Researchers have found a new way to look for gravitational waves, the ripples in space-time caused by massive celestial objects exploding, rotating or merging. Physicists first sensed the waves in 2015 using laser-based detectors, and other scientists have been chasing them with ground-based radio telescopes. Now, the search moved to space. A new study reveals that data from the Fermi Gamma-ray Space Telescope could, in theory, also sense a passing wave. Although this technique is not accurate enough to make an actual detection, it is already helping other researchers to fine-tune their analyses.
Discovering Fermi’s ability to do this “was a huge surprise to us,” says team leader Matthew Kerr, a gamma-ray astronomer at the Naval Research Laboratory. When the telescope was launched nearly 14 years ago, “this was off the radar.”
Gravitational waves, predicted by Albert Einstein’s general theory of relativity, occur when massive masses – such as black holes or neutrons, the dense cores of burning stars – move violently, spin around and collide with each other. Since 2015, two large Earth-based detectors, the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US and Virgo in Europe, have detected dozens of black hole mergers and one pair of neutron stars. The detectors shoot lasers through kilometers of vacuum tubes. When the wave passes, it changes the length of the tube by no more than 1/10000 the width of the proton, which is then detected by the laser.
Radio astronomers are hunting for prey bigger than LIGO and Virgo: they’re looking for the gigantic giants, the union of supermassive black holes each weighing billions of dollars from the Sun. These black holes lurk in the centers of galaxies. When two galaxies merge, black holes are thought to orbit each other and slowly coalesce. Traditional telescopes would never choose such a pair in a distant galaxy, says Chiara Mingarelli, a gravitational-wave theorist of the University of Connecticut, Storrs. Gravitational waves “may be the only evidence we’ll ever see.”
Because the waves produced by such a spiraling duo are long — one cycle takes years to pass — catching them requires a galactic-wide network. Instead of using lasers and vacuum tubes, radio astronomers turn to pulsars, which are neutron stars that spew radiation from their poles. As it rotates, this radiation rushes across the sky like a supercharged beacon beam. On Earth, astronomers see flashes from some pulsars hundreds of times per second, arriving regularly like the chimes of an atomic clock. A passing gravitational wave will change the distance between the pulsar and Earth slightly, so by monitoring the arrival times of pulses from a group of pulsars across the Milky Way over many years — known as the Pulsar Timing Array (PTA) — astronomers hope to detect changes Minor refers to the passage of gravitational waves.
Last year, using data collected over a dozen years, PTA teams in North America and Europe announced that they had picked up faint statistical signals suggestive of something known as the gravitational wave background, which is a reflection of all supermassive black hole mergers across a large area. patch of the universe. Analyzing a few more years of data, which teams are now doing, may solidify these claims.
Now Fermi has entered the fray. Pulsars emit gamma rays, as well as flood radio waves. But many astronomers doubted that their instruments would sense enough to detect gravitational waves. Kerr and his colleagues decided to find out. They searched through 12.5 years of Fermi archives for gamma-ray photons from about 30 suitable pulsars. Unlike radio PTAs, which must target pulsars for short periods of time, Fermi constantly monitors a large patch of the sky, so many pulsars are always visible. But photons in the gamma range are so rare, “Fermi can look all week and not see any photons,” Kerr says.
Still, the team reports today in Science, their nets through the Fermi archive revealed enough photons to make PTA gamma rays. Like their radio colleagues, Kerr and his team have not been able to definitively reveal the background of the gravitational wave. But they managed to put an upper limit on the value of their signal. Kerr acknowledges that the gamma-dependent limit is about a third as narrow as that of the PTAs, but it will improve as Fermi collects more data. “So, if Fermi doesn’t fall from the sky, we’ll have a similar sensitivity” within 5 to 10 years, he says.
“This is a really interesting paper,” says Maura McLaughlin of West Virginia University, NANOGrav lead, one of the PTA radio teams. Although the gamma ray effort still plays a catch-up role, it can actually contribute. “One of the very useful things that gamma-ray data can do is help us understand the influence of the interstellar medium,” McLaughlin says, a major source of noise in PTA searches. This strand of particles and radiation can bend the path of radio waves and slow down some frequencies more than others, smearing the signal. But gamma rays get free passage, and by comparing signals from radio pulsars and gamma rays, researchers can better understand interstellar noise, and possibly learn about the signature of gravitational waves. Mingarelli, who is also a member of the NANOGrav team, says the gamma-ray signals are an “independent scale.” “It adds to the gravitational wave detection toolkit.”
Once the PTAs – radio and gamma rays – determine the gravitational wave background, the next target will be individual supermassive black hole binaries to discover how these swirling giant planets affect the galaxies around them. “It’s a whole new way of observing the universe,” Mingarelli says. “Who knows what we’ll find?”