Mystery of the Missing Mass

In their mind-stretching efforts to explain the universe, cosmologists have been troubled by a glaring inconsistency: the universe behaves as if it is much more massive than it appears.

The total mass of the galaxies, which contain all the stars and thus most of the known matter in the universe, is far too small to account for the declining rate at which the universe is expanding. The missing mass may now have been discovered. A Naval Research Laboratory scientist has found evidence that it exists in the form of a hot gas that fills the void between galaxies.

Astrophysicist Richard Henry and his NRL associates crammed X-ray detectors into the nose of an Aerobee rock et last September and fired it high above the atmosphere, which absorbs X rays before they reach the earth. Telemetry from one detector designed to spot "soft" or low-energy X rays emanating from intergalactic space showed unexpectedly high readings. What-- in apparently empty space -- was producing this radiation?

Decelerating Bang. Recalling an ear lier suggestion that soft X rays would be given off by hot, thinly dispersed hydrogen gas, the NRL scientists turned to their computers. To produce the detected radiation, they calculated, the temperature of the gas would have to be 1,400,000DEG F. and its density only one atom of hydrogen in every 3.5 cu. ft. of space. But even at this low density, says Astrophysicist Henry, the hydrogen gas would constitute 100 times as much matter as there is in all of the galaxies combined and thus solve the mystery of the universe's missing mass.

The NRL finding has profound implications for cosmologists, most of whom now believe that the universe began about 10 billion years ago with the explosion of a primordial clump of densely packed matter. The fragments of the original "big bang" have been expanding outward ever since, but at a rate that is steadily decelerating be cause of gravitational pull of each particle of matter on all the others. If the total mass of the universe is less than a certain value, cosmologists say, gravitational pull will never fully overcome the momentum imparted by the big bang; the universe will then continue to expand forever. But if the universal mass is large enough, the outrushing galaxies will eventually be slowed to a stop and then will begin falling back together again.

Cataclysmic Collision. To determine the actual deceleration rate of universal expansion, Allan Sandage, of the Mt. Wilson and Palomar observatories, and other astronomers have been plotting graphs of the brightness of both nearby and distant galaxies versus their red shift.* From the resulting curves, they have approximated the deceleration of the galaxies as they recede from the earth. If his data is correct, says Sandage, galaxies have been racing away from each other for 10 billion years or so, but are slowing down rapidly enough to bring them to a halt in another 30 billion years. From that time, it will take another 40 billion years for the galaxies to fall back again in a cataclysmic collision.

Long before they heard about the NRL X-ray findings, Sandage and other astronomers had tentatively calculated the total amount of matter in the universe necessary to produce the deceleration they had observed: an average of one gram of matter in every 100 octillion (100 followed by 27 zeroes) cu. cm. This week they are more certain of their figure. It is the equivalent of one hydrogen atom in every 3.5 cu. ft. of space--the density of the hot gas that NRL scientists believe makes up the universe's missing mass.

* The shift in the frequency of light toward the red end of the spectrum, an indication of both the recessional speed and the distance of the galaxy.

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