NASA scientists have reached a breakthrough in computer modeling that
allows them to simulate what gravitational waves from merging black
holes look like. The three-dimensional simulations, the largest
astrophysical calculations ever performed on a NASA supercomputer,
provide the foundation to explore the universe in an entirely new
way.
According to Einstein's math, when two massive black holes merge, all
of space jiggles like a bowl of Jell-O as gravitational waves race
out from the collision at light speed.
Previous simulations had been plagued by computer crashes. The
necessary equations, based on Einstein's theory of general
relativity, were far too complex. But scientists at NASA's Goddard
Space Flight Center in Greenbelt, Md., have found a method to
translate Einstein's math in a way that computers can understand.
"These mergers are by far the most powerful events occurring in the
universe, with each one generating more energy than all of the stars
in the universe combined. Now we have realistic simulations to guide
gravitational wave detectors coming online," said Joan Centrella,
head of the Gravitational Astrophysics Laboratory at Goddard.
The simulations were performed on the Columbia supercomputer at NASA's
Ames Research Center near Mountain View, Calif. This work appears in
the March 26 issue of Physical Review Letters and will appear in an
upcoming issue of Physical Review D. The lead author is John Baker of
Goddard.
Similar to ripples on a pond, gravitational waves are ripples in space
and time, a four-dimensional concept that Einstein called spacetime.
They haven't yet been directly detected.
Gravitational waves hardly interact with matter and thus can penetrate
the dust and gas that blocks our view of black holes and other
objects. They offer a new window to explore the universe and provide
a precise test for Einstein's theory of general relativity. The
National Science Foundation's ground-based Laser Interferometer
Gravitational-Wave Observatory and the proposed Laser Interferometer
Space Antenna, a joint NASA - European Space Agency project, hope to
detect these subtle waves, which would alter the shape of a human
from head to toe by far less than the width of an atom.
Black hole mergers produce copious gravitational waves, sometimes for
years, as the black holes approach each other and collide. Black
holes are regions where gravity is so extreme that nothing, not even
light, can escape their pull. They alter spacetime. Therein lies the
difficulty in creating black hole models: space and time shift,
density becomes infinite and time can come to a standstill. Such
variables cause computer simulations to crash.
These massive, colliding objects produce gravitational waves of
differing wavelengths and strengths, depending on the masses
involved. The Goddard team has perfected the simulation of merging,
equal-mass, non-spinning black holes starting at various positions
corresponding to the last two to five orbits before their merger.
With each simulation run, regardless of the starting point, the black
holes orbited stably and produced identical waveforms during the
collision and its aftermath. This unprecedented combination of
stability and reproducibility assured the scientists that the
simulations were true to Einstein's equations. The team has since
moved on to simulating mergers of non-equal-mass black holes.
Einstein's theory of general relativity employs a type of mathematics
called tensor calculus, which cannot easily be turned into computer
instructions. The equations need to be translated, which greatly
expands them. The simplest tensor calculus equations require
thousands of lines of computer code. The expansions, called
formulations, can be written in many ways. Through mathematical
intuition, the Goddard team found the appropriate formulations that
led to suitable simulations.
Progress also has been made independently by several groups, including
researchers at the Center for Gravitational Wave Astronomy at the
University of Texas, Brownsville, which is supported by the NASA
Minority University Research and Education Program.
Ranked the fourth fastest supercomputer in the world on the November 2005 Top500 list, Columbia has increased the NASA’s total high-end computing, storage, and network capacity tenfold. This has enabled advances in science not previously possible on NASA’s high-end systems. It sits at the NASA Advanced Supercomputing (NAS) Facility at the Ames Research Facility. It consists of a 10,240-processor SGI Altix system comprised of 20 nodes, each with 512 Intel Itanium 2 processors, and running a Linux operating system.
