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Last Updated: Wednesday, 19 April 2006, 16:57 GMT 17:57 UK
Black hole mergers modelled in 3D
By Jonathan Amos
BBC News science reporter

Simulations are the "fingerprint" to look for in real data

Simulations on a supercomputer have allowed Nasa scientists to understand finally the pattern of gravitational waves produced by merging black holes.

The work should help the worldwide effort that is currently underway to make the first detection of these "ripples" in the fabric of space-time.

Ultra-sensitive equipment set up in the US and Europe is expected to achieve the breakthrough observation very soon.

The new research will make it easier to recognise the correct signals.

"With these calculations, we are now able to know what will be the distinctive gravitational wave signature that comes out from just outside merging black holes," commented Professor Peter Saulson, who is part of the Laser Interferometer Gravitational Wave Observatory (Ligo) Scientific Collaboration.

"And by looking for this signal, we will be able to learn whether Einstein's Theory of General Relativity is correct or whether there is even stranger physics ahead for us in the future."

Tell-tale sign

Researchers believe their first detection of gravitational waves is imminent. Confirmation would be regarded as a major scientific advance, and would usher in a new way of studying the Universe.

Any accelerating object should send these waves of energy radiating outwards at the speed of light; but only truly massive bodies, such as exploding stars and coalescing black holes, would disturb space-time sufficiently for our technology to pick up the signal.

Ligo lab (NSF/Ligo Laboratory)
Laser interferometers bounce light beams back and forth down long tunnels
Passing gravitational waves should produce small disturbances in the light arms
The set-ups are sensitive to deviations that are fractions of the width of a proton
Different events, such as merging black holes, should have a unique signature
Scientists hope eventually to launch space-based laser interferometers
Observatories - such as Ligo, based in Louisiana and Washington states, and GEO 600 in Germany - bounce lasers down long tunnels, hoping to pick up the fantastically small disturbances the waves should generate as they pass through the Earth.

The new simulations, performed on the US space agency's Columbia supercomputer at the Ames Research Center in California, give the wave hunters a clear profile to look for in their data.

Nasa astrophysicist Joan Centrella described the simulation results as the "fingerprint" that would betray the existence of gravitational waves.

"To get this fingerprint, we have simulated the merger of two black holes by translating Einstein's equations into a way that computers can understand them. It's easy for me to say that but this has been a 'Holy Grail' quest for the last 30 years," she told a news conference.

Results 'confidence'

Previous efforts to model black hole mergers had "crashed and burned", said Dr Centrella, whose team reports the simulations in the journals Physical Review Letters and Physical Review D.

Black holes - extreme regions of space where even light cannot escape the pull of gravity - were so exotic that computers had enormous difficulty grappling with the calculations involved, she added.

Columbia supercomputer (Nasa)
Columbia used new formulations of Einstein's calculations

But Columbia's enormous power has finally cracked the problem. The 3D simulations used more than 2,000 of the machine's 10,000 64-bit processors, running over a period of 80 hours.

They modelled the merging of equal-mass, non-spinning black holes, starting at various positions corresponding to the last two to five orbits before the holes fell on to each other.

With each simulation run, regardless of the starting point, the black holes orbited stably and produced identical waveforms during the collision and its aftermath.

This unprecedented combination of stability and reproducibility assured the scientists that the simulations were true to Einstein's equations, Nasa said in a statement.

The results apply both to the smaller black holes created when giant stars collapse and the supermassive black holes that observations show lurk at the centres of galaxies.

Big prize

"Right now, Ligo is searching the sky. It can see the stellar-mass version of these calculations - two black holes about the mass of the Sun out to about 150 million light-years away, a distance that includes several thousand galaxies," said Professor Saulson.

Map of laser interferometers (BBC)
Laser interferometers are being constructed across the world
The installations will see into the cores of exploding stars
They will make it possible to trace the outline of black holes
Space observatories will probe the first moments of creation
The new knowledge may lead to a unified theory of physics
"The Ligo collaboration is searching through the data for this and other signals, and for all we know one of those events could go off tomorrow."

And Professor Jim Hough, from the University of Glasgow, UK, commented: "Centrella's work is superb. This is the first time there have been really good simulations for when the gravitational field is so strong. This is terribly useful because it means 'templates' can be developed for data analysis in the signals picked up by our detectors."

Unlike electromagnetic waves - the light seen by traditional telescopes - gravitational waves are extremely weak. If one were to pass through your body it would alternately stretch your space in one dimension while squashing it in another; but the changes are tiny.

Laser interferometers are looking for disturbances in their experimental set-ups that are equivalent to mere fractions of the diameter of a proton, one of the particles that make up the nucleus of an atom.

If the technology can be made to work effectively, it will allow scientists to probe the Universe in a way that is not dependent on light, and should, theoretically, allow them to look right back to the first moments after the Big Bang.

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