April 26, 2000
BERKELEY, CA -- On April 26, 2000, the international BOOMERANG
consortium led by Andrew Lange of the California Institute of Technology
and Paolo de Bernardis of Università 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 one 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 NERSC. From the dataset, the BOOMERANG team
was able to make the most detailed map of the CMB's temperature
fluctuations ever seen.
From a map of these temperature fluctuations, the researchers derived
a "power spectrum," a curve that registers the strength of the 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."
strength of the CMB fluctuations on different angular scales
as measured by BOOMERANG: points with error bars are the data,
while the curve corresponds to the best-fitting cosmological
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 three 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.
The BOOMERANG data (region rising toward
the left) constrain the sum of the fractions of matter (Wm)
and dark energy (WL) in the universe. Complementary
observations of type Ia supernovae (region rising toward the
right) constrain the difference of these fractions. Taken
together the results point to a flat universe that will expand
forever, made up of about 1/3 matter and 2/3 dark energy.
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 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
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.
Reprinted by permission from Nature
404:27 April (2000), copyright 2000 Macmillan Magazines Ltd.
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
"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. BOOMERANG's website
is at http://www.physics.ucsb.edu/~boomerang/
(mirrored in Europe at http://oberon.roma1.infn.it/boomerang/).
NERSC's website may be found at http://www.nersc.gov/.
Borrill's MADCAP program is publicly available at http://www.nersc.gov/~borrill/cmb/madcap.html.
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located in Berkeley, California. It conducts unclassified scientific
research and is managed by the University of California. Visit our
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