On April
26, 2000, the international BOOMERANG consortium led by Andrew Lange of
the California Institute of Technology and Paolo Bernardis of Universit·
de Roma, "La Sapienza," announced results of the most detailed
measurement yet made of the cosmic microwave background radiation (CMB).

Findings of BOOMERANG, which stands for "balloon observations of
millimetric extragalactic radiation and geophysics," strongly
indicate that the curvature of the universe is not positive or negative
but flat. Much of the data analysis was performed at the Department of
Energy’s National Energy Research Scientific Computing Center (NERSC) at
Lawrence Berkeley National Laboratory.

"From studying our universe to studying the human genome,
scientists are generating incredible amounts of data — but it takes the
capabilities of supercomputing facilities such as the Energy Department’s
National Energy Research Scientific Computing Center to make sense of and
learn from that data,’ said Secretary of Energy Bill Richardson."

In January 1999, the BOOMERANG Long Duration Ballooning mission
completed its circumnavigation of the South Pole after ten and a half days
aloft. Instruments suspended beneath the balloon made close to 1 billion
measurements of tiny variations in the temperature of the CMB across a
wide swath of the sky.

"This is the largest and most precise set of CMB data yet
collected," says BOOMERANG team member Julian Borrill, an
astrophysicist and computer scientist with the National Energy Research
Scientific Computing Center (NERSC) at the Department of Energy’s Lawrence
Berkeley National Laboratory. "From the dataset, the BOOMERANG team
was able to make the most detailed map of the CMB’s temperature
fluctuations ever seen."

From the map of these temperature fluctuations, the researchers derived
a "power spectrum," a curve that registers the strength of these
fluctuations on different angular scales, and which contains information
on such characteristics of the universe as its geometry and how much
matter and energy it contains.

"CMB data is remarkably clean," says astrophysicist Andrew
Jaffe of the University of California at Berkeley, a BOOMERANG team member
and, like NERSC’s Borrill, a member of the university’s Center for
Particle Astrophysics and its Space Sciences Laboratory. "You can
write down everything you know about the data and then calculate the most
likely power spectrum — a task that is conceptually simple but
computationally challenging."

To derive the spectrum, Borrill used the parallel processing power of
NERSC’s 696-node Cray T3E supercomputer, employing a software package he
developed called MADCAP ("microwave anisotropy dataset computational
analysis package"). The calculation required 50,000 hours of
processor time and would have taken almost six years to complete if run on
a desktop personal computer. On the Cray T3E, however, processing time
over the life of the project totaled less than 3 weeks. The power spectrum
from the BOOMERANG Antarctic flight data is detailed enough to allow the
determination of fundamental cosmic parameters to within a few percent.

All CMB experiments seek to determine the state of the universe some
300,000 years after the Big Bang, when the universe cooled enough for
protons and electrons to form hydrogen atoms. At that moment, photons were
freed from what had been a hot primordial soup of subatomic particles.
Ever since that time these energetic photons have been traveling through
space, their wavelength now stretched to microwave scale and their
frequency reduced to the equivalent of radiation from a black body at only
2.73 degrees Kelvin.

The first step in deriving information from CMB observations is to map
the tiny fluctuations in this background radiation — temperature
differences of no more than 1 part in 100,000 — which reflect the equally
tiny inhomogeneities in the early universe, a time when the universe was
in a much simpler state than it is today.

"We basically have to separate the three components of the
temperature at each point we look in the sky," Borrill explains.
"There is instrument noise. There are foreground sources of microwave
radiation, such as dust. And finally there are the intrinsic variations in
the temperature of the CMB — which is what are we are trying to
measure."

The observations made as the telescope sweeps across the sky — 50
million observations for each of 16 channels at four frequencies, in the
whole BOOMERANG dataset — are not independent of one another, and the
different kinds of information are related differently.

"Starting with millions of observations, we must separate out the
different components," says Borrill. "Each of them is best
expressed in its own distinct way — but to express them jointly we need a
common frame. Usually we choose the most manageable, which is the pixel
domain — in other words, we make a map."

Every one of the map’s tens of thousands of pixels is made by combining
information from hundreds of observations taken at different times
throughout the balloon flight. In the resulting map it is easy to identify
foreground sources such as quasars or the plane of the galaxy, and dust
can be detected by its spectral signature, "which is why we make maps
at various frequencies," Borrill says. Comparing observations made at
different times improves the signal-to-noise ratio, but this is a
computationally expensive operation.

Deriving the power spectrum from the map, the next major step in
analysis, is even more challenging. The characteristic power of the
microwave background at various angular scales must be determined.

"The idea is to ignore all the other stuff and find just the
contribution of the cosmic microwave background," says Andrew Jaffe.
"We have to reduce the thousands of pixels in the map to a dozen or
so numbers, representing different points along the power spectrum curve,
and see how they fit to curves characteristic of different models of the
universe."

"The MADCAP program finds the power at each angular scale at each
of these points along the curve," Borrill says. "We’re asking
what the CMB would look like on that patch of sky if the universe had
such-and-such a shape and history." When the right curve is found, it
allows astrophysicists to distinguish between competing models of the
universe’s origin, evolution, and present make-up.

Although both map-making and power-spectrum derivation require
comparing each pixel in the chosen dataset to every other pixel, in
principle this only has to be done once to make a map, whereas it must be
done a dozen times or more — for each chosen point on the curve — to
derive the power spectrum.

"We are at the limit of what is manageable with today’s algorithms
on today’s supercomputers," Borrill says. "It’s a job that gets
harder with each experiment."

Analysis of the BOOMERANG Antarctic flight data has produced an
impressive degree of certainty about some of the most fundamental cosmic
parameters. BOOMERANG’s power spectrum of the CMB establishes that the
universe is flat — that its geometry is Euclidean, not curved. Combined
with other cosmological measurements, such as studies of distant
supernovae by the Supernova Cosmology Project headquartered at Berkeley
Lab, the BOOMERANG results support the emerging "concordance
model" of a flat universe filled with dark energy — dark energy that
may correspond to the cosmological constant first proposed by Albert
Einstein in 1917.

But datasets tens to hundreds of times bigger than BOOMERANG’s will be
produced by NASA’s MAP satellite, to be launched later this year, and the
European Space Agency’s PLANCK, to be launched in 2007. For these massive
datasets, new computational strategies will be necessary.

"We could make a very high-resolution map but analyze only a very
small part of it, or" — and this is the alternative Borrill plainly
prefers — "we could come up with better algorithms. Now that we have
a method that works, we can test new ideas against it."

Borrill adds that "because of the power of our parallel machines
and the depth of our experience with cosmic microwave background studies,
NERSC is becoming the computing center of choice for analyzing CMB data
from experiments all over the world. We want to maintain that status, but
it will take hard work and fresh ideas."

The BOOMERANG results are reported in the April 27, 2000, issue of the
journal Nature.

The Berkeley Lab is a U.S. Department of Energy national laboratory
located in Berkeley, California. It conducts unclassified scientific
research and is managed by the University of California.

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