The collision of two supermassive black holes in the aftermath of a galaxy merger is the ultimate cosmic cataclysm, throwing out more energy for about an hour than all the visible stars in the entire universe combined. Moreover, in order to form the billions of majestic galaxies scattered across the sky, astronomers think that such events must have happened frequently as the first galaxies consolidated and grew.
Yet, though the heavens are strewn with innumerable coalescing galaxies, few super-massive black hole binary systems — the natural precursors to these titanic mergers — have been detected. As noted by a team of astronomers in one paper, "It has been somewhat embarrassing that very few cases of binary black hole systems have been confirmed." Fear not. At this very moment scientists and engineers are attacking the problem on multiple fronts. Not only are astronomers developing techniques for discovering super-massive black hole binaries, they even hope to someday watch them in real time, as the black holes themselves crash and merge.
Big Binaries
Astronomers define supermassive black holes as having a mass of at least ahalf million Suns. Observers have found these monsters in the center of everylarge galaxy that has been searched. The black holes formed as the galaxybuilt up by repeated mergers over billions of years. The merger process shouldalso have produced many galaxies with two such behemoths at their centers,with the black holes naturally sinking toward each other to form a binary. But after much searching, astronomers have only managed to culltogether a very short list of black hole binary candidates.
What makes these binaries so hard to identify has everything to do with their basic nature. They are located in distant galaxies, making them faintand difficult to see. More important, black holes, by definition, are invisibleand can only be traced by the effects their gravity produces on theirsurroundings. "It’s hard enough to find a single super-massive black hole,"notes David Merritt (Rochester Institute of Technology). "The chance thatyou would find two in the same galaxy is even smaller."
THE MAIN EVENT in these computer simulation frames from the Caltech/Cornell
Numerical Relativity Collaboration, two black holes plunge inward and merge, emitting
gravitational waves throughout the process.
gravitational waves throughout the process.
The dark objects at the top of each panel show the shapes of the black
holes; the black curve illustrates the inspiraling orbit of one of the holes.
The colored area below depicts the extreme curvature of space-time
produced by these massive objects. The depth of the indentation represents
the curvature of space, colors represent the rate at which time flows,
and arrows represent the velocity of the flow of space itself.
The most convincing binary black hole candidates show two energeticcomponents in the galaxy’s central regions, as if the galaxy had a doublenucleus. Of these, only NGC 6240 and 0402+379 clearly exhibit evidence oftwo super-massive black holes. In order to see both monster black holes in theother systems, the black holes have to be separated enough that theyaren'treally part of a binary system at all. "In NGC 6240, for example, the two blackholes are still thousands of light-years apart," explains Christopher Reynolds(University of Maryland). "They don’t feel each other’s gravity yet."With 0402+379, the two active nuclei are only about 24light-years apart, with an orbital period roughly estimatedas 150,000 years (S&T: August 2006, page 17).
As this issue went to press, Todd Boroson and Tod Lauer(National Optical Astronomy Observatory) reported a possiblebinary in a distant quasar. The black holes are separatedby less than half a light-year, and have an orbital period ofabout 100 years. This finding awaits confirmation.
Other possible binary candidates include "X-shaped"radio galaxies, of which only about a 100 are known. Thesesources exhibit two separate radio-emitting lobes at two different angles from their nucleus, forming an X. Theselobes are created by jets originating very close to thecentral black hole. One theory suggests that this X-shaped structure results from a rapid flip in the direction of the jet.Since jets are launched along a super-massive black hole’sspin axis, a black hole merger is one of the few ways to producesuch a rapid flip. But this theory remains unproven.
Then there is OJ287, possibly the most exciting super-massiveblack hole binary candidate of all. Every 12 yearsor so this active galaxy suddenly comes to life, emittingenergy across many wavelengths in two outbursts aboutone to three years apart. Astronomers have tracked theseoutbursts for the past century with increasing accuracy.Scientists theorize that the galaxy has two super-massive black holes, weighing approximately 18 billion and 20million solar masses, with the smaller black hole orbitingthe larger in an elongated orbit.
The double outbursts arethought to be caused when the smaller black hole makesits close approach every 12 years, diving through the centralblack hole’s accretion disk on its way in and out.In November 2005 the first of the double outburststook place, occurring almost 10 months earlier than predicted by Newtonian physics. But according to Einstein,when two such huge masses orbit each other, they bendspace itself, and as the bending changes from the orbitalmotion, it propagates outward from the system as ripplesof gravitational waves. The waves rob the system of orbitalenergy, causing the black holes to draw nearer, with theorbital period shrinking each orbit. It is this process thatcauses such black holes to merge.
Then there is OJ287, possibly the most exciting super-massiveblack hole binary candidate of all. Every 12 yearsor so this active galaxy suddenly comes to life, emittingenergy across many wavelengths in two outbursts aboutone to three years apart. Astronomers have tracked theseoutbursts for the past century with increasing accuracy.Scientists theorize that the galaxy has two super-massive black holes, weighing approximately 18 billion and 20million solar masses, with the smaller black hole orbitingthe larger in an elongated orbit.
