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WRITING THE BOOK OF THE UNIVERSE
By Trish Barker, NCSA Senior Science Writer
The universe is so vast and vastly old that it seems to defy observation,
explanation and imagination. Scientists can't put the universe under a
microscope or create stars and galaxies in test tubes.
Rather than throwing their hands up in despair, cosmologists turn to
calculation and simulation to answer fundamental questions about the history
and evolution of the universe. As NCSA Director Dan Reed testified to the U.S.
House Science Committee in July, "One of the unique capabilities of
large-scale scientific simulation [is] the ability to model phenomena where
experiments are not otherwise possible."
Quasars are among the phenomena that simulation is helping to demystify.
Discovered just 40 years ago, quasars are prodigious energy producers.
Although they are not much bigger than our solar system, quasars pour out 100
to 1,000 times as much light as a galaxy containing a hundred billion stars.
Spinning black holes are thought to be the engines powering quasars. The
prevailing theory is that the spinning draws streams of gases spiraling into
the black hole's center, just as an eddy in a stream pulls water into its
depths. As these gas streams collide, intense friction results; vast amounts
of heat and energy are emitted before the gases finally collapse into the
black hole. Quasars are believed to be this final blast of energy.
All of the quasars that have been identified are billions of light-years away,
so their energy has traveled immense distances to reach our solar system.
Therefore, observing quasars is a way for cosmologists to travel back in time.
It's similar to the way paleontologists study our planet's past by digging
down through layers of earth, uncovering fossils that provide clues about the
animals and plants of long-past epochs. Just as paleontologists use fossilized
bones to build theories about the muscle, skin, characteristics and behavior
of dinosaurs, cosmologists use the clues gathered through observation to
envision, simulate and build theories about the evolution of the universe.
Michael Norman, professor of physics and director of the Laboratory for
Computational Astrophysics at the Center for Astrophysics and Space Sciences
at UCSD, has traveled back to one era of the universe. Norman's research team
used NCSA's Titan cluster to complete a ground-breaking simulation of how
ultraviolet radiation from quasars propagates through intergalactic space,
reionizing helium gas.
Evolution Of The Universe
To understand the significance of helium reionization, it's important to
understand how the universe is thought to have begun. The widely accepted Big
Bang Theory suggests that the universe originated in an extremely dense,
extremely hot state that exploded outward in the so-called Big Bang.
Initially, the universe was a super-hot soup of electrons, quarks and other
sub-atomic particles; eventually, the mixture cooled enough for hydrogen and
helium nuclei to form. Free electrons and photons still roamed. It took about
400,000 years for the universe to cool down enough for the ionized nuclei and
electrons to unite to form atoms.
Driven by gravity, these pioneering hydrogen and helium atoms began to collect
into huge clouds of gas from which stars and galaxies formed. As the first
stars and quasars formed, their ultraviolet energy ripped electrons away from
their atoms. Hydrogen atoms were affected first, and then helium atoms (which
hold their two electrons more tightly). The waves of ultraviolet radiation
from stars and quasars rippled outward, gradually ionizing the intergalactic
medium. This reionization epoch filled the universe with an intense
ultraviolet background and heated the intergalactic medium.
Ripples Of Radiation
It is this reionization epoch -- beginning about 2 billion years after the Big
Bang -- that Norman has simulated.
Computing the ripple-like effects of quasar radiation began with ENZO, a
structured adaptive mesh refinement cosmological hydrodynamics code developed
by Norman and two colleagues -- Thomas Abel of Cambridge University, and Greg
Bryan of Oxford University. Starting with the standards and constants that
define the behavior of the universe and then plotting in quasars at specific
points, Norman used ENZO and computational resources at the San Diego
Supercomputing Center to simulate the changes taking place in a section of the
universe, yielding a series of about 50 snapshots.
Each snapshot contained a huge amount of data (50 to 100 gigabytes), all of
which had to be transferred to the Titan cluster. Then a cosmological
radiation transfer code (CRT) written by University of Illinois astronomy
graduate student Pascalis Paschos computed how radiation moved out from
quasars to reionize the cosmic medium. These calculations required more than
40,000 compute hours on the Titan cluster.
Norman was impressed with the cluster's capability to rise to the
computational challenge. "This is an I/O-intensive calculation as well as
being a compute-intensive application," he said. "This is what convinced me
that I could do big science on these clusters."
The results of the CRT calculations were then translated into visualizations,
which showed a sphere of ionization bubbling out from each quasar, followed by
the collision and merger of the growing spheres.
Comparing Observation And Simulation
In addition to according well with the standard cosmological theory, Norman's
simulation, which is being written up for publication in the Astrophysical
Journal, also matched recent observation. In 1999, NASA launched the FUSE (Far
Ultraviolet Spectroscopic Explorer) satellite. Orbiting nearly 500 miles above
the Earth, FUSE examines light in the far ultraviolet range of the
electromagnetic spectrum, a range that can't be observed by other satellites
and that can't be observed from the planet's surface because our atmosphere
would disperse the ultraviolet light.
When directed toward a quasar, FUSE receives its ultraviolet radiation. As
this radiation passes through the intergalactic medium, some of it is
absorbed. FUSE measures the resulting absorption spectrum, which provides
information about the medium through which the energy has passed, including
its degree of ionization. The data gathered by FUSE corresponds well with
Norman's simulation, showing clouds of ionization billowing out from quasars.
While the simulation sheds valuable light on one era of the universe,
questions still remain. Norman could not simulate the origin of quasars,
because how and why they form is still a mystery.
"What's missing is a theory that would allow us to do a simulation of galaxy
formation and decide that this galaxy gets a quasar at its center with these
properties," he said. "What is still somewhat uncertain is how it all works
together to produce the universe we see today."
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