When Giant Black Holes Collide

,,When giant black holes collide, the surrounding space-time trembles,,

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.

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.

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

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

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.

A computer generated simulation shows the event

of 2 Spiral Galaxies with Super-massive Black Holes Collide.

Computer Power

Scientists need a reasonably accurate approximation of what LISA will see when it finally begins observing gravitational waves. The waves of these events will be passing through our solar system from many directions, at many frequencies, with their waveforms exhibiting many different shapes. Astronomers need to understand what these waves will look like and how to separate the waves of different phenomena from one another. To accomplish this, several research groups have used computer simulations to predict the gravitational-wave signals.

These simulations employ several hundred high speed processors linked together and working in parallel. Still, the simulations that re-create the orbits and merger of two black holes can take days. If you tried to run one of the longer simulations on your own desktop computer, it would need to go nonstop for almost two decades.These simulations are so complex that even these super computers routinely crash. For one thing, black holes contain singularities, the central point where the density is infinity. Unfortunately, computers don’t handle infinity particularly well.

Scientists tried a variety of techniques to get around this problem. One trick was to remove the black holes from the equation. Another was to freeze the black hole binaries in position and allow the coordinate system to move about them. In addition, the equations in these simulations naturally produce many solutions, most of which are physically implausible. Unfortunately, teaching computers to distinguish between what is realistic and what is not is problematic. Worse, the unreasonable solutions are usually unstable and tend to increase in number. "Because the unstable answers grow so rapidly, they dominate," explains Joan Centrella (NASA/Goddard Space Flight Center). As a result, past attempts to simulate the orbits of black hole binaries, beginning in the 1960s and continuing through the mid-1990s, were unable to simulate even the beginnings of a binary's first orbit.
Though there were incremental gains throughout these years, it wasn't until 2005 that physicists suddenly succeeded in overcoming these difficulties. In April 2005 Frans Pretorius (now at Princeton University) created a simulation that lasted five full orbits. Unfortunately, his system of equations was different than everyone else's, making it difficult for others to adopt it. Then, in November 2005, the group led by Joan Centrella and John Baker at NASA/Goddard, and Manuela Campanelli's team at the University of Texas at Brownsville,independently came up with almost identical solutions that worked and could be used by almost everyone.

Both groups found that the presence of black holes themselves in the equations was less of a problem than expected, and that their complicated efforts to deal with them had actually made things more unstable. The two teams accounted for the fact that the only part of the black hole that interacts with the real world is the outer boundary,also known as the event horizon. They could essentially round off the singularity to a very small number with no consequences. Moreover, both groups found a method for allowing the black holes to move through the coordinate grid.

COLLIDING GALAXIES The Hubble Space Telescope

captured this image of The Mice, two galaxies in the process of

merging. Each galaxy probably has a supermassive black hole

at its center, and eventually the two black holes will sink to the

center of the merged galaxy and collide.

"With a very small change, suddenly all the codes in the world were working," says Centrella. "Every body was in the game. It had a huge impact." Or as Campanelli(who has since moved to the Rochester Institute of Technology) jokes, "It spread like a disease." Suddenly, it became possible for computer groups worldwide to simulate black hole mergers of all kinds, including black holes of equal and unequal masses, some spinning, some not,and with orbits both circular and eccentric. 

The results have been illuminating, when translated into a sinusoidal waveform. Physicists divide the merger process into three stages: inspiral, merger, and ring down. As the black holes spiral inward, the waves steadily increase in frequency. Then, as the merger stage begins,the amplitude increases suddenly and drastically.

The merger stage, however, is very short. Almost immediately the black holes absorb each other and the ring down phase takes over. Much like the fading of a ringing bell, the wave pattern dies off until it's gone. The simulations produced some interesting surprises. Though scientists had a good idea of what the wave forms would look like during inspiral and ring down, they had not known what the waveform of the merger stage would look like at the moment the two black holes collided. They had suspected that it would produce a complex pattern. To their surprise, the merger waveform is amazingly simple. 

Also, the waveforms of almost all types of binary mergers are surprisingly similar. Though the spins and relative sizes of the black holes affect the shape of the waveforms in the inspiral stage, once merger begins the wave forms all look remarkably alike. "In fact, the results don't depend on the initial conditions," notes Baker.

The Big Kick

These simulations have already begun telling astronomers something about the real world, at least a decade before LISA is launched. As two black holes plunge toward merger, the computer simulations suggest that - depending on their orbit, masses, and spins - the gravitational waves will fly outward asymmetrically, giving the merged black hole aside ways kick.

In some cases,such as when the two black holes are spinning in opposite directions, this kick could be as high as 2,500 miles (4,000km) per second - easily fast enough to fling the merged black hole from its host galaxy.

Not only should space be littered with invisible gigantic matter-sucking black holes flying about at great velocities, most galaxies shouldn't have super-massive beasts in their nucleus at all, according to these simulations. Since astronomers routinely find super-massive black holes at the center of galaxies, what keeps the merged black holes where they are? According to a model developed by Tamara Bogdanovic and her University of Maryland colleagues (including Reynolds), it all depends on the alignment of the black hole spins, combined with how much gas the host galaxies contain, prior to merger.

When two gas-rich galaxies merge, the gas acts to align the spins of the two central black holes, thus slowing the kick down sufficiently when they finally merge. In gas-poor mergers, however, there is nothing to align the spins, and the merged black hole ends up with a huge kick (S&T: September 2007, page 14)Astronomers at the Max Planck Institute in Germany have identified one possible candidate, a quasar showing a blue shift in its nucleus of about 1,650 miles per second relative to the rest of the quasar, suggesting that something very large there is flying outward at great speed.

"Most of us started out in general relativity, an Ivory Tower kind of business," says Baker. "Now we're rolling up our sleeves doing astrophysics. It's very exciting."..

BINARY BLACK HOLES?
Left: NASA’s Chandra X-ray Observatory captured this image of the center 
of galaxy NGC 6240, which is the product of a recent galaxy merger.

The two bright blue areas represent a double nucleus; each one is presumably powered

by a super-massive black hole. The black holes are about 3,000 light-years apart, so it will take 
millions of years for them to spiral inward and collide.

Right: This high-resolution radio image shows the center of the active galaxy 0402+379 (named for
its sky coordinates). The two bright spots, C1 and C2, are presumably powered by
super-massive black holes. The monsters are only about 24 light-years apart, meaning
they are close enough to feel each other’s gravity.

Source: Sky and Telescope Magazine, April 2009 and sources.

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