The double outbursts arethought to be caused when the smaller black hole makesits close approach every 12 years, diving through the centralblack hole’s accretion disk on its way in and out.In November 2005 the first of the double outburststook place, occurring almost 10 months earlier than predicted by Newtonian physics. But according to Einstein,when two such huge masses orbit each other, they bendspace itself, and as the bending changes from the orbitalmotion, it propagates outward from the system as ripplesof gravitational waves. The waves rob the system of orbitalenergy, causing the black holes to draw nearer, with theorbital period shrinking each orbit. It is this process thatcauses such black holes to merge.
WARPING "SPACE-TIME"!!
A Rochester Institute of Technology team is one of several
groups that have successfully managed to get black holes
to collide inside a computer. The resulting
groups that have successfully managed to get black holes
to collide inside a computer. The resulting
calculations give scientists keen insight into what to
expect if experiments are devised to detect low-frequency
gravitational waves.
Assuming this was happening at OJ287, a team led byMauri Valtonen (University of Turku, Finland), predicted that the second outburst would occur on September 13,2007, about 20 days earlier than predicted by Newton'sequations. To their delight, the outburst began within oneday of their prediction, on September 12, suggesting thatOJ287 might be the first confirmed super-massive blackhole binary (S&T: April 2008, page 16).Yet there are doubts.
In order for this theory to work,the heavier black hole must be around 18 billion solarmasses, by far the most massive black hole yet observed."Whenever you have an indirect determination of somethinglike that and if it gives you a number that is thisextreme, you worry," says Reynolds. Worse, the evidence for all of these binary candidates merely includes measurements of the effects of the blackholes on nearby material.
In no case have scientists measured the black hole masses directly. To actually look at black holes and the effect they have on space itself will require gravitational-wave detectors.
Big Detectors
Big Detectors
The initial development of such a detector, the LaserInterferometer Space Antenna (LISA), has begun, though its funding prospects remain uncertain, and its launchis more than a decade away. A joint project of the European Space Agency and NASA, LISA will consist of threespacecraft flying approximately 3 million miles apart in atriangular formation, orbiting the Sun and trailing about20º behind Earth. Inside each spacecraft will be a cubes lightly smaller than 2 inches square and made of an alloy70% platinum and 30% gold, sealed inside a vacuum andshielded as much as possible from any solar and cosmicradiation that might impart an acceleration.
These cubes, called proof masses, will be allowed to float freely, with sensors carefully tracking each cube's location in order to keep the spacecraft centered around it. Lasers will also track the position of the three proof masses. When a gravitational wave sweeps through the solar system, the positions of the three masses will shift relative to one another by less than the width of a uranium nucleus.Like buoys on the ocean, one will bob upward before the next as the gravitational wave passes by. By measuring these incredibly tiny motions, LISA will detect the strength, direction, and frequency of the gravitational waves.
Moreover, because of LISA's location and the large distance between its three spacecraft, the mission will be specifically tuned to detect low-frequency gravitational waves.Since the larger-mass binaries tend to produce the lowest frequencies, super-massive black hole binaries will produce bass notes, making LISA ideally suited to study the in spirals and collisions of these titanic objects. "We would expect to see mostly massive black hole mergers early in the era of galaxy formation," explains Robin "Tuck" Stebbins(NASA/Goddard Space Flight Center). "We'll catch perhaps as many as 200 per year. "The technology to keep a proof mass floating independentlyinside a spacecraft, while also measuring its position, lies at the cutting edge of aerospace engineering. ESA — with some contribution from NASA — plans to first fly a prototype mission, dubbed LISA Pathfinder, scheduled for launch in early 2011.
If LISA Pathfinder works, and the full mission is then built and deployed, astronomers will have an instrument capable of measuring the gravitational waves emanating from inspiraling black holes near the beginning of the universe. And by measuring the waves from these events,astronomers will be able to track their orbits and actually predict the moment of collision.
Moreover, because of LISA's location and the large distance between its three spacecraft, the mission will be specifically tuned to detect low-frequency gravitational waves.Since the larger-mass binaries tend to produce the lowest frequencies, super-massive black hole binaries will produce bass notes, making LISA ideally suited to study the in spirals and collisions of these titanic objects. "We would expect to see mostly massive black hole mergers early in the era of galaxy formation," explains Robin "Tuck" Stebbins(NASA/Goddard Space Flight Center). "We'll catch perhaps as many as 200 per year. "The technology to keep a proof mass floating independentlyinside a spacecraft, while also measuring its position, lies at the cutting edge of aerospace engineering. ESA — with some contribution from NASA — plans to first fly a prototype mission, dubbed LISA Pathfinder, scheduled for launch in early 2011.
If LISA Pathfinder works, and the full mission is then built and deployed, astronomers will have an instrument capable of measuring the gravitational waves emanating from inspiraling black holes near the beginning of the universe. And by measuring the waves from these events,astronomers will be able to track their orbits and actually predict the moment of collision.