Seven years ago, researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) reported the first detection of gravitational waves. Now, the list of gravitational wave candidates number nearly 100.
And astronomers are hopeful they can find more of these ripples in the fabric of spacetime, caused by the acceleration of massive objects, such as two black holes spiral towards each other for a catastrophic meltdown.
May 24 marked the start of observation cycle 4 (O4), the most recent effort of the LIGO-Virgo-KAGRA (LVK) collaboration. With newly upgraded gravitational-wave detectors, astronomers hope the O4 can see gravitational waves – and the objects that produce them – a daily occurrence.
Related: Hunting for gravitational waves: the LIGO laser interferometer project in the photo
“We expect to go by what we had in our previous run – one neutron star every two months, one [binary] black hole every week or 10 days,” said Salvatore Vitale, an astrophysicist at the Massachusetts Institute of Technology (MIT), “to get a binary black hole every day or two, get a neutron star every week.”
Gravitational waves are a byproduct of general relativityas expected from Albert Einstein a century ago. General relativity holds that space and time are like a fabric. Every object leaves a dent in that fabric, which we perceive as gravity. In that world, disturbances, like two black holes colliding, can create ripples in the fabric. Astronomers can use laser-based detectors to pinpoint those ripples.
As LVK’s name suggests, the collaboration is a multi-pronged effort, combining four detectors on three continents: LIGOthe two detectors, one in Livingston, Louisiana and another in Hanford, Washington; Virgin in Europe, extending across the Tuscan plains southeast of Pisa, Italy; AND KAGRAbelow the mountains of central Japan.
Alas, at the start of O4, only the LIGO pair is fully operational. Virgin must undergo repairs to a damaged mirror and will remain disabled for an uncertain amount of time. KAGRA, meanwhile, will observe only for a month before going offline; it has not reached its target sensitivity and its operators hope to restart it again in late 2024.
Astronomers want more detectors because a single gravitational-wave detector doesn’t provide details about the direction the waves travel. So, they need more detectors to actually triangulate the source of the gravitational waves. With all four, astronomers could track a source within a few square degrees of the sky. With only two detectors, they’re stuck with a much larger wedge of sky.
“It will be more difficult for us to tell our friends with telescopes where to point their telescope,” Vitale said.
But even two detectors could reap a bounty of science. With increased sensitivity, detectors can detect weaker or more distant gravitational waves. This means that scientists can collect more events.
And with more events, they hope to begin answering a looming question: Where did the black holes they’re seeing form?
Maybe black holes formed inside galaxies; perhaps they formed outside, in globular clusters or dwarf galaxies. Or, perhaps, they are primordial, having formed in raw space at the beginning of the universe.
“To answer this question, you need a large dataset,” Vitale said.
The current of LVK plan it requires O4 to operate for 18 months, until 2025. After that, the gravitational-wave detectors will shut down for upgrades and engineering and start up again around 2027 for a fifth longer observing period.
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