can see. Lin calculates that our halo of dark matter is equivalent to 600-800 billion solar masses, compared to the only 100 billion solar masses of visible matter.

(Flam, Faye; "Spinning in the Dark," Science, 260:1593, 1993. Also: Anonymous; "'Dark Matter' Is Observed 'Cannibalizing' a Galaxy," Baltimore Sun, p. 8A, June 8, 1993.)

The dark matter surrounding a galaxy will, according to the Theory of Relativity, act as a gravitational lens that will deflect light rays passing near it. This dark matter, acting like a telescope, should increase the number of quasars counted in the sky near galactic clusters. Such larger quasar counts are indeed observed, but these increases are much larger than expected. The implication is that there is much more dark matter in the universe than previously thought. (Cowen, Ron; "Quasar Count Poses Dark-Matter Puzzle," Science News, 143:397, 1993.)

Finally, dark matter is forcing scientists to reexamine the Equivalence Principle, which asserts that gravitational mass (as in Newton's Law of Gravitation) is identical to inertial mass (as in Newton's Force = Mass X Acceleration). In terrestrial experiments, the two kinds of mass are equal, but on a cosmological scale, they may not be. There could be an extra, small (10%), longrange, non-gravitational force exerted between massive objects.

(Frieman, Joshua A., and Gradwohl, Ben-Ami; "Dark Matter and the Equivalence Principle," Science, 260:1441, 1993.)

Reference. "Dark matter" or "missing mass" is cataloged in AWB5 in our book: Stars, Galaxies, Cosmos. Details of this book may be seen here.

From Science Frontiers #89, SEP-OCT 1993. 1993-2000 William R. Corliss