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Above: Paul Saylor (left) and Doug Swesty discuss the simulated merger of two 1.4 solar mass neutron stars. by Jarrett Cohen |
Issue 3, September 1997 Welcome to the on-line version of NASA's Insights Newsletter.
Insights is published by the High Performance Computing and Communications (HPCC) Program Office. Address changes to Judy Conlon or write to: NASA HPCC Insights, Mail Stop 269-3, Moffett Field, California 94035-1000, USA
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In the three snapshots
above, the lighter color signifies higher matter density and
darker color, lower density. Embodying some gravitational
effects, simulations show that gravitational radiation
losses influence instabilities during the merger. Only full
relativistic calculations, now underway, can predict the
gravitational wave signal and probe the possibility of black
holes resulting. | ||
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"The Einstein field equations
are an elegant creation like the score of a Mozart concerto
waiting to be played," said Paul Saylor, professor of
computer science at the University of Illinois at
Urbana-Champaign (UIUC). With terms numbering in the
thousands, these equations make up the general theory of
relativity, which states that space and time are dynamic.
Saylor's orchestra of astrophysicists, relativity experts
and computer scientists are testing the theory's limits by
simulating neutron star mergers. Neutron stars result from supernova explosions: the core of an old, bloated star collapses and violently spits out the envelope. Left behind is a city-sized atomic nucleus with the mass of our sun. "The atoms are squeezed so closely together that their boundaries disappear; even the nuclei merge," said Jim Lattimer, professor of physics and astronomy at the State University of New York at Stony Brook. He related that a liter of neutron star matter would weigh as much as Halley's Comet. Neutron star coalescence occurs in binary systems where
orbiting stars have undergone supernovas. Affirmed by
observations, "Einstein's theory of gravity predicts that
two objects in orbit around each other will emit
gravitational radiation," said Doug Swesty, research
scientist at UIUC's National Center for Supercomputing
Applications (NCSA). The radiation causes the orbit to
decay. After about one billion years, an ever-accelerating inspiral draws the stars together, "like two gigantic sumo wrestlers circling each other," Saylor said. Only this match occurs at near-light speeds. In the final round, the orbital period is about one millisecond, and "these massive objects . . . are orbiting around each other at 60,000 revolutions-per-minute!" Swesty exclaimed. The merger itself takes only one to two milliseconds. The velocities involved make neutron star mergers
relativistic, and only general relativity can describe
gravitational radiation and its effects. This radiation is
essentially gravitational waves, or ripples of space-time,
that approximate cosmic Jell-O, quipped Wai-Mo Suen. "If you
punch Jell-O, it will vibrate," said the associate professor
of physics at Washington University in St. Louis. "At the
end, the neutron stars collide in a big punch and send out a
burst of gravitational waves."
--Wai-Mo Suen Updating Newton Most computational approaches have at best made some
corrections to Newtonian gravity, now over 300 years old.
Saylor's team is one of the few doing a "full relativistic
calculation," Swesty said. |
Their "Cactus" general relativity code builds on four years of simulating the more massive black holes by investigators at UIUC, Washington University and the Max Planck Institute for Gravitational Physics in Potsdam, Germany. Max Planck research scientists Joan Massó and Paul Walker were Cactus's main developers. Since black holes exist in a vacuum, Cactus is coupled to a new hydrodynamics code depicting changes in matter. Obtaining fast performance on microprocessor-based supercomputers such as the CRAY T3E requires high utilization of each processor's memory cache. Keeping the operands in cache as much as possible before doing a load from main memory will result in a higher percentage of peak speed, team members said. As of this writing, additional efficiency improvements brought the coupled code to approximately 40 billion floating-point operations per second on the High Performance Computing and Communications' 512-processor CRAY T3E at Goddard Space Flight Center.
Gazing through gravitational waves For neutron star coalescence, such simulations intertwined with contemporary observations will begin answering questions about the dense stars' properties and merger outcomes. Neutron stars are primarily detected at radio or X-ray frequencies, with three binary systems known to exist in our galaxy, but the most telling and direct signature is gravitational waves. The central mystery gravitational waves can unravel is neutron star matter's equation of state, which relates pressure to density and temperature, Lattimer said. It is challenging to understand the equation of state because scientists cannot yet replicate the density conditions on Earth. Each equation of state may produce a unique gravitational wave signal. In the models "we vary the properties of matter at high densities to see if the emitted signal could constrain the matter's behavior," Swesty said. A strong function of the equation of state is the maximum mass a neutron star can have; the estimated range is 1.4 to 3.2 solar masses. Swesty explained that mergers have three likely outcomes: a larger neutron star, a black hole or an object that will undergo a delayed collapse to a black hole. A relativistic simulation generating anything but the first option would point to an upper mass limit. The most ambitious effort to measure the predicted gravitational waves is the Laser Interferometric Gravitational Observatory (LIGO), coming on-line in 2000. [See the LIGO web site] It may locate as many as 300 neutron star mergers per year, but "a single observation would be phenomenal," Swesty said. "LIGO, long prior to coalescense, will detect masses of the individual objects. That, combined with what we learn from the coalescense signal in the models, could really help us to pin down the structure of neutron stars." With the additional potential of using gravitational wave strength to mark the universe's expansion rate, "this is an auspicious era for astronomical research," Saylor said. "It is like when Galileo looked through the universe with his telescope. We will be looking at the universe through the lens of gravitational waves." Other co-investigators include Steven Ashby, Lawrence Livermore National Laboratory; Ian Foster, Argonne National Laboratory; Michael Norman, NCSA/UIUC; and Clifford Will, Washington University. For more information about the project, visit their web site.
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