“Our LIGO teams have worked through hardship during the past two-plus years to be ready for this moment, and we are indeed ready,” Caltech physicist Albert Lazzarini, the deputy director of the LIGO Laboratory, said in a news release.
Lazzarini said the engineering tests leading up to today’s official start of Observing Run 4, or O4, have already revealed a number of candidate events that have been shared with the astronomical community.
“Most of these involve black hole binary systems, although one may include a neutron star,” he said. “The rates appear to be consistent with expectations.”
One such event, called S230518h, was detected last week. Researchers say that if they can confirm the data, the event was most likely caused by the merger of a faraway black hole and a neutron star.
The twin LIGO gravitational-wave detectors at Hanford, Wash., and Livingston, La., will be joined for O4 by the Virgo detector in Italy as well as the KAGRA observatory in Japan. Virgo is scheduled to take part in the run starting later this year. KAGRA will parallel LIGO’s observations for the next month, take a break for some upgrades, and then rejoin the run.
Gravitational-wave astronomers are confident that they’ve filled out their repertoire of cataclysmic collisions, thanks to the detection of two cosmic crashes that each involved a black hole and a neutron star.
“With this new discovery of neutron star-black hole mergers outside our galaxy, we have found the missing type of binary. We can finally begin to understand how many of these systems exist, how often they merge, and why we have not yet seen examples in the Milky Way,” Astrid Lamberts, a member of the Virgo collaboration who works at the Observatoire de la Côte d’Azur in France, said in a news release.
There’s still some mystery surrounding the detections.
“This doesn’t look much like a chirp, which is what we typically detect,” Nelson Christensen, an astrophysicist at the French National Center for Scientific Research, said in a news release. “This is more like something that goes ‘bang,’ and it’s the most massive signal LIGO and Virgo have seen.”
Christensen and his colleagues say the signal, known as GW190521, appears to have come from the violent collision of two spinning black holes that were about 85 and 66 times as massive as our sun.
The merger created an even bigger black hole that’s about 142 times as massive as the sun. It also released the equivalent of eight solar masses in the form of gravitational-wave energy, in accordance with Albert Einstein’s E=mc2 formula, the scientists said.
For decades, physicists have been fleshing out their theories regarding the nature and origin of black holes — concentrations of matter so massive and compact that nothing, not even light, can escape their gravitational grip.
Some black holes are thought to be created when stars up to 130 times as massive as our sun run out of their fusion fuel and collapse inward, producing black holes as big as 65 solar masses. There are also scenarios in which stars that weigh more than 200 solar masses can collapse into black holes in the range of 120 solar masses.
Not that long ago, scientists thought the physics behind gravitational collapse ruled out the creation of black holes between 65 and 120 solar masses. Instead, the collapse of midsize stars was thought to produce instability through the creation of electron-antielectron pairs — and as a result, the stars were supposed to blow themselves completely apart.
Now, the fact that one of the black holes involved in last year’s smashup was measured at 85 solar masses is complicating claims for the existence of a pair instability mass gap.
“The fact that we’re seeing a black hole in this mass gap will make a lot of astrophysicists scratch their heads and try to figure out how these black holes are made,” said Christensen, who is the director of the Artemis Laboratory at the Nice Observatory in France.
The authors of the paper published in the Astrophysical Journal Letters have already come up with one possibility: Perhaps the midsize black hole was not directly produced by a stellar collapse, but instead by an earlier merger of smaller black holes — just the sort of merger that LIGO and Virgo have been detecting over the past five years.
“This event opens more questions than in provides answers,” said Caltech physicist Alan Weinstein, a member of the LIGO collaboration. “From the perspective of discovery and physics, it’s a very exciting thing.”
Weinstein cautioned that there’s still some uncertainty about the current explanation for GW190521.
“Since we first turned on LIGO, everything we’ve observed with confidence has been a collision of black holes or neutron stars,” he said. “This is the one event where our analysis allows the possibility that this event is not such a collision. Although this event is consistent with being from an exceptionally massive binary black hole merger, and alternative explanations are disfavored, it is pushing the boundaries of our confidence. And that potentially makes it extremely exciting.”
