After a whirlwind tour of press conferences, parties and awards following their Feb. 11 announcement of the world’s first direct detection of gravitational waves, the team of scientists at the Laser Interferometer Gravitational-Wave Observatory are preparing to begin another round of listening for cosmic collisions.
Sometime this fall — likely September or October — the twin detectors in Livingston Parish and Hanford, Washington, will again begin collecting data in the search for signals similar to those detected on Sept. 14 from a 1.3 billion-year-old merger of two black holes.
Gravitational waves are distortions, or ripples, in the fabric of space and time caused by violent and energetic events such as colliding black holes or neutron stars, exploding supernovae or the birth of the universe itself. The ripples travel through the universe at the speed of light, carrying with them information about the cataclysmic events that created them.
LIGO’s detection of gravitational waves won the team numerous accolades, including a Special Breakthrough Prize in Fundamental Physics and the 2016 Gruber Prize in Cosmology, as well as recognition from the White House, U.S. Congress, the Louisiana Legislature and the LSU Board of Supervisors.
Team members also have seen their detected waveform transformed into popular culture: turned into music; printed on scarves, dresses and graduation caps; iced onto cakes; worn by an Olympic swimmer; inked as a tattoo and given its own Twitter account, @iamGW150914.
Brimming with confidence since last fall’s discovery, LIGO officials said they are almost certain to make detections from other black-hole mergers during the next six-month observing run. What they hope for, though, is a set of waves from something different — maybe a pair of neutron stars, maybe something entirely unexpected.
“If you ask my dream, what I would really like to see is an unknown,” said Gabriela Gonzalez, spokeswoman for the LIGO Scientific Collaboration and an LSU physics and astronomy professor. “A loud gravitational wave that we see in Hanford and Livingston and that we can tell it’s a gravitational wave — a distortion of space-time — but we don’t know what produced it. I think that would be the most exciting.”
The Sept. 14 detection was not only the world’s first direct observation of gravitational waves but also the first confirmation of a merger of black holes, regions of space with gravitational pulls so strong that even light disappears into them.
The detection answered questions about the existence of such systems and whether LIGO’s upgraded interferometers — instruments that measure differences in the time it takes a laser beam to travel down 2.5-mile-long, perpendicular tubes — were sensitive enough to detect them.
The detection also prompted a flurry of scientific inquiry and discussion: Were the large black holes that LIGO detected — each about 30 times the mass of our sun — actually primordial black holes formed at the dawn of time? Were they some of the dark matter scientists say accounts for most of the material universe?
Is it possible that although black holes do not allow even light to escape their grasp, this merger produced the weak burst of high-energy rays that NASA’s Fermi Gamma-ray Space Telescope detected in the same region of the sky, within a half second of the gravitational wave detection?
“Very few models predicted a gamma ray that would be produced by a coalescence of black holes,” Gonzalez said. “I think the attitude of both Fermi and us is we’ll wait and see. If it’s a real coincidence (coinciding event), it should happen again with some other gravitational wave events in the future.”
Waiting to see
How many events the team will detect during the next round of observations is a matter of both extensive calculation and a bit of guesswork. The members can estimate a rate of detection based on known variables, but the events that cause gravitational waves are sometimes themselves unknown.
Before the Sept. 14 detection, LIGO scientists had focused their calculations on the mergers of neutron stars, not black holes. That’s because neutron stars — the dense remnants of collapsed stars — had been observed already through other means, like electromagnetic radiation, and were, thus, more predictable, said Joseph Giaime, head of the LIGO Livingston Observatory and a professor of physics and astronomy at LSU.
“If you’d asked any of us before, we probably would’ve said a binary neutron star (detection) was more likely because we had no idea how likely binary black holes were. This one really landed on our lap,” Giaime said. “With black holes, we knew we would have stronger signals, but we had no idea on a rate because no black hole binaries in that mass range had ever been observed by any means before ours.”
Until many detections are made, scientists will have difficulty predicting how often they are likely to occur, Gonzalez said.
“It’s very difficult to extrapolate from just one detection,” she said.
The team has not yet published all of its results from Advanced LIGO’s initial observing run, which lasted from Sept. 12 to Jan. 12, but it did record a second signal that raised a few eyebrows.
The second signal, recorded on Oct. 12, appeared to be from another black hole merger, but it was much weaker and statistically less significant than the Sept. 14 gravitational wave signal.
“It just wasn’t large enough that we were confident in telling the world it’s a gravitational wave,” Giaime said. “For the main signal (Sept. 14), we calculated a false-alarm rate — how often by random chance we would see it if there was no gravitational wave — and it was once every 200,000 years. Like, never. But with this one (Oct. 12), it was like one in two years, which is a little uncomfortably likely for scientists to say there was a gravitational wave.”
The signal appears to have come from a pair of black holes, roughly 23 and 13 solar masses apiece, colliding about 3.5 billion light years away, according to a paper the team published April 27.
Gonzalez said there’s an 85 percent likelihood that the signal was from gravitational waves, “but our standards are a lot higher. Only when you have many events can you consider these lower significance ones.”
The scientists are now fine-tuning the interferometers in Livingston and Hanford to prepare for the next observing period this fall.
The instruments, which underwent a five-year, $205 million upgrade before collecting data last year, have the potential to eventually see 10 times farther into space. They had reached about a third of their total target sensitivity before the first observing run and could be about halfway to the goal this time, according to figures Giaime provided.
LIGO officials hope their colleagues with the VIRGO Collaboration in Europe will have completed upgrades to their own detector near Pisa, Italy, in time to join LIGO’s observing period. Three detectors scanning the skies at the same time would allow the scientists to better triangulate the source position for any gravitational waves they might detect.
A new LIGO facility is in the works, as well, after the National Science Foundation signed an agreement with the Indian government March 31 for construction of a detector there. If all goes well, that detector could be built and operating in 2024 or thereabouts, Gonzalez said.
A similar project is underway in Japan, where scientists have built an interferometer in an old mine site. Kagra, as its called, has the potential to really advance the state of the art, Giaime said, because it is underground, away from many of the surface vibrations that interfere with detector readings and because the instrument will use cryogenic test masses, which also reduce noise.
“It’s an advanced technique we’re not doing in Advanced LIGO, and we’re grateful someone’s doing it,” Giaime said.
While ground-based detectors listen for the high-frequency gravitational waves produced by black hole and neutron star mergers, supernovae and pulsars, the European Space Agency is working on a project to put a detector into space that could listen for lower-frequency waves caused by much more massive objects.
“That detector, out of all of them, is the most exciting one, even though I work on LIGO, because the predictions are a lot firmer,” Gonzalez said. “They would see loads of coalescenses of these big black holes in the center of galaxies with very, very strong signals. … Like seeing galaxies collide.”
The Evolved Laser Interferometer Space Antenna, or eLISA, would be able to hear those signals because of its relative size. Where LIGO detectors have 2.5-mile long arms, eLISA would suspend its beam-bouncing mirrors a million kilometers apart, Gonzalez said.
In December, the LISA Pathfinder launched into space to test some of the concepts eLISA will use for its space-based detector. The team will announce the results of its first science mission June 7.
At each step, gravitational wave astronomy is pushing the boundaries of science and technology.
As LIGO Executive Director David Reitze, of Caltech, told the U.S. House Committee on Science, Space and Technology in February, “To make LIGO work, we had to develop the world’s most stable lasers, the world’s best mirrors and optics, some of the world’s largest vacuum systems, as well as push the frontiers of quantum measurement science and high-performance computing. We use a lot of technology, and all the technology we use, we advance.”
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