The image is a slice of the entire Bolshoi simulation, 1 billion light-years across and about 30 million light-years thick. Image: Stefan Gottlober (AIP)
The Bolshoi supercomputer simulation, the most accurate and
detailed large cosmological simulation run to date, gives physicists and
astronomers a powerful new tool for understanding such cosmic mysteries as
galaxy formation, dark matter, and dark energy.
The simulation traces the evolution of the large-scale
structure of the universe, including the evolution and distribution of the dark
matter halos in which galaxies coalesced and grew. Initial studies show good
agreement between the simulation’s predictions and astronomers’ observations.
“In one sense, you might think the initial results are
a little boring, because they basically show that our standard cosmological
model works,” says Joel Primack, distinguished professor of physics at the
University of California,
“What’s exciting is that we now have this highly accurate simulation that
will provide the basis for lots of important new studies in the months and
years to come.”
Primack and Anatoly Klypin, professor of astronomy at New Mexico State University,
lead the team that produced the Bolshoi simulation. Klypin wrote the computer
code for the simulation, which was run on the Pleiades supercomputer at NASA Ames
“These huge cosmological simulations are essential for interpreting the
results of ongoing astronomical observations and for planning the new large
surveys of the universe that are expected to help determine the nature of the
mysterious dark energy,” Klypin says.
Primack, who directs the University of California High-Performance
Astrocomputing Center (UC-HIPACC), says the initial release of data from the
Bolshoi simulation began in early September. “We’ve released a lot of the
data so that other astrophysicists can start to use it,” he said. “So
far it’s less than one percent of the actual output, because the total output
is so huge, but there will be additional releases in the future.”
The previous benchmark for large-scale cosmological
simulations, known as the Millennium Run, has been the basis for some 400
papers since 2005. But the fundamental parameters used as the input for the
Millennium Run are now known to be inaccurate. Produced by the Virgo Consortium
of mostly European scientists, the Millennium simulation used cosmological
parameters based on the first release of data from NASA’s Wilkinson Microwave
Anisotropy Probe (WMAP). WMAP provided a detailed map of subtle variations in
the cosmic microwave background radiation, the primordial radiation left over
from the Big Bang. But the initial WMAP1 parameters have been superseded by
subsequent releases: WMAP5 (five-year results released in 2008) and WMAP7
(seven-year results released in 2010).
The image shows the biggest dark-matter halo in the simulated volume. It would host a giant cluster of galaxies. The image is about 30 million light-years across. Image: Stefan Gottlober (AIP)
The Bolshoi simulation is based on WMAP5 parameters, which
are consistent with the later WMAP7 results. “The WMAP1 cosmological
parameters on which the Millennium simulation is based are now known to be
wrong,” Primack says. “Moreover, advances in supercomputer technology
allow us to do a much better simulation with higher resolution by almost an
order of magnitude. So I expect the Bolshoi simulation will have a big impact
on the field.”
The standard explanation for how the universe evolved after
the Big Bang is known as the Lambda Cold Dark Matter model, and it is the
theoretical basis for the Bolshoi simulation. According to this model, gravity
acted initially on slight density fluctuations present shortly after the Big
Bang to pull together the first clumps of dark matter. These grew into larger
and larger clumps through the hierarchical merging of smaller progenitors.
Although the nature of dark matter remains a mystery, it accounts for about 82%
of the matter in the universe. As a result, the evolution of structure in the
universe has been driven by the gravitational interactions of dark matter. The
ordinary matter that forms stars and planets has fallen into the
“gravitational wells” created by clumps of dark matter, giving rise
to galaxies in the centers of dark matter halos.
A principal purpose of the Bolshoi simulation is to compute
and model the evolution of dark matter halos. The characteristics of the halos
and subhalos in the Bolshoi simulation are presented in a paper that has been accepted for
publication in the Astrophysical Journal
and is now available online.
A second paper,
also accepted for publication in the Astrophysical
Journal and available online, presents the abundance and properties of
galaxies predicted by the Bolshoi simulation of dark matter.
The Bolshoi simulation focused on a representative section
of the universe, computing the evolution of a cubic volume measuring about one
billion light-years on a side and following the interactions of 8.6 billion
particles of dark matter. It took 6 million CPU-hours to run the full
computation on the Pleiades supercomputer, recently ranked as the seventh
fastest supercomputer in the world.
A variant of the Bolshoi simulation, known as BigBolshoi or
MultiDark, was run on the same supercomputer with the same number of particles,
but this time in a volume 64 times larger. BigBolshoi was run to predict the
properties and distribution of galaxy clusters and other very large structures
in the universe, as well as to help with dark energy projects such as the Baryon
Oscillation Spectroscopic Survey (BOSS).
Another variant, called MiniBolshoi, is currently being run
on the Pleiades supercomputer. MiniBolshoi focuses on a smaller portion of the
universe and provides even higher resolution than Bolshoi.
Primack, Klypin, and their collaborators are continuing to
analyze the results of the Bolshoi simulation and submit papers for
publication. Among their findings are results showing that the simulation
correctly predicts the number of galaxies as bright as the Milky Way that have
satellite galaxies as bright as the Milky Way’s major satellites, the Large and
Small Magellanic Clouds.
“A lot more papers are on the way,” Primack says.