It's not just the postman who rings twice.
For the second time this year, scientists working with a sensitive gravitational wave experiment say they have picked up the vibrations of two distant black holes colliding in space.
The find confirms what scientists were hoping for when they switched on the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) last fall: Space is aquiver with signals generated by the violent motion of massive objects, and their detector is quite capable of picking them up.
LIGO made headlines in February when scientists reported for the first time that they had measured gravitational waves directly – triumphantly verifying a century-old prediction by Albert Einstein.
At the time, physicists noted that the detection may have been a lucky break because it involved the collision of two unexpectedly massive black holes, each about 30 times heavier than the sun. The event was detected on Sept. 14, 2015. As they spiralled in toward each other and collided, their gravitational energy rattled spacetime far and wide, producing a textbook signal that was immediately recognized by LIGO 1.3 billion light years away.
The second event, detected on Dec. 26 but only reported on Wednesday, was both more subtle and more typical of what advanced LIGO was built to detect. In this case, the two black holes measured about 14.2 and 7.5 times the sun's mass, and they produced a pulse of gravitational waves that was only one-third as strong as the first event, even though they were coming from about the same distance.
"We had to dig a little bit deeper into our noise to be able to see it," said David Reitze, executive director for the LIGO experiment, headquartered at the California Institute of Technology.
The team also announced a third possible signal spotted in October at the limit of the experiment's sensitivity. Although it was too weak to report as a definitive detection, scientists are enthusiastic that the universe appears to be giving LIGO plenty to listen to, with the prospect of more to come when science observations resume later this year.
"It's definitely important and encouraging that these weaker signals are there and are beginning to show up," said Harald Pfeiffer, a researcher at the Canadian Institute for Theoretical Astrophysics in Toronto and a member of the LIGO team.
The more events that the experiment is ultimately able to detect, the more researchers say they will be able to learn about the nature of black holes – bizarre objects with gravity so strong that not even light can escape them – and the history of their formation in the universe.
Gravitational waves offer the only direct way of observing what happens when two black holes merge into one, an extreme event that pushes the limits of current theory.
The waves are ripples in the fabric of space that cause everything they encounter to momentarily stretch and squeeze as they pass by. To detect them, researchers supported by the U.S. National Science Foundation built two identical facilities – one in Washington State and one in Louisiana – where laser light is reflected back and forth through four-kilometre-long tunnels that are constructed at right angles to each other.
A gravitational wave appears as a momentary difference in the time it takes light to travel down one tunnel relative to the other. By comparing data from the two facilities, scientists can rule out local effects that might mimic a gravitational wave signal and only focus on events that are seen by both.
A third gravitational wave detector, dubbed VIRGO and located near Pisa, Italy, is expected to come online by the end of the year. As VIRGO's sensitivity improves to match the LIGO detectors, the three-part network should vastly improve researchers' ability to pinpoint where in the sky different bursts of gravitational waves are coming from – and so improve the odds that astronomers may have time to zero in on a distant flash of light that corresponds to a black-hole collision or some other event.
Those other events might include the collisions of smaller but nearer objects, such as neutron stars in our own Milky Way galaxy. But even more tantalizing would be a signal unlike anything that physicists have predicted LIGO will see.
"That would be just wonderful," said David Shoemaker, a senior team member and director of the LIGO Laboratory at the Massachusetts Institute of Technology. "For me, that would be the proof that we were doing something that was going to lead science forward rather than verify science that has been done."