In the Dark Matter Wars, Wimps Beat Machos

By James Glanz
New York Times
February 29, 2000

MARINA DEL REY, Calif., Feb. 27 -- The last hopes for a universe filled with familiar stuff behaving in comprehensible ways died here in Los Angeles County last week. It showed in the head-turning attire of at least two of the scientists who carried the news to a major conference on cosmology in a hotel across the street from a restaurant called Killer Shrimp.

But never mind the shrimp. Dr. Joel Primack spoke about the overall contents of the universe while wearing a midnight blue jacket and a tie that bore the likeness of Van Gogh's "Starry Night," in which some ominous presence between the stars overwhelms the visible bodies themselves. In her own talk, Dr. Katherine Freese heralded "The Death of Baryonic Dark Matter" in an all-black pantsuit.

Baryons are the ordinary particles, like protons and neutrons, of which stars, asteroids, comets, planets, people and textiles are made. By measuring the gravitational pull of some unknown "dark matter" on the visible stars and galaxies, astronomers have determined that this mysterious material which seems to permeate the universe has a weight that is 60 times that of the stars and 7 times that of all baryons, including gas and solid material in space.

By combining calculations with observational data, Dr. Freese reported that she had eliminated the last shred of possibility that the dark matter is ordinary, baryonic material. Dr. Primack, his tie providing mute acknowledgment, added that an unknown form of dark energy permeating space is apparently pushing against gravity on large scales. There is so much of this dark energy, sometimes called quintessence or the cosmological constant, that it in effect has a weight almost twice that of all matter, dark or visible.

Cosmologists have seen evidence for the dark matter and dark energy for some time. What has changed is that for now, there seems to be little chance of escaping the conclusion that the stuff of which we are made amounts to no more than cufflinks on the cosmic tuxedo, as it were.

"The bulk of the energy density in the universe seems to be dark energy," said Dr. Primack, who is at the University of California, Santa Cruz. "And the big question is, What is the dark energy?"

A similar state of affairs holds for the dark matter, he said. But despite the lingering mysteries, he said, most cosmologists are far from put off by a universe filled with dark, strange and unknown stuff. "Most professionals in this field have had a long time to get used to these ideas," Dr. Primack said.

He added that the professionals were pleased that reigning theories of how the universe was born, generated chemical elements and formed structures like galaxies mesh remarkably well with the observed amounts of ordinary matter, dark matter and dark energy in space. Many of those theories deal only with the overall balance sheets of matter and energy and not their details.

While it is true that those dark details remain to be explained, he said, "I tend to think of this more as a problem for fundamental physicists, and less for me." As recently as a few years ago, a group of astronomers called the Macho collaboration thought they had found the dark matter in chunks of presumably baryonic material orbiting in a huge cloud, or halo, around the Milky Way. (Macho is an acronym for the "massive compact halo objects" in question.

The main evidence came from observing brief brightenings of stars outside the Milky Way. The Macho collaboration concluded that these brightenings had occurred when the objects, assumed to be in the halo, passed directly between Earth and distant stars and magnified the light of the stars behind them. The gravity of the objects, each roughly the mass of our Sun, acted as a kind of lens, just as Einstein's equations of relativity say that they should.

The discoveries seemed to fit nicely with the longstanding observation that galaxies like the Milky Way spin too fast to be held together only by the gravity of visible matter -- a main argument for the existence of dark matter in the universe. Was it possible that the dim objects, suggested by the lensing observations, could be the galactic dark matter?

The answer turns out to be no, Dr. Freese said in her talk. The so-called "white dwarf" stars, which those objects would almost certainly have to be, would have to be the remnants of a population of stars that over their lifetimes would have spewed out substantial amounts of gas, heavy elements and infrared radiation -- none of which are seen.

In addition, she said, such a large quantity of baryons in galaxies like the Milky Way could not be reconciled with accepted theories of how elements formed in the Big Bang explosion in which the universe is thought to have been born.

"It's looking very likely that 50 to 90 percent of our galaxy is nonbaryonic," said Dr. Freese, a University of Michigan physicist.

In the same session, which was held at the Fourth International Symposium on Sources and Detection of Dark Matter in the Universe, members of the Macho collaboration and a related French collaboration called EROS conceded that the high rate at which they initially observed lensing events was probably a statistical fluke, at best.

"There are not enough machos in the galactic halo," said Dr. Éric Aubourg of EROS. "It looks like everybody agrees on this."

So what is the dark matter? As tempting as the supposition may be, it could not be killer shrimp, which would presumably be baryonic. Theorists believe it could exist in clouds of nonbaryonic particles called Wimps, for weakly interacting massive particles. At the same conference, two experimental groups -- one based in Rome and one at 10 American institutions -- presented conflicting evidence on whether Wimps have been detected. But that argument remains for the future.

While some astronomers presented fresh evidence that dark energy is filling the space between galaxies, counteracting their gravity and accelerating the expansion of the universe, Dr. Edwin L. Turner of Princeton University pointed out that a few observations can still be found that do not fit the concordance that cosmologists have found with their theories.

And not everyone is impressed with the elegance of the dark new universe. "It's a bit little ugly, isn't it?" said Dr. Turner. "A little of this, a little of that -- I think this would have been considered appalling a decade ago."

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