The merger of one black hole (artist’s illustration) with another created gravitational waves detected in January by the LIGO observatory.Credit: Aurore Simonnet (SSU/EdEon)/LVK/URI
The songs of the cosmos, now that we can finally hear them, might seem anti-climactic — more faint chirp than grand symphonic melody. But, over the past decade, those songs have helped scientists to fathom some of the biggest, strangest and most powerful events in the Universe.
Almost exactly ten years ago, the Laser Interferometer Gravitational-Wave Observatory (LIGO) became the first detector to ‘hear’ gravitational waves: ripples that waft through the fabric of space-time after being unleashed by the Universe’s most-violent collisions. The first detection, on 14 September 2015, recorded the tune sung by two black holes coalescing into one, and it confirmed Albert Einstein’s prediction that such gravitational waves exist. Now, LIGO, alongside other detectors, has detected some 300 gravitational-wave events, including one announced today that supports a theorem by cosmologist Stephen Hawking in the 1970s.
At each of LIGO’s twin installations — in Livingston, Louisiana, and Hanford, Washington — laser beams bounce down two arms, arranged in an L shape, that are each 4 kilometres long. A hairsbreadth misalignment of the beams indicates a squeezing and stretching of space-time, which registers as a wave on a computer read-out. When rendered as an audible sound, the wave resembles a bird’s chirp.
To celebrate LIGO’s first decade of discoveries, Nature asked gravitational-wave researchers about their favourite detections so far. Here are LIGO’s greatest hits, according to specialists.
The ring tone
Reported today in Physical Review Letters1, LIGO’s latest finding stems from the clearest gravitational-wave signal yet.
As the telltale signs of a gravitational wave on 14 January emerged from the noise of LIGO’s observations, researchers listened to two black holes fusing together. This time, however, they could also detect the vibrations produced by the single black hole spawned by the collision.
A technician inspects one of LIGO’s mirrors.Credit: Caltech/MIT/LIGO Lab/Matt Heintze
Typically, it’s difficult to pick out this ‘ringdown’ from the merger itself. But thanks to the ten years spent fine-tuning LIGO’s instruments, scientists made their most-sensitive measurements so far.
Analysis of the ringdown showed that the two parent black holes, which had a combined surface area of 240,000 square kilometres, birthed a much larger one, with a surface area of 400,000 square kilometres. The clear increase in size lends more credence to Hawking’s black-hole-area theorem, which posits that the surface areas these bodies can never decrease. Although Hawking passed away before he could witness this confirmation, he said in 2016 that he thought LIGO would be capable of such a feat.
This detection will quickly “become one of my favourites”, says David Reitze, executive director of the LIGO laboratory at the California Institute of Technology in Pasadena. “It was the perfect ten-year anniversary gift,” adds his colleague Katerina Chatziioannou, a LIGO physicist.
The first chirp
LIGO began operations in 2002, but it wasn’t until after an upgrade that wrapped up in 2015 that it picked up the first-ever murmurs of a gravitational-wave event. And it was an event, indeed. After five months of checking repeatedly that the signal wasn’t just background noise or a fake chirp, planted to test the detector, researchers announced their discovery with much jubilation at press conferences around the world. For the first time, scientists had proved that they could “listen to the thundering cosmos”, says Reitze, “even though the thunder was a tiny little blip by the time it got to Earth”.
Rainer Weiss (centre) and Kip Thorne (right), who were among the co-founders of LIGO, attend the 2016 news conference when scientists announced that the observatory had become the first to detect a gravitational wave.Credit: Gary Cameron/Reuters
That blip came from a black-hole merger 1.3 billion years ago2. That was how long it took for the collision’s subsequent ripples in space-time to cascade all the way to Earth. The detection “was the first observational evidence for the existence of binary black-hole systems”, the pairs of black holes that produce these cosmic crashes, says Gabriela González, a physicist at Louisiana State University in Baton Rouge and LIGO’s spokesperson at the time of the discovery.
The detection won the 2017 Nobel Prize for confirming the century-old prediction of gravitational waves from Einstein’s general theory of relativity. Einstein got one thing wrong, however: he thought gravitational waves would never actually be detected.
The light show
Mere seconds after LIGO felt the rumble of a gravitational-wave event on 17 August 2017, NASA’s Fermi Gamma-Ray Space Telescope spotted a flash of high-energy photons 40 million parsecs away. The two detections were anything but a coincidence.