If one wasn’t enough to baffle the scientists, imagine a double whammy!! Collision of black holes. For more than 20 years, astronomers have considered an intriguing question: What happens when two black holes meet? Inside a galaxy, black holes that formed from dead, massive stars might encounter each other, especially in double or multiple star systems. No one has yet seen such a collision take place, but the subject is becoming a hot topic of theoretical astrophysics.
Studying the possibilities took a big step forward in 2004, when a team of astronomers — Marc Favata of Cornell University, Scott Hughes of the Massachusetts Institute of Technology, and Daniel Holz of the University of Chicago — authored a study that appeared in Astrophysical Journal Letters; this kicked off a number of other studies to create a burgeoning field. It turns out astronomers think a funny thing happens when black holes collide. They spiral toward each other and merge into a single entity. A gravitational “sling- shot” effect then violently whips them outside their host galaxies into intergalactic space.
The ejection mechanism results from a byproduct of the merger: gravitational waves. The gravitational waves actually shoot the merged black hole far away from the site of its merger. 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 currently on the table, and researchers are looking to build their knowledge of the subject.
A real breakthrough would come from observing a binary black hole — a black-hole merger in the making. “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. 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. Such 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, Santz Cruz. The only way to detect intergalactic black holes would be from gravitational-lensing effects, and current telescopes are unable to do that. Intergalactic black holes could absorb material without radiating, and so, continue along below the radar. “For that reason,” says Mitch Begelman of the University of Colorado, “We can’t rule out the possibility that black holes outside galaxies contain more mass than black holes inside galaxies.” The evidence for small black holes gone missing from normal galaxies does hold some potential.
According to David Merritt of the Rochester Institute of Technology in New York, “As we look at ever smaller galaxies, there’s a point where you stop seeing black holes.” Merritt and other astronomers wonder if smaller galaxies may have had their small stellar black holes shot out into intergalactic space.
Studying black-hole mergers could pay off big dividends when it comes to understanding large black holes in the early universe. Do they form by the process of mergers or by gradual accretion? Observational tests for determining how big black holes formed in the early cosmos are lacking; perhaps looking at how smaller black holes behave in more recent times will shed light on this question.