Gravitational wave hunters bag fourth black-hole detection

The VIRGO detector in Italy has helped narrow the search on the sky

Scientists have detected another burst of gravitational waves coming from the merger of two black holes.

The collision occurred nearly 2 billion years ago, but it was so far away that its shockwave has only just reached us.

This is the fourth confirmed detection made by an international team investigating Einstein's Theory of General Relativity.

Sheila Rowan of Glasgow University, UK, said the team was now on the threshold of a new understanding of black holes.

"It is tantalising to see this new story of how black holes formed and evolved through history of the cosmos," she told BBC News.

"This information is almost within our grasp but we are not quite there yet."

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Gravitational waves are ripples in space and time caused by cataclysmic events in the Universe such as the collision of two black holes or the explosion of a giant star.

They can be thought of as ripples in a pond caused by a pebble. But the pebble is the cataclysmic event and the fabric of the Universe, including everything and everyone on Earth is the water. Just as the water ripples, all matter is momentarily distorted as the gravitational wave passes through. But the distortions are minute - the entire Earth is stretched and squeezed by less than the width of an atom.

Gravitational detectors in the US and more recently in Italy have been able to detect this tiny warping effect - which was predicted by Albert Einstein 102 years ago.

Two labs in the US operated by the LIGO scientific collaboration made the initial discoveries in September and December 2015 and in January this year.

Event triangulation

These facilities were built a long way apart - one in Livingston, Louisiana, and the other in Hanford, Washington State - so that there would be a slight difference in the time when each sensed the passing waves. This delay enabled researchers to calculate very approximately where on the sky the collisions had occurred.

But the area of uncertainty was huge - a region that 3,000 full moons would occupy.

This has now changed with the addition of a third detection system based near Pisa in Italy run by the VIRGO scientific collaboration.

Having all three labs sense the waves narrows down the source to an area of just 300 full moons.

Although that still seems very uncertain, it makes all the difference to astronomers who want to try to see if there is any electromagnetic radiation (light at all wavelengths) also emanating from a collision - information that could yield further insights on the event.

Challenged expectations

The latest detection was the first to include the VIRGO detector and so the first for which astronomers could swing their telescopes into a more focused search zone.

Giovanni Losurdo, who led the VIRGO project to completion, described the inclusion of the Italian based-detector as a "milestone".

"The whole enterprise was based, since its start, on a visionary goal: the creation of a network capable of localising the sources in the sky and to start the era of investigation of the Universe. And finally, after decades, we are there."

The new collision, picked up at 10:30 GMT on 14 August, was of two black holes that were approximately 25 and 31 times the mass of our Sun. The trio of labs put the origin of the waves in the direction of the constellation Eridanus.

The binary black hole system was more than 10,000 billion billion miles away.

A signal from far, far away

Artwork: The merger of the two black holes - one 25 times the mass of our Sun, the other 31 times the mass of our star - produced a new spinning black hole with 53 times the mass of the Sun. This means that about three solar masses were converted into gravitational-wave energy during the coalescence. It is this energy that was detected by LIGO and VIRGO on 14 August. The merger took place 1.8 billion light-years from Earth, meaning the signal took 1.8 billion years to reach us.

As it turned out astronomers were not able to detect any light from Eridanus that could be ascribed to the merger, which has been given the designation GW170814.

This null result is actually extremely interesting because electromagnetic radiation would be emitted by matter in the area of the collision, and this suggests that the single black hole created in the union sucks up all the matter around it - as many theories predict.

Scientists were also, for the first time, able to measure a property of gravitational waves called polarisation. This describes the direction in which the waves vibrate. It was only possible because the VIRGO detection system was added to the network. It was able to measure the gravitational waves form a completely different direction to the US labs.

All four collisions detected to date involve black holes of a similar size. This has come as a surprise to many astronomers, including Prof Martin Hendry of Glasgow University.

"They are more massive than we may have previously expected," he told BBC News.

"We were expecting to get some black holes that were 20 or 30 solar masses, but now it's beginning to look as if this is the norm rather than being unusual.

"We can't read too much into just four confirmed events but what we have is starting to hint at pathways to form such systems which might allow us to rule out some models. We can't do that just yet but it would appear that if they are always as massive as this, we may not need too many more [detections before we can draw some conclusions about how black holes form].

The LIGO system is to be upgraded to make it even more sensitive and will be back online in a year's time. And there are plans to add detectors in Japan and India which will enable researchers to detect less violent gravitational distortions and perhaps discover objects and phenomena in the Universe that we cannot yet even imagine.

• A laser is fed into the machine and its beam is split along two paths

• The separate paths bounce back and forth between damped mirrors

• Eventually, the two beams are recombined and sent to a detector

• Gravitational waves passing through the lab should disturb the set-up

• Theory holds they should very subtly stretch and squeeze its space

• This ought to show itself as a change in the lengths of the light arms

• The photodetector captures this signal in the recombined beams

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