Like sharks swimming beneath dark waters, black holes remain almost undetectable until they begin a feeding frenzy, swallowing stars and unleashing energetic radiation. Although those outbursts were most common in the early universe, new research reveals black holes as unpredictable predators. "We are finding that black holes are active even at cosmologically recent times," says Amy Barger, an astronomer at the University of Hawaii's Institute for Astronomy.



A giant black hole probably lies at the heart of the active galaxy Centaurus A, above. Long ago, the hole may have aided in the galaxy's formation.
Photo by European Southern Observatory
She and her colleagues made their discovery while studying the diffuse glow of X rays from countless black holes across the sky. Recently the researchers probed 20 of these X-ray sources in detail. At any given time around 10 percent of black holes are actively pulling in vast amounts of matter and spitting out radiation. Many of those that are active seem to remain so for an unexpectedly long 1 billion to 2 billion years. That means the population of massive black holes is several times larger than what astronomers can detect. And based on the new accounting, black holes generate nearly as much radiation energy as all the stars in the cosmos combined.

The prevalence and longevity of massive black holes imply that they played a significant role in sculpting the universe. Early on, gravity and radiation from the holes could have aided the formation of stars at the heart of galaxies. But some black holes refuse to settle down. "We need to learn what triggers some of them to turn on and start consuming matter at relatively late times," Barger says.