Tiny echo of a tumultuous event

The void is rocking and rolling with invisible cataclysms. Astronomers revealed recently that they had felt space-time vibrations known as gravitational waves from the merger of a pair of mammoth black holes resulting in a pit of infinitely deep darkness weighing as much as 49 suns, some three billion light-years from here. This is the third black-hole smashup that astronomers have detected since they started keeping watch on the cosmos back in September 2015, with LIGO, the Laser Interferometer Gravitational-Wave Observatory. All of them are more massive than the black holes that astronomers had previously identified as the remnants of dead stars.

In less than two short years, the observatory has wrought twin revolutions. It validated Einstein’s long-standing prediction that space-time can shake like a bowlful of jelly when massive objects swing their weight around, and it has put astronomers on intimate terms with the most extreme objects in his cosmic zoo and the ones so far doing the shaking: massive black holes. “We are moving in a substantial way away from novelty towards where we can seriously say we are developing black-hole astronomy,” said David Shoemaker, a physicist at the Massachusetts Institute of Technology and spokesman for the LIGO Scientific Collaboration, an international network of about 1,000 astronomers and physicists who use LIGO data.

They and a similar European group named Virgo are collectively the 1,300 authors of a report on the most recent event that was published in the journal Physical Review Letters. “We’re starting to fill in the mass spectrum of black holes in the universe,” said David Reitze, director of the LIGO Laboratory, a smaller group of scientists headquartered at Caltech who built and run the observatory. The National Science Foundation (NSF), which poured $1 billion into LIGO over 40 years, responded with pride.

Remnants of stars

In the latest LIGO event, a black hole 19 times the mass of the sun and another black hole 31 times the sun’s mass, married to make a single hole of 49 solar masses. During the last frantic moments of the merger, they were shedding more energy in the form of gravitational waves than all the stars in the observable universe. After a journey lasting three billion years, that is to say, a quarter of the age of the universe, those waves started jiggling LIGO’s mirrors back and forth by a fraction of an atomic diameter 20 times a second. The pitch rose to 180 cycles per second in about a tenth of a second before cutting off.

Zsuzsanna Marka, an astronomer at Columbia University, USA, was sitting in an office on the morning of January 4 when she got an email alert. She started to smile but then remembered she was not alone and the other person was not a member of LIGO, so she couldn’t say why she was smiling. “I just kept smiling,” she said. Upon further analysis it proved to be a perfect chirp, as predicted by Einstein’s equations. Because of the merger’s great distance, the LIGO scientists were able to verify that different frequencies of gravity waves all travel at the same speed, presumably the speed of light.

As David said, “Once again Einstein triumphs.” Black holes were an entirely unwelcome consequence of his theory of general relativity that ascribes gravity to the warping of space-time geometry by matter and energy. Too much mass in one place, the equations said, could cause space to wrap itself around in a ball too tight and dense for even light to escape. In effect, Einstein’s theory suggested, matter, say a dead star, could disappear from the universe, leaving behind nothing but its gravitational ghost. Einstein thought that nature would have more sense than that.

But astronomers now agree that the sky is dotted with the dense dark remnants of stars that have burned up all their fuel and collapsed, often in gigantic supernova explosions. Until now, they were detectable only indirectly by the glow of X-rays or other radiation from doomed matter heated to stupendous degrees as it swirls around a cosmic drain. But what telescopes cannot see, gadgets like LIGO now can feel, or ‘hear’.

Gravitational waves alternately stretch and squeeze space as they travel along at the speed of light. LIGO was designed to look for these changes by using lasers to monitor the distances between mirrors in a pair of L-shaped  antennas in Hanford, Washington, and in Livingston, Louisiana. There is another antenna in Italy known as Virgo now undergoing its final testing. When it is online, having three detectors will greatly improve astronomers’ ability to tell where the gravitational waves are coming from.

The detectors were designed and built and rebuilt over 40 years to be able to detect collisions of neutron stars — the superdense remnants of some kinds of supernova explosions. Astronomers know such pairs exist in abundance, doomed someday for a fiery ending. Colliding black holes, being more massive, would be even easier to detect, but LIGO’s founders and funders at the NSF mostly did not know if there were any around to detect. Now they know.

The burning question

The current version of the observatory, known as Advanced LIGO, was still preparing for its first official observing run, in September 2015, when it recorded the collision of a pair of black holes 36 and 29 times as massive as the sun. A second collision, on December 26, 2015, was also confirmed to be massive black holes. A third event in October of that year was probably a black hole merger, the collaboration said. The burning question now is: Where did such massive black holes come from?
One possibility is that they were born that way, from a pair of massive stars orbiting each other that evolved, died, blew up and then collapsed again into black holes — all without either star getting kicked out of the system during one of those episodes of stellar violence. Another idea is that two pre-existing black holes came together by chance and captured each other gravitationally in some crowded part of the galaxy, such as near the centre, where black holes might naturally collect.

Astronomers won’t say which explanation is preferred, pending more data, but what David calls a “tantalising hint” has emerged from analysis of the January 4 chirp, namely how the black holes were spinning. If the stars that gave rise to these black holes had been lifting and evolving together in a binary system, their spins should be aligned, spinning on parallel axes like a pair of gold medal skating dancers at the Olympics, David explained. Examination of the January chirp, David said, gives hints that the spins of the black holes were not aligned, complicating the last motions of their mating dance. “It was not a simple waltz, it was more like a couple of break dancers,” he said.