More gravitational waves as black holes clash

Two black holes, crashing into each other 1.4 billion light-years away, sent gravitational waves which hit us late last year.

The universe gave astrophysicists an unexpected Christmas gift last year. The super sensitive detectors in the US that, in September, directly picked up the first ever gravitational wave signal, received another on the night of December 25, local time.

An international team of researchers presented their second gravitational wave – an event named GW151226 – at the American Astronomical Society in San Diego today. It will be published in the journal Physical Review Letters.

“The fact that we have so quickly detected gravitational waves from a second pair of colliding black holes is very exciting as it suggests these events are more numerous than many researchers previously believed," says Eric Thrane, an astrophysicist at Monash University in Melbourne and part of the LIGO team.

Only a few months ago, in February, the Laser Interferometer Gravitational-wave Observatory (LIGO) team broke the news that their ultra-sensitive gravitational wave detectors picked up the minute ripples in space-time generated by colliding black holes just a few months after they were switched on.

Two black holes, one 29 times the mass of the sun and another 36, crashed together in a cataclysmic explosion that sent waves through space-time.

The two LIGO detectors, one on the east of the US and the other in the west, were stretched a tiny fraction of an atom's width as the waves rippled through Earth. And in doing so, they confirmed a major prediction of Albert Einstein's 1916 general theory of relativity. 

This more recent event involved much smaller black holes – only eight and 14 times the mass of the sun – to produce a final black hole 21 times the sun's mass. The remaining mass was converted into energy to produce gravitational waves.

While the December wave signal was not as strong as the more energetic September blast, it lasted several seconds as opposed to only around half a second. This is because the lighter black holes "fell in" relatively slowly, so emitted a weaker signal for a longer period.

This happened around 1.4 billion years ago. Since the collision, the waves have been travelling through space to reach Earth on 25 December 2015, US time. They first washed over the Louisiana LIGO detector, then the identical detector in Washington state 1.1 milliseconds later.

It took just over a minute for supercomputers to sift out the background noise. But when it did, it presented the wave signal with a confidence level of 99.99999%.

And there are hints of a third gravitational wave on 2 October, but with a lower degree of certainty.

The timing of the signal – 1.1 milliseconds – lets researchers estimate which part of the universe the black holes clashed. This will only improve when the Virgo instrument in Italy, due to come online later this year. will help triangulate future signals.

And researchers are aiming to further increase the instruments' sensitivity.

David McClelland from Australian National University, for instance, is working on quantum optical devices to "triple our searchable volume of the universe", he says.

So strap yourself in – you'll see a lot more from gravitational waves in years to come.