But what role does this process play in building black holes? Could large numbers of black holes exist outside galaxies, where their presence would be extremely difficult to detect? These questions and others are still on the table. Although scientists have already detected gravitational waves from five merging pairs of stellar-mass black holes, researchers are tuning up their detectors for another observing run in early 2019, hoping more data will further illuminate the subject.
“Almost all large galaxies contain black holes,” says Hughes, “and galaxies merge like mad — especially a couple billion years ago.” Hughes believes binary black holes could have formed and be forming yet today, but detecting them observationally will be difficult. “We’re talking about two incredibly small bodies separated by a parsec,” he says, merely 3.26 light-years in galaxies that span hundreds of thousands of light-years across.
“Almost all large galaxies contain black holes,” says Hughes, “and galaxies merge like mad — especially a couple billion years ago.” However, detecting them is still difficult.
Black holes escaping their parent galaxies would be shot out at high velocities, probably 685,000 mph (1.1 million km/h). Such high-speed objects eventually might join other nomadic black holes in deep space. These freeform black holes would prove elusive. “If they’re not shining [from radiation produced by swallowing nearby bright material], it’s hard to know where to look,” according to Piero Madau of the University of California, Santa Cruz. One way to detect intergalactic black holes would be from gravitational-lensing effects. Another method, first outlined by University of Cambridge researchers in 2016, would be to measure Doppler shifts in gravitational-wave signals, which would reveal how hard a black hole is kicked.