Physicists have detected ripples in the fabric of spacetime that were set in motion by the collision of two black holes far across the universe more than a billion years ago.

The event marks only the second time that scientists have spotted gravitational waves, the tenuous stretching and squeezing of spacetime predicted by Einstein more a century ago.



The faint signal received by the twin instruments of the Laser Interferometer Gravitational Wave Observatory (LIGO) in the US revealed two black holes circling one another 27 times before finally smashing together at half the speed of light.



The cataclysmic event saw the black holes, one eight times more massive than the sun, the other 14 times more massive, merge into one about 21 times heavier than the sun. In the process, energy equivalent to the mass of the sun radiated into space as gravitational waves.

An animated gravitational waves explainer from the University of Birmingham.

“This is confirmation that there’s a real population of black holes out there waiting to be detected in the future,” said John Veitch, an astrophysicist on the LIGO team at the University of Birmingham.



In February, researchers on the instrument made the historic announcement that they had picked up gravitational waves for the first time. The twin pieces of equipment, one in Hanford, Washington state, the other in Livingston, Louisiana, recorded the ripples in September 2015 as minuscule distortions in laser beams sent down 4km-long tubes. The detectors are so sensitive they can pick up changes in length one thousandth the diameter of a proton.



Writing in the journal Physical Review Letters on Wednesday, the LIGO team describes how a second rush of gravitational waves showed up in their instrument a few months after the first, at 3.38am UK time on Boxing Day morning 2015. An automatic search detected the signals and emailed the LIGO scientists within minutes to alert them.



“Everyone was still flabbergasted by the first discovery. We were writing up papers and preparing for them to be released when we had this second one. We thought ‘phew!’, it’s definitely real!” said Veitch. Another apparent signal received in October has not yet been verified.



The latest signals arrived at the Livingston detector 1.1milliseconds before they hit the Hanford detector, allowing scientists on the team to roughly work out the position of the collision in the sky. At least one of the black holes was spinning.



Closed for upgrade work in January this year, LIGO is expected to switch back on in the autumn with improvements that will nearly double the amount of the universe it can observe.

According to Will Farr, another LIGO researcher at Birmingham, pairs of black holes slam into one another on average once every 15 minutes in the observable universe.



“For me, the first detection broke everything wide open, but there was always the possibility that we had got phenomenally lucky. With the second signal it’s clear we are starting to see a population. We are going to see many of more of these in the next run,” he said.