Further observations from LIGO and Virgo could turn up something completely new in the gravitational-wave menagerie — for example, evidence for the creation of primordial cosmic strings.
The LIGO project is funded by the National Science Foundation and operated by Caltech and MIT. It relies on two gravitational-wave detectors that have been built at Hanford, Wash., and at Livingston, La., with about 2,000 miles of separation to provide a double-check on the detectors’ results.
Each detector consists of an L-shaped network of tunnels, measuring 2.5 miles on a side, with laser beams reflected back and forth within the tunnels. Gravitational waves from far-off cataclysms disturb the fabric of spacetime ever so slightly — but the detectors are sensitive enough to pick up such disturbances to within the width of a proton.
Europe’s Virgo detector, designed according to a similar scheme, provides further verification for LIGO’s gravitational-wave data and makes it easier to triangulate on the sources of the waves.
In the case of GW190521, the waves are thought to have been thrown off by a source that’s now roughly 17 billion light-years (5 gigaparsecs) away from us — at a time when the universe was about half its current age of 13.8 billion years. That makes it one of the most distant gravitational-wave sources detected so far.
Because light travels at a finite speed, it may seem counterintuitive for a signal that was sent out 7 billion years ago to be received from an object that’s 17 billion light-years away.
But in an email exchange, Weinstein told me there’s a relatively simple explanation for the seeming mismatch. “The universe has been expanding since the gravitational waves were emitted,” he wrote. “So physical distances are tricky to interpret in an expanding universe, and must be treated with care.”
Japan’s Kamioka Gravitational-Wave Detector, or KAGRA, is due to start teaming up with similar detectors in Washington state, Louisiana and Italy in December, boosting scientists’ ability to triangulate on the origins of cataclysmic cosmic events such as black hole smash-ups.
Representatives of KAGRA, the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO) and Europe’s Virgo detector signed a memorandum of agreement today in Toyama, Japan, to confirm their collaboration. The agreement includes plans for joint observations and data sharing.
“This is a great example of international scientific cooperation,” Caltech’s David Reitze, executive director of the LIGO Laboratory, said in a news release. “Having KAGRA join our network of gravitational-wave observatories will significantly enhance the science in the coming decade.”
Nobel-winning physicist Takaaki Kajita, principal investigator of the KAGRA project, said “we are looking forward to joining the network of gravitational-wave observations later this year.”
Astronomers and their fans have been talking about the detections for days, thanks to the fact that LIGO and Virgo are quickly sharing the raw results from their current observing run. But today’s statements provided the most authoritative views from researchers running the two gravitational-wave detectors.
The April 26 detection of a cosmic collision known as S190426c is the most intriguing event. The subtle signal of a far-off disturbance in the gravitational force was picked up by LIGO’s twin detectors at Hanford in Eastern Washington and at Livingston in Louisiana. The Virgo detector in Italy also detected the signal.
The signal is consistent with what might be expected if a black hole were to swallow a neutron star, roughly 1.2 billion light-years from Earth. Such an event has never been observed before.
Instead, they’ll all be bearing down for the most serious search ever conducted for signs of merging black holes, colliding neutron stars — and perhaps the first detection of a mashup involving both those exotic phenomena.
Both experiments have been upgraded significantly since their last observational runs, resulting in a combined increase of about 40 percent in sensitivity. That means even more cosmic smashups should be detected, at distances farther out. There’s also a better chance of determining precisely where cosmic collisions occur, increasing the chances of following up with other types of observations.
The Aug. 14 event, known as GW170814, showed that the ripples in spacetime were emitted by the smash-up of two black holes about 31 times and 25 times as massive as the sun, located about 1.8 billion light-years away. The merger created a single black hole about 53 times the sun’s mass.
Three solar masses were converted directly into gravitational-wave energy, in accordance with Albert Einstein’s famous equation E=mc2.
All that follows the model set by LIGO with its three previous detections since September 2015. The new twist involves folding in the data from Virgo, which started its first full-fledged advanced run in league with LIGO on Aug. 1.