Gravitational wave hunters bag fourth black-hole detection

 In Science

Virgo DetectorImage copyright
Virgo Collaboration

Image caption

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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Media captionFive reasons why gravitational waves matter

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.

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