April 29, 2001

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BERKELEY, CA -- 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.

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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