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3 Stars Provide First Evidence of Dark Matter

TIMES SCIENCE WRITER

Sudden, stunning flashes of brilliance by three nearby stars have led scientists to conclude that 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% or more of all mass in the universe.

Two teams of scientists--one American and Australian, the other French--reported Monday at conferences in Italy that three stars in the nearby 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.

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

The unusually intense increase in brightness has led some scientists to wonder if the star was simply some unusual type of “variable” star, a class of stellar objects whose brightness waxes and wanes because of internal instabilities or other factors.

“Extraordinary results require extraordinary proofs,” University of Chicago astrophysicist Michael S. Turner said. “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.

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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.

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Indeed, theoretical astrophysicists believe that visible matter may account for only 0.1% to 10% of the universe. The rest is dark matter, whether planet-size 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 if MACHOs are the primary form of dark matter or only a fraction of it, they said.

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

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“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, the reaches of space are so vast that the chances of seeing the 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.

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Fritz Zwicky of the California Institute of Technology first observed this phenomenon in the 1930s. Forty years later, Vera Rubin of the Carnegie Institution of Washington realized that dark matter played a central role in holding galaxies together and encouraging them to clump together--in short, making the universe work.

Ever since, astronomers have been trying to explain dark matter. Some have suggested that it is made up of exotic subatomic particles called weakly interacting massive particles, or WIMPs. Others suggested clumps of listless ordinary matter, which they called MACHOs.

Although dark matter cannot be seen in any conventional sense, it can be “observed” through its gravitational pull on other objects, as when it bends the light from distant stars. To the observer on Earth, this makes the star briefly appear brighter.

Glimpse of Darkness

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Scientists recently saw the first sign of so-called dark matter when they noticed an ordinary star suddenly brighten. Unlike ordinary matter, which may comprise 10% or less of everything in the universe, dark matter cannot be seen directly because it does not emit radiation. But it has mass, so it exerts gravitational pull on everything from stars to the light emitted by stars.

1) Normally, a star radiates light evenly in all directions. A certain percentage reaches Earth, and that is the brightess level normally observed by astronomers.

2) If a clump of dark matter passes between the star and Earth, its strong gravity bends the star’s light so that more light reaches Earth. The result: the star suddenly appears brighter.


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