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Scientists Report Evidence of Galactic Dark Matter

By Mark A. Stein
Los Angeles Times

Sudden, stunning flashes of brilliance by three nearby stars have led scientists to conclude they have seen the first direct evidence of one form of dark matter, the mysterious and until now unseen phenomenon that is believed to account for 90 percent or more of all mass in the universe.

Two teams of scientists, one American and Australian and the other French, reported Monday at conferences in Italy that three stars in the Large Magellanic Cloud briefly grew brighter than usual, almost certainly because gravity from dark matter bent the star's light rays into focus on Earth.

Scientists said this particular type of dark matter most likely is in the form of a "brown dwarf," a blob of cold, listless gas the size of Jupiter. The example they detected is one of what is thought to be an ocean of billions of brown dwarfs engulfing the Milky Way galaxy.

The American and Australian researchers, led by Charles Alcock of the Lawrence Livermore National Laboratory near San Francisco, said they used a computerized, automated telescope to find a star that grew seven times as bright as usual before returning to normal.

"This is an exciting result from a very elegant experiment," said P.J.E. "Jim" Peebles, a Princeton University astrophysicist.

The unusually intense increased brightness has led some scientists to wonder if the star was simply some unusual type of "variable" star, a class of stellar objects whose luminations wax and wane because of internal instabilities or other factors.

"Extraordinary results require extraordinary proofs," said University of Chicago astrophysicist Michael S. Turner. "They're going to have to prove to their colleagues that this is not some weird sort of variable."

But the independent French findings seem to strengthen the American and Australian results.

The European team, led by Michel Spiro of the French national laboratory in Saclay outside Paris, recorded stars on old-fashioned photographic plates, then used electronic cameras to analyze the plates. They found two much-dimmer examples of microlensing in a different part of the Large Magellanic Cloud.

The Large Magellanic Cloud is the larger of two small, irregular galaxies that orbit the spiral-shaped Milky Way. It is about 169,000 light-years from Earth.

Even though only three events have been reported, the direct discovery of dark matter is a significant advance in cosmology because the gravity attributed to dark matter is essential to current theories about how the universe works.

According to what is known about the universe, ordinary visible matter -- stars, gas, dust and planets -- simply do not have sufficient mass by themselves to cause gas to condense into stars, stars to clump into galaxies and galaxies to gather into clusters.

Indeed, theoretical astrophysicists believe visible matter may account for only 0.1 percent to 10 percent of the universe. The rest is dark matter, whether planet-sized gas blobs like those reported Monday or exotic forms of subatomic matter predicted by theories but never seen.

Alcock and other scientists stressed that if the findings reported Monday eventually are confirmed as evidence of brown dwarfs -- also known as massive compact halo objects, or MACHOs -- that would not rule out other varieties of dark matter.

Nor would it determine whether MACHOs are the primary form of dark matter or only a fraction of it, they said.

But confirmation would indicate that current theories about the nature of dark matter are valid as are current approaches to detecting it.

"One doesn't make the case for having solved the dark matter problem by finding one single MACHO," said Peebles, a leading theoretician in the field. "The important thing is that they have shown they know how to find them. Now they need to go out and see how many there are, and how much mass they account for."

Although there are believed to be billions of MACHOs swarming the Milky Way, space is so vast that the chances of seeing this "microlensing" phenomenon are slim -- one in 2 million at any one moment.

To overcome those odds, Alcock and his colleagues used a state-of-the-art electronic camera attached to an automated telescope to record the brightness of 3.3 million stars every night. A computer then automatically compared these digitized readings over many months.

As its name suggests, dark matter does not emit light or any other radiation and so cannot be seen directly. But something in the universe is exerting extraordinary gravitational pull to make stars and galaxies move as they do; the gravity of the visible matter is not nearly enough by itself.