A New Window on the Universe
UT astrophysicist, Richard Matzner, uses Ranger to simulate binary black hole mergers
Between 1907 and 1915, Albert Einstein developed the theory of general relativity, which provides a geometric explanation of gravity. Beyond its precise description of easily observable phenomena, like planets revolving around the Sun, the theory predicts the existence of gravitational waves — ripples on the surface of a cosmic pond — caused by massive disturbances in space-time.
A description of the initial data for the collision of rotating black holes.
The science of relativity studies these gravitational disturbances, particularly those that accompany interacting black holes and neutron stars — objects a million times more compact than our Sun. Once the province of science fiction, black holes are turning out to be much more prevalent and important than previously thought. Every galaxy that has been investigated closely has a supermassive black hole at its core, and the size and rotation of the black hole appears to influence how the galaxy around it evolves.
"The merger of two black holes is the strongest possible gravitational wave source," Richard Matzner, professor of astrophysics and director of the Center for Relativity at The University of Texas at Austin, explained. "Anything extreme about relativity will happen in a black hole. That's why people are fascinated with these objects."
Matzner believes that science now stands on the verge of detecting gravitational waves for the first time. The NSF-funded Laser Interferometer Gravitational Wave Observatory (or LIGO), a large physics experiment attempting to detect subtle gravitational signals from great distances, has been operating at full sensitivity for the last four years. The discovery of gravitational waves, when it occurs, will open up brand new avenues of astrophysical study. "It will be a new way of looking at very distant parts of the universe," Matzner said, "and we anticipate that those results will be very important for cosmology."
The observation of gravitational waves will provide scientists on Earth with information about black holes and other mysterious objects in the distant Universe that cannot be observed with normal, light-based telescopes. It will also prove once again the validity of Einstein's theory of general relativity.
Solving the Einstein Equations
When Einstein postulated his theory, he created a series of mathematical formulae (the ‘Einstein equations'), which quantify the relativistic description of gravity and allow for the precise explanation of everyday occurrences, as well as the most gravitationally-intense events in the universe.
The complicated orbits of two black holes merging, as studied by Matzner and his collaborators.
These formulae are simple enough for scientists to write down and estimate solutions on a piece of paper. But to solve them precisely, and to realistically depict the behavior of two black holes circling each other and eventually colliding, requires the power of massive supercomputers. In fact, despite decades of study, it wasn't proven to be computationally possible until 2005, when F. Pretorius at Caltech in July, and two teams of scientists, from NASA's Goddard Space Flight Center and The University of Texas at Brownsville, in November, managed, by different methods, to simulate a binary black hole merger.
This breakthrough led to a flurry of research on the most powerful supercomputers to predict the different configurations that binary mergers might take: spinning or non-spinning black holes in various orientations and with different ratios of masses. Matzner, who has been working on black hole simulations for 15 years, is part of this large effort to conduct a survey of mergers to understand the gravitational waves they emit.
"It's considered a Grand Challenge problem because it has more variables to keep track of and compute than most things that scientists deal with. Also, it's nonlinear, and requires high resolution," Matzner said. "It keeps the biggest computers busy."
On Ranger, Matzner studies the merger of two black holes spinning at different speeds and in different directions. His simulations suggest that such a merger would cause a ‘kick'; the energy released would act like a rocket and propel the black hole from the center of a galaxy (where they are almost always found) to either an offset location or out of the galaxy altogether. Both scenarios represent new possibilities for cosmology.
"People have claimed that they've seen offset black holes in galaxies, but we haven't seen galaxies without a central black hole," Matzner said. The simulations lead Matzner to believe that black holes might regenerate quickly, a theory that would reconcile his models with astronomical observations. "That's not consistent with what most people think about how the center of a galaxy would behave, but it's possible."
The schematic shows the way that the openGR system distributes the computational load across processor cores.
In addition to driving new theories of the universe, Matzner's survey of black hole configurations produces gravitational waveform signatures. These signatures are characteristic of what the signal of a black hole merger might look like.
"This knowledge is crucial to make the gravitational wave observations possible and to separate the wheat from the chaff," said Lars Koesterke, a research scientist at TACC specializing in astrophysics.
To detect the presence of black hole mergers hundreds of millions of light-years from Earth, researchers need to know something about the signals these events produce. The LIGO observatory depends on these so-called ‘templates' to discover the signal through the noise. Researchers predict the signal from a black hole merger will induce a length change in the gravitational wave observatory less than a fraction of the size of a proton (10-19 meters over the course of LIGO's 4-kilometer-long arms) — a needle in space's haystack. "There aren't that many strong binary sources in the universe," Matzner said. "You have to look out very far in order to encompass enough galaxies to have a possible source, which makes the signals weak at the detector."
The data that come from LIGO's laser interferometers are currently being checked against a database of templates created by a combination of computational and approximation methods, to determine if an event has already been detected. It is unlikely that what is found will be a perfect match to any template, but a close fit will signify the high-likelihood of an observed event. Then the question will be: what was the signal a match for? And what kind of an event does that signal signify?
Such questions will send astrophysicists back to the supercomputers to perform high-resolution simulations that can reproduce such a measured signal.
Pushing Gravitational Codes to the Petascale
Another aspect of Matzner's research involves creating ever-higher resolution relativistic simulations of black hole mergers in order to coax more information out of scientists' virtual models of these events.
Richard Matzner, professor of astrophysics and director of the Center for Relativity at The University of Texas at Austin
The major breakthrough that occurred in computational relativity four years ago wouldn't have been possible without adequately powerful high-performance computing (HPC) systems. As supercomputers enter the multi-Petascale Era, growing exponentially more powerful, astrophysicists anticipate that new details about black holes will emerge, prompting fresh insights and predictions… as long as their codes can keep up with the pace of ever-expanding HPC systems.
Matzner's finite difference black hole simulations are arguably some of the highest resolution in the world. However, every advance in resolution leads to new insights about the nature and characteristics of black holes, he says. To that end, Matzner has developed a new simulation code, openGR, capable of parallel simulations up to 32,000 cores on Ranger. Though openGR's scaling isn't yet ideal, this kind of massive parallelism will be necessary to get further resolution out of simulations and help push the whole field of computational relativity forward.
It's a golden age of gravitational astronomy. With the LIGO observatory undergoing upgrades that will allow it to look at a volume of space 1000 times larger than it currently does, and a space observatory, LISA (the Laser Interferometer Space Antenna) — with the potential to detect black hole mergers anywhere in the universe — planned for the next decade, the discovery of gravitational waves is virtually assured.
But even the experts can't say for sure where this might lead. "It will be a new picture, a new window on the universe," Matzner said, "and whatever we detect will probably be a strange astrophysical object. Even if it's black holes, it will likely be a configuration that we're surprised by."
Matzner's collaborators on this work are: Andrea Nerozzi and Ulrich Sperhake, Jena Germany; Matt Anderson, BYU; Jon Allen, Paul Walter, Brandon DiNunno, and Sarah Miller, UT Austin; and James Healy, Frank Herrmann, Ian Hinder, Deirdre M. Shoemaker and Pablo Laguna, Penn State.
The work was supported by NSF grant PHY-0354842 and NASA grant NNG-04GL37G .
May 18, 2009
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