Black holes collided two billion years ago and it was just detected on Earth

Another set of gravitational waves has been picked up by astronomers – the fourth time the phenomenon has been detected.

The waves – ripples in the fabric of space and time – were generated by the cataclysmic collision of two black holes nearly two billion years ago, and were captured by super-sensitive detectors in the US and Italy.

It is the first time the Virgo facility, near Pisa, has picked up a significant gravitational wave signal, marking a new turning point for scientists.

An aerial view of Virgo, near Pisa (The Virgo collaboration/CCO 1.0)
An aerial view of Virgo, near Pisa (The Virgo collaboration/CCO 1.0)

Gravitational waves are distortions in the fabric of space-time created by some of the most violent events in the universe.

Predicted by Albert Einstein 100 years ago, they cause anything in their path to stretch and compress by an unimaginably tiny degree.

It is this minuscule change – amounting to a distance 1,000 times smaller than the width of a proton, a sub-atomic particle found in the nucleus of every atom – the scientists are looking for.

Using a network of American and European detectors for the first time, the international team traced the latest source of gravitational waves to the merger of two black holes 1.8 billion light years away.

It has taken 1.8 billion years for the waves travelling at the speed of light to reach the Earth.

The event, code-named GW170814, produced a new spinning black hole with 53 times the mass of the sun. During its violent birth, the equivalent of three solar masses was converted into gravitational wave energy.

British scientists played a key role in the discovery, as they did in the first confirmed detection of gravitational waves in September 2015.

Professor Andreas Freise, from the University of Birmingham’s Institute of Gravitational Wave Astronomy, said: “Once again, we have detected echoes from colliding black holes but this time we can pinpoint the position of the black holes much more accurately thanks to the addition of the Virgo detector to the advanced detector network.

“Around 10 years ago I was in charge of designing the core interferometer of the Advanced Virgo project. To see that instrument become a reality, and now helping to deliver significant results, is really special.”

The LIGO in Washington (LIGO/Caltech/MIT/Sonoma State/Aurore Simonnet)
The Ligo in Washington (Ligo/Caltech/MIT/Sonoma State/Aurore Simonnet)

Dr John Veitch, from the University of Glasgow’s School of Physics and Astronomy, who co-led the team analysing the signal, said: “The addition to the network of a signal from Virgo provided us with a lot of useful data.

“Having a third detector means that we can now triangulate the position of the source, and much more accurately determine the exact spot in the cosmos where the signal came from.

“We go through multiple stages of analysis. The first is filtering the data from the detectors, which provides us with triggers for possible detections, which are then checked against the data from the other detectors.

“When a match between detectors is found, we can begin looking in more detail at the data to determine the mass and the position of the source, and start sharing data with other partners across the world.”

The first gravitational wave detection in 2015 was made by the Laser Interferometer Gravitational Wave Observatory (Ligo) in the US. It too was the result of a pair of colliding black holes wrenching the fabric of space-time.

Two more Ligo detections quickly followed, also traced to merging black holes.

Ligo consists of two L-shaped detectors 1,865 miles apart in Louisiana and Washington. Each arm of the L is a 2.5-mile pipe containing a system of mirrors.

A passing gravitational wave will cause a tiny mismatch in the length of the two arms. Laser beams fired through the pipes and bouncing off the mirrors are used to spot the discrepancy and alert the scientists.

The Virgo detector, based in Cascina, near Pisa, works the same way and has two 1.8-mile arms. A first version of the detector began operating in 2007 before a major upgrade to Advanced Virgo that was completed this year.

The new findings have been accepted for publication in the journal Physical Review Letters.


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