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News Spring 2006 |
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NASA HPC Centers Support Gravitational Wave Breakthrough
Black holes with masses
millions of times the mass of a star have been found
in the core of our Milky Way and many other galaxies.
If they collide, Einstein's equations from the general
theory of relativity predict that gravitational radiation—waves
of displacement of spacetime—will be radiated
away from the collision. Detection of these waves has
never been achieved directly, but new detectors are
being operated and planned to make this important fundamental
test of Einstein's general relativity.
Image
at right : Gravitational Astrophysics
Laboratory Team. From left to right: Michael
Koppitz, Jim van Meter, Joan Centrella, and John
Baker. Not pictured: Dae-Il “Dale” Choi (Photo
credit: Chris Gunn, NASA GSFC).
GSFC’s Gravitational Astrophysics Laboratory
achieved a breakthrough recently: computation of the
signature gravitational wave pattern that is radiated
when two black holes that orbit one another experience
orbital decay and merge. The solution of this very
difficult three-dimensional, time-variable problem
in numerical relativity has been a “holy grail” of
the field, according to Joan Centrella, Chief of
the Gravitational Astrophysics Laboratory and contributing
author of a publication of the results that appeared
in Physical Review Letters, 96, 111102 (2006).
The computations were performed on the NCCS’s
SGI Altix 3700 BX2 supercomputer and on NASA Ames Research
Center (ARC)’s Columbia system.
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| Einstein's equations for the
problem, beginning with two moving black holes and ending
with one (the product of their merger), are extremely
complex. The end results are several useful predictions
for gravitational wave astronomers. Determining the amount
of the energy in the system that gets converted into
gravitational wave energy is crucial for observers to
know how sensitive their detectors must be. Centrella's
team predicts that approximately 4% of the mass-energy
in the initial black hole pair system can be converted
into wave energy, and this gives observers some confidence
that the waves will not be too weak to detect, even if
the black hole collision occurs somewhere far away from
our galaxy. The other important product of the simulations
is the pattern of the wave disturbance in space and time,
as a function of direction in space from the orbital
plane of the initial pair of black holes.The National
Science Foundation has funded an observatory system called
the Laser Interferometer Gravitational-Wave Observatory
(LIGO), with component detectors in Louisiana and Washington.
LIGO began full operations in November 2005. With the
expected wave intensity, LIGO is estimated to have a
25% chance of detecting gravitational radiation from
a celestial source in the time that it operates. |
NASA has a space-based mission planned
to detect gravitation radiation, called the Laser Interferometer
Space Antenna (LISA), currently planned for launch
around 2017. Centrella's team has provided valuable
parameters for the LIGO and LISA scientists to use
in their operations and mission planning.
For the computations
Centrella's team made some simplifying assumptions.
The two black holes were assumed to have the same
mass, and neither was spinning. Future computations
can explore the effects of varying the masses in the
pair and of assuming various rotational properties
of one or more of the pair. The effects of these variations
in initial conditions could be significant, changing
the gravitational radiation output intensity and space-time
distribution pattern. There is much more work to be
done in this numerical relativity field.
The work at the NCCS was described by James van Meter
(NRC). The team made many code development runs on
the Altix, each using 64 processors for 5 hours. Many
of these development runs served as the prelude to
a 3-day run on Columbia with 2,032 processors, modeling
five orbits of the binary black hole merger. Van Meter
looks forward to operation of the NCCS’s forthcoming
Linux cluster (see “NCCS to Install Next-Generation
Supercomputer,” p.1), as he hopes to carry out
more simulations typically using 500 processors.
http://www.nasa.gov/centers/goddard/
universe/gwave_feature.html
http://gsfctechnology.gsfc.nasa.gov/HolyGrail.html |
Image
above: Centrella's team crunched Einstein’s
theory of general relativity equations on the Columbia
supercomputer to create a three-dimensional simulation
of merging black holes. This was the largest astrophysical
calculation ever performed on a NASA supercomputer.
The simulation provides the foundation to explore
the universe in an entirely new way, through the
detection of gravitational waves (Image credit: Chris
Henze, NASA ARC).. |
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