SPACE.com -- Possible First Direct Evidence That Black Holes Exist

Dolan's data, and an artist's rendering of the matter falling beyond the event horizon.

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	Possible First Direct Evidence That Black Holes Exist By Robert Roy Britt Senior Science Writer posted: 12:39 pm ET 11 January 2001

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A scientist announced today what he thinks is evidence of a hot blob of gas slipping past an event horizon, a theoretical point of no return that surrounds a black hole. If further research supports the finding, it would be the first direct evidence that black holes exist.

But the researcher, Joseph Dolan of NASA's Goddard Space Flight Center, cautioned that the small amount of data used in the study could contain a statistical error, and further research will be needed to confirm the finding.

"If we were trying to convict Cygnus XR-1 of being a black hole in court, we'd win a civil case that only needs a preponderance of the evidence, but nota criminal case, that requires beyond a reasonable doubt."

Other scientists urged caution in interpreting the results, which if proved true, would be a milestone achievement in the hunt for one of nature's most enigmatic creations.

Huge and hidden

A stellar black hole, like the one asserted in the new study, is theorized to be the collapsed corpse of a star and is relatively small by black hole standards. But it can still pack the mass of several Suns into an area no larger than a planet. This incredible density distorts time and space, sucking all nearby matter and energy inward, and letting nothing, not even light, escape -- which explains why black holes are utterly dark.

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No one has ever seen a black hole, but they are predicted by Einstein's general theory of relativity, and few scientists doubt their existence.

Finding an event horizon, a sphere around a black hole that represents the last gasp of "normal" activity, would prove the whole concept. So Dolan spent years in search of one, poring over old Hubble Space Telescope data of an object called Cygnus XR-1, a stellar black hole in our own galaxy, roughly 6,000 light-years away.

What he found in images from 1992 were bright flashes of ultraviolet light that sputtered and then disappeared. Dolan suggests that the signals -- he found two separate instances of them -- represent hot blobs of gas orbiting inward and spiraling through the event horizon.

Dolan presented his findings at a meeting of the American Astronomical Society meeting in San Diego, California.

The twilight zone

Cygnus XR-1, with a mass estimated at three to seven times that of our Sun compressed into an area no larger than Earth, is thought to be part of a binary system. The black hole sucks hot gas off of a large companion star, which is roughly 30 times as massive as our Sun.

This gas streams toward the black hole, going into an ever-tighter orbit. Along with other material, the gas forms a flat disk called an accretion disk.

Here's how the gas blobs would disappear, as predicted by the theory of black holes:

As a blob of hot gas approaches the point of no return -- the event horizon -- immense gravity stretches the light waves emanating from the hot gas. At a certain point, the wavelengths are stretched so far that they are no longer visible. The gas then crosses over into a sort of "twilight zone," where time and space no longer obey rules we understand.

Pulse trains

The sequence of pulses Dolan spotted -- six in one event and seven in the other -- each lasted just 0.2 seconds, diminishing in strength with each pulse. And then they were gone. Such an event, if this was one, is called a dying pulse train. (See click-to-enlarge image near the top-right of this page.)

"If pulse trains have clearly been detected, then it is indeed a milestone," said Niel Brandt, assistant professor of astronomy and astrophysics at Penn State. "However, the difficult part is in proving that they have indeed been detected. It is generally difficult to use a complex system, such as an accretion disk around a black hole, as a springboard to address fundamental issues."

The difficulty becomes more pronounced when working with limited data, Brandt told SPACE.com, adding that the finding could spur new research, using similar methods, into Cygnus XR-1 and other black holes in our galaxy.

In an e-mail interview, Dolan emphasized the preliminary nature of the findings.

"If we were trying to convict Cygnus XR-1 of being a black hole in court, we'd win a civil case that only needs a preponderance of the evidence, but not a criminal case, that requires beyond a reasonable doubt," Dolan said. "Finding an even horizon would put the evidence beyond a reasonable doubt."

Dolan notes that observations of many binary star systems suspected of having a black hole have proved that the dense central object is not a white dwarf star or a neutron star. Though these are also very dense objects, they are not massive enough to account for the activity scientists see.

"But we haven't ruled out the possibility of their being something more exotic than a black hole," Dolan said, admitting the possibility that black holes don't exist at all. But this would mean that Einstein was no Einstein, and a new theory of gravity would have to explain the complex activity around objects like Cygnus XR-1.

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