For almost 14 years, the Fermi Gamma-ray Space Telescope has orbited Earth at an altitude roughly a hundred miles higher than the International Space Station, quietly collecting evidence of gamma rays for researchers across the world.“When the gamma-ray comes into our detector, we don't focus it or anything like that. It hits an atom nucleus somewhere and basically explodes,” Kerr says.
“We know that at the heart of massive galaxies is a supermassive black hole,” Parthasarathy says. “In the early Universe, massive galaxies merged, which means that [their] supermassive black holes also merged.” “Each time a pair of supermassive black holes merge, they create ripples in spacetime,” Parthasarathy says.Strictly speaking, it isn’t just supermassive black hole mergers that cause such a disturbance. “When matter accelerates, it generates gravitational waves,” Kerr says. Even a person waving their hand generates an infinitesimally small gravitational wave.
“The longer the gravitational waves you're trying to search for, the bigger the detector you need,” Parthasarathy says.“If you want to detect these things, you need detectors that are galaxy-sized. We don't have the technology… to build detectors which are galaxy-sized,” Parthasarathy says. “When the gravitational wave background passes through one of the lines of sight [between a pulsar and Earth], the arrival time from that pulsar [will be] earlier or later by a billionth of a second,” Parthasarathy says.