Scientists get a glimpse of universe's 'invisible' matter

For nearly 70 years, astronomers have struggled to explain "dark matter," which accounts for nearly all the matter in the universe. Although they can't see it, astronomers know it's there, because its gravity affects the matter astronomers do see.

Now, a research team claims it has found direct evidence of a portion of the dark matter affecting the Milky Way.

What have they found? A sizable number of dying, once-sunlike stars lurking in the vast expanse of dim, ancient stars in the Milky Way's halo. Called "white dwarfs," these objects could account for up to 35 percent of the galaxy's dark matter. The scientists say the unexpectedly large number they found challenges notions about the conditions that spawned stars during the galaxy's infancy.

"Our discovery ... has all sorts of implications that astronomers have tended to ignore. Now we have to deal with the implications," says Ben Oppenheimer, an astronomer at the University of California at Berkeley and the lead author of the paper reporting the results in today's issue of the journal Science.

Decades-old puzzle

The idea that the Milky Way was affected by unseen matter first appeared in 1932, when Dutch astronomer Jan Hendrik Oort proposed it to explain what appeared to him to be unusual motions among stars in the galaxy's disk. A year later, Cal-Tech astronomer Fritz Zwicky noted that galaxies he was observing didn't hold enough visible matter to explain their motion - the whirling galaxies should have been hurling stars into intergalactic space with abandon, but weren't.

Meanwhile, as researchers unraveled the story of the Big Bang, the enormous explosion out of which cosmologists conclude the universe emerged some 10 to 15 billion years ago, they calculated that the vast majority of the universe's mass must be in a form other than protons, neutrons, and electrons.

The search for dark matter became a quest to find WIMPs, weakly interacting massive particles that are not built from protons, neutrons, or electrons. In addition, they also sought some of the mass in the form of MACHOs, or massive compact halo objects made of ordinary matter.

Until 1995, astronomers suggested a range of candidates for MACHOs, Dr. Oppenheimer says. Brown dwarves, star-wannabes that failed to gain enough mass to ignite, mini black holes, and even orphaned giant planets headed the list of MACHO candidates.

In a halo that stretches as far as 160,000 light years from the Milky Way's center, such objects would be far too dim to detect directly. So two teams of scientists searched for indirect effects, such as gravitational lensing, in which light from distant objects is bent as is passes strong gravitational fields. Both teams uncovered "microlensing" by objects in the Milky Way's halo. From the effects, they calculated that the objects were about half as massive as the sun.

"Mention half a solar mass, and astronomers immediately think white dwarf," Oppenheimer says.

Many astronomers didn't take to the idea of a halo full of white dwarfs, he continues, because it threatened to upend their ideas about star formation in the early years of the galaxy, some 10 billion to 13 billion years ago. But the issue popped back up within the past two years, as scientists deepened their understanding of the phases white dwarves go through as they fade.

Last year, a team led by British astronomer Simon Hodgkin reported the discovery of a particularly cool class of white dwarf that also appeared to belong to the galaxy's halo, not its spiral disk, where a handful of others had been found.

This led the team, which included Oppenheimer, Dr. Hodgkin, and three others, to comb digitized photos of the sky for other candidates in the search for white dwarves in the halo.

After selecting 92 objects, the astronomers found that 38 of them were white dwarves. All but four of these had reached a stage in cooling that roughly translates to an age of from 9 billion to 10 billion years.

More mass in early galaxy

"There's something very exciting about finding ancient relics of the early parts of the galaxy," Oppenheimer offers. "It makes ancient Egyptian artifacts seem a bit young."

And like the Rosetta Stone, these dying stars could help unlock the secrets of the early galaxy, the researchers say.

The unexpectedly large number of white dwarves in the halo inferred from the team's results suggests that during the galaxy's birth, the nascent halo contained 20 times more high-mass stars than current star-formation theories allow.

Noting that the work bears on the question of dark matter, dramatically confirms new ideas about the evolution of white dwarves, and is likely to yield new insights into the galaxy's infancy, adds team member Didier Saumon of Vanderbilt University. "That's why it's such an exciting result. It's got so many implications."

(c) Copyright 2001. The Christian Science Monitor