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New CMB Measurements Further Support Inflationary Universe

four false-color maps
A sample of four false-color maps of intensity variations in the cosmic microwave background (CMB) from different parts of the sky created from data collected by the University of Chicago's Degree Angular Scale Interferometer (DASI). The maps depict tiny deviations, on the order of one hundred thousandth of one degree, in the otherwise uniform 2.73 degree Kelvin background. The maps are a snapshot of the universe as it looked 14 billion years ago. DASI is funded by the National Science Foundation. (Graphic courtesy of the DASI Collaboration)
Major new cosmic microwave background (CMB) measurements uphold the idea of an early "inflationary" era during which the observable universe expanded with superluminal speed and tiny quantum fluctuations in the density of matter were amplified into much larger structures, according to results presented at the APS April meeting in Washington, DC. These structures are imprinted in the CMB as faint variations in the temperature across the microwave sky. The CMB, the curtain of photons set free when the expanding universe became cool enough to permit the existence of neutral atoms, is the earliest, largest, and furthest observable thing in all of science.

The best way to extract cosmological information from the CMB is to plot the observed microwave power as a function of the angular size of regions contributing to the CMB. The inflation model predicts that this spectrum should feature a number of peaks. The first peak, at an angular size of about 1 degree, corresponds to the largest blobs of matter in the primordial plasma at the time of the CMB, about 400,000 years after the big bang. Subsequent peaks should correspond to blobs that had come together under the action of gravity but had then rebounded outward because of radiation pressure, and later still had condensed for a second or third time.

A year ago the Boomerang collaboration, which used a balloon-based detector floating over Antarctica, provided a detailed map of the first peak which, besides falling at the 1-degree size predicted by inflation, also determined that the overall curvature of the universe was zero. But Boomerang and another detector group, MAXIMA, saw scant evidence of any other peaks, and this puzzled astronomers.

All this changed at the APS April meeting in Washington, DC, where the Degree Angular Scale Interferometer (DASI) collaboration, which parks its microwave detector on the roof of NSF's South Pole station, presented solid evidence for a second and third peak. The DASI results, according to John Carlstrom of the University of Chicago, were largely in concert with Boomerang's presentation at the meeting; Boomerang used a new type of analysis and reported 14 times more data than last year. The microwave spectra for the two groups were similar, as were the values of various cosmological parameters. For example, the position of the first peak yields the total energy of the universe. Boomerang and DASI found values of 1.03 and 1.04, respectively, with about a 6% uncertainty.

The presence of harmonic peaks bolsters the theory that the universe grew from a tiny subatomic region during a period of violent expansion a split second after the Big Bang. "Just as the difference in harmonic content allows us to distinguish between a flute and or trumpet playing the same note, so the details of the harmonic content imprinted in the CMB allows us to understand the detailed nature of the universe," said Barth Netterfield (University of Toronto), lead author of the Boomerang paper.

Comparing the height of the first and second peaks, one can calculate the expected percentage of all energy in the universe that exists in the form of ordinary matter (baryons). This turns out to be about 5% for both groups, a fact that agrees well with predictions made by the independent "big bang nucleosynthesis" theory. It is harder to nail down other cosmological parameters, such as the percentage of energy in the form of dark matter or dark energy-energy lurking in the vacuum and responsible for the newly discovered net acceleration in the cosmological expansion. The new CMB measurements suggest values of about 30% and 65%, respectively.

The estimate of ordinary matter in the universe agrees with the theoretical predictions made in 1998 by noted University of Chicago cosmologists Michael Turner and the late David Schramm, using a different method based on the amount of deuterium produced in the Big Bang. Schramm, who died that same year, was the first to realize that deuterium was a sensitive indicator of the density of ordinary matter in the universe. "These results strengthen the case that most of the mysterious dark matter is comprised of some new form of matter," said Turner. "We may be made of star stuff, but we are not made of the stuff of the cosmos."

New MAXