The biggest crashes in the Universe

Scientists from the Albert Einstein Institute have simulated grazing collisions of two black holes. Such results will ultimately improve the search for gravitational waves.

September 06, 1999

For the first time scientists from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Golm near Potsdam, Germany, have simulated how two black holes merged into each other in a grazing collision. Fully three-dimensional simulations on supercomputers are essential for the planned detection of gravitational waves emited by two coalescing black holes.

Black holes are so dense that even light can’t escape them. That’s why it isn't easy to discover them. But in some years scientists hope to make progress when they open a new window in space. Gravitational waves which are literaly ripples in the fabric of space should be detectable with new instruments at the beginning of the next century.

The simulation shows the apparent horizons – the boundary (at a given time of the evolution) separating the outside world from the inside of the black hole, from which no light can escape to the outside. The horizons can be seen as bubble-like surfaces in the center of the simulation; the different colours indicate surface patches of various curvature.

At the Albert Einstein Institute scientists around Professor Ed Seidel are preparing for this search with numerical simulations which can provide the observers with reliable ways of recognizing the waves produced by black holes. "Colliding black holes are one of the hottest candidates for gravitational waves" says Prof. Seidel. In recents years he and others succeeded in simulating gravitational waveforms produced by disturbed black holes and in head-on collisions.

But interactions of two spinning black holes as they spiral in and coalesce are much more common and important in astronomy than directly head-on collisions. Such grazing collisions were calculated by Dr. Bernd Brügmann of the Albert Einstein Institute for the first time. However, due to the limited computer power available to him at that time, he was not able to calculate crucial details such as the precise signature of the gravitational waves emitted. This signature carried important information about the nature of the black holes in the collision. Brügmann has published the results recently in the International Journal of Modern Physics (D8 /1999/p. 85-100).

For the earlier calculation, Brügmann used a powerful Origin 2000 supercomputer at the institute. It has 32 separate computer processors working in parallel performing 3 billion computations per second. But this June, an international team including Brügmann, Seidel, and many others virtually owned a much larger 256-processor Origin 2000-Computer at the National Center for Supercomputing Applications (NCSA) at the University of Illinois. The team includes also researchers at the Washington University in St. Louis (USA) and the Konrad-Zuse-Zentrum in Berlin (Germany). The machine was used to provide the first detailed simulations of the kinds of grazing collisions of black holes of unequal mass, and spin that Brügmann had studied previously. Werner Benger at the Konrad-Zuse-Zentrum was able to create stunning visualizations of the collision process. They show how the "black monsters" from one to some hundred million solar masses merge, creating bursts of gravitational waves that may soon be detect by special instruments.

Visualisation of two colliding black holes, now also shown with gravitational waves generated by the merger process, displayed in colors from white (maximum of intensity) to red.

During the last moments, the black holes spiral inward, emitting weak gravitational signals periodicly. The symmetrical horizon of each object, from which light itself can’t escape, is stretched. In a very short time of some milliseconds or less, the two horizons coalesce like waterdrops. The amplitude and frequency of the gravitational waves are increasing strongly. What happens after that the scientist called ringing down - a decreasing signal like the latests sounds of a churchbell. The two united black holes form a new common horizon, which oscillates as it rings down and settles to a quiet final black hole.

One of the important results of the research work is the very huge amount of energy coalescing black holes emit in the form of gravitational waves. If two objects with 10 and 15 solar masses get closer the 30 miles and collide, the amount of gravitational energie may be around one percent of its mass. "This is thousand times more than the energy our sun emits during the latest five billion years", Dr. Brügmann says. But most of the biggest crashes in the universe scientist occur far away from earth. The signals should be extremely weak by the time they arrive here. It is expected to induce a space strain, which would jerk masses spaced at 0,7 miles by one thousandth of the diameter of a proton!

Construction of several detectors has started around the world. One is the German-British GEO600-Project, a 0,7 miles long laser interferometer, built by the Max Planck Institute for Quantum Optics and the University Hannover near Hannover. The scientists hope to measure the short transit of gravitational perturbations from inspiral black hole collisions but they expect only one event per year in a distance around 600 million lightyears. The computer models are needed to provide observers with reliable data of recognizing the waves produced by black holes. With the advancement of such supercomputer simulations, scientists now stand on the threshold of a new kind of experimental physics. "Astronomers now tell us that they know the locations of many thousands of black holes, but we can't do any experiments with them on earth. The only way we will learn the details is to build numerical substitutes for them inside our computers and watch what they do" explains Prof. Bernard Schutz, a Director of the Albert Einstein Institute. "I believe that studying black holes will be a key theme of astronomy in the first decade of the next century."

One of the long term goals is to simulate the interactions of two spinning black holes as they orbit around each other years before collision, but it will take much more computation time and needs new ideas to solve the equations of Einstein's General Theory of Relativity.

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