New evidence derived from measurements of minute
variations in the temperature of the cosmic microwave background
(CMB) have produced a new diagram of sound waves in the dense
early universe. The graph, called a CMB "power spectrum," not
only shows a primary resonance but is consistent with two more
harmonics, or peaks; their position and amplitude strongly
support one model of how the universe came to have its present
structure -- the inflationary Big Bang model -- while ruling out
competing models.

The new results were reported at the American Physical Society
meeting in Washington, D.C. on Saturday and Sunday, April 28 and
29, by researchers in the MAXIMA collaboration. Two other groups
that measure variations in the CMB, the BOOMERANG and DASI
collaborations, also reported new findings at the meeting;
BOOMERANG, like MAXIMA, gathered its data with a balloon-borne
instrument, while the DASI collaboration was ground-based.

The three groups are in broad agreement that the universe is
flat; that its structure is due to early, rapid inflation (and
not to topological defects in the early universe); and that it
probably contains a bit more ordinary matter than is suggested by
models of particle formation in the Big Bang.

MAXIMA's new findings are the result of a greatly refined
analysis of data from a 1998 balloon flight over East Texas,
initially reported one year ago. In the new analysis, noise has
been stringently removed to produce a high-resolution map of a
portion of the microwave sky with pixels just one twentieth of a
degree wide, resolution nearly twice as fine as that of the map
initially reported.

Last year's reports from both MAXIMA and BOOMERANG clearly
showed numerous fluctuations on a scale of about one degree --
the first "peak" in the CMB power spectrum -- but only hinted at
a second peak. In the new MAXIMA analysis, the power in the
second peak region is clearly shown and the height of a third
peak is suggested.

The peaks indicate harmonics in the sound waves that filled the
early, dense universe. Until some 300,000 years after the Big
Bang, the universe was so hot that matter and radiation were
entangled in a kind of soup in which sound waves, or pressure
waves, could vibrate. The CMB is a relic of the moment when the
universe had cooled enough so that photons could "decouple" from
electrons, protons, and neutrons; then atoms formed and light
went on its way.

At the moment of decoupling, the pressure waves left telltale
traces of their existence in the form of slight temperature
variations in the CMB, which in the intervening 10 billion years
or so has cooled to a mere three degrees Kelvin. CMB experiments
are designed to detect these variations, whose spacing and
magnitude, mapped on the sky, can reveal fundamental properties
of the universe.

In 1992, George Smoot of the Physics Division at the Department
of Energy's Lawrence Berkeley National Laboratory, who is a
professor of physics at the University of California at Berkeley,
led the team that first detected fluctuations in the CMB with an
experiment aboard the Cosmic Background Explorer satellite, COBE.

"Since the initial COBE mapping, many ground and balloon-based
experiments have shown that the fluctuations have a peak in power
at about one angular degree," says Smoot, a member of the MAXIMA
collaboration. "Most notably the MAXIMA-1 and BOOMERANG results
reported one year ago defined this first peak quite well."

Analogous to the "first harmonic" of a vibrating string, the
first peak showed prominent features of one angular degree ??
suggesting that the universe is flat, having Euclidean geometry.
Had the variations been smaller or larger than a degree, they
would have indicated a universe whose geometry is negatively or
positively curved, like the surface of a saddle or a sphere.

The width and position of the first peak suggests that
fluctuations on all scales were already in place at the earliest
moments of the universe. A period of rapid expansion in the early
moments after the Big Bang would have set these perturbations in
place by blowing up microscopic quantum fluctuations to
astrophysical scales -- seeding the galaxies and nets of galaxies
we see today.

This explanation implies that there should be fluctuations at
other scales of the CMB as well, forming additional peaks on the
power spectrum at half the fundamental scale, a third the
fundamental scale, and so on.

Had the structure of the cosmos been seeded not by inflation but
by topological defects, introduced by phase changes in the
extreme energies of the early universe, neither of the observed
peaks would have been prominent, and the third would be much
lower than the second.

But the new MAXIMA power spectrum suggests that in fact the
second peak is suppressed, and the third peak appears to be
elevated. The best explanation is that the universe contains
slightly more baryons -- ordinary matter -- than is predicted by
models of the synthesis of light elements in the Big Bang.

The MAXIMA collaboration is led by Paul Richards, a member of
the Materials Sciences Division of Berkeley Lab and a professor
of physics at UC Berkeley, Adrian T. Lee, a member of the Physics
Division at Berkeley Lab and an assistant professor of physics at
UC Berkeley, and Shaul Hanany, assistant professor of physics at
the University of Minnesota. Lee is the lead author of the paper
describing the new MAXIMA results, to be published in
Astrophysical Journal Letters.

MAXIMA data was analyzed by Radek Stompor of UCB at the
Department of Energy's National Energy Research Scientific
Computing Center (NERSC) at Berkeley Lab and at the University of
Minnesota, using in part the MADCAP software devised by Julian
Borrill of NERSC.

The MAXIMA collaboration began at the National Science
Foundation's Center for Particle Astrophysics at UC Berkeley.

MAXIMA has received support from three US federal agencies: NSF,
NASA and the Department of Energy. In addition to UC Berkeley and
the University of Minnesota, collaborating institutions include
CalTech, the University of Rome, and the IROE-CNR in Florence.


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