Space-Time "Ripples" Created by Merging Black Holes?

Brian Handwerk
for National Geographic News
March 4, 2009
Two newfound black holes on the verge of crashing into each other could be creating "ripples" in the fabric of space-time, astronomers suggest.

The two supermassive black holes orbit each other about once a century, separated by just a third of a light-year—and they seem to be getting closer, a new study says.

Such systems should be relatively common, but so far they have proven elusive, said study co-author Todd Boroson of the National Optical Astronomy Observatory in Tucson, Arizona.

"We believe that galaxies grow primarily by merging with other galaxies," he explained.

"If you have a black hole in the center of each, you'd expect to find systems where you have two black holes that gradually merge."

If confirmed, scientists will likely want to study this system intensely to figure out why they haven't seen others like it.

The binary black holes also could provide a unique environment to test aspects of Einstein's theory of general relativity, astronomers note.

The theory predicts that compact, massive bodies—such as black holes orbiting one another—should produce ripples in space-time that move at the speed of light.

Space-Time Ripples

Boroson and colleague Tod Lauer spotted the newfound binary black holes by examining data on more than 17,000 quasars from the Sloan Digital Sky Survey, a major mapping project that has so far imaged more than a quarter of the cosmos.

Each quasar is thought to represent a supermassive black hole at the center of a distant galaxy. Quasars are extremely bright because matter gets superheated as it falls into a black hole, sending out huge amounts of energy.

(Related: "First Triple Quasar Found, May Shed Light on Early Universe.")

But one quasar in the survey displayed two distinct sets of energy emissions moving at different speeds. The astronomers interpreted this to be two black holes spinning around each other.

According to Einstein's theory, such a system might be producing an effect called gravitational radiation, noted Jon Miller of the University of Michigan in Ann Arbor. Miller was unaffiliated with the research but reviewed it for publication in this week's issue of the journal Nature.

Although cosmologists have yet to see the effect directly, they think gravitational radiation propagates across the universe like waves on the surface of a puddle and remains unchanged as it travels.

Because the ripples reach us in an unaltered state, their strength, direction, and frequency could tell us more about the distant, dramatic events that created them, such as supernovae and black hole mergers.

Supermassive Merger?

Study author Boroson added that this particular pair of black holes may be in a key phase of a union.

Theoretically, gravitational interaction with stars and gases will help push two nearby black holes closer together.

But at a certain point there may be nothing left for the black holes to interact with, because they will have swallowed or thrown out all nearby matter. Then they become stuck in close orbit until something gives them a nudge.

The black holes in question now seem to be close enough to have gotten at least partially past this sticking point, Boroson said.

According to the researcher, the "endgame story" of black hole pairs like this one is to merge into a single massive object.

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