January 21, 1996

Dark Matter Lights the Void

By GEORGE JOHNSON

MANY moons from now, when extraterrestrial archeologists sift through the records of our brief civilization, they might be amused to stumble across the proceedings of an annual convention of stargazers called the American Astronomical Society. They would be right in concluding that 1996 was, in one way or another, a landmark year.

Last week, at this cosmological jamboree in San Antonio, astronomers unveiled photographs from the Hubble Space Telescope that were so jammed with stars that the estimated number of galaxies in the universe quintupled overnight, to 50 billion. Before we earthlings had time to absorb this stunning revelation, we were hit with still more.

By analyzing the wobble of distant stars, astronomers found deviations that could be caused by large, orbiting planets.

Scrutinizing the wiggles within the wobbles, they even dared speculate that the two unseen planets would be close enough to their suns to soak up the rays needed for life.

But perhaps the single revelation that the aliens would find most amusing involves a strange hypothetical substance called dark matter, which by its very nature cannot be seen. Over the years it has become clear that if the prevailing cosmological theories are right, some 90 percent of the universe must be made up of dark matter. Last week, astronomers said they may have identified half of this unseen stuff. But the implications remain as mysterious as ever.

And the aliens might wonder: How was it that this ancient race was forced to conclude that almost all of the universe is invisible?

Trouble in the Galaxy

The earthlings, the anthropologists might report, kept track of time by counting the trips their planet had taken around its sun since the time of a highly regarded prophet named Jesus. It was after about 1,930 of these revolutions that the planet's stargazers began to notice something seriously wrong with the galaxies. Even their own Milky Way was violating what earthlings, with characteristic lack of humility, called a Law of the Universe.

This particular law had been laid down by another prophet called Newton. Newton's law did such a fine job describing how apples fell from trees and arrows flew through the air that the earthlings were sure that the stars themselves must obey it.

How shocking then that the Milky Way seemed to be spinning much faster than Newton decreed -- so fast, in fact, that it should have flown apart long before. Could it be that the great prophet was wrong, that his laws were no more universal than such earthly concoctions as British common law and the Internal Revenue Code?

But these stargazers were far more clever than that. Perhaps, they proposed, some kind of unseen cosmic glue is holding the Milky Way together. If 90 percent of the galaxy consists of this invisible stuff, then Newton's laws would again reign supreme.

In the annals of astronomy, this was duly recorded as the "discovery" of dark matter.

This dark matter -- whatever it was -- turned out to be quite useful. Later on, when Earth's inhabitants were having problems with their creation myth, the Big Bang, all it took was a hefty dose of dark matter to set it straight. The elders had taught that the universe began 10 billion earth orbits earlier with a great explosion that still reverberated. Just one problem. The debris from the explosion -- all this hurtling star stuff -- would have been flying too fast to clump together to form galaxies and clusters of galaxies. The prophet Newton's gravity just wasn't strong enough.

Then dark matter came to the rescue. If most of the universe had simply escaped the astronomers' notice, then there would be enough mass -- and therefore enough gravity -- to cause the congealing.

Before long, nearly every stargazer on earth had joined the cult of dark matter. But then there came a schism. Two factions, called the Wimps and the Machos, went to war over the true nature of the invisible cosmic glue.

"Wimp" stood for Weakly Interacting Massive Particles. No one had ever seen such things. But the believers in Wimps took the radical view that these exotic particles, which would neither emit nor absorb light, accounted for most of the dark matter. According to this daring heresy, everything made from "normal" matter, including the earthlings themselves, was an aberration.

Against this disheartening view arose a more orthodox faith: the believers in Machos, or Massive Compact Halo Objects. They insisted that the dark matter needed to preserve the laws was simply ordinary matter -- dead stars called white dwarfs that emitted too little light to be detected.

In decree after decree, the Wimps and the Machos fought to a standstill. Then the Machos suddenly gained the upper hand.

On the 16th earthly spin of the first lunar cycle in trip No. 1,996 around the sun, some stargazers announced that they had "found" the dead stars the Machos so coveted.

Gazing at a nearby stellar cluster -- called the Magellanic Clouds, after a dead explorer who never even left the planet -- the astronomers saw that some stars would suddenly become brighter, then dim again. What if, the Machos proposed, this brightening was caused by great lenses, magnifying the light? No, they weren't crazy enough to think there were big pieces of polished glass floating around in the sky, like those in their telescopes. These were gravitational lenses. Starlight was bent and magnified by the gravity of invisible dead stars hiding in the halo of their own Milky Way.

If you did the numbers right, you could show that maybe half the galaxy was made of this stuff. And if this were true of all the galaxies (what confidence these people had!), then half the dark matter would be found.

What about the other half? Did the Machos eventually explain it away? Or did the Wimps lay claim to it?

That is not yet clear. There were apparently some interesting revelations in the next great meeting, during the cycle they called 1997.

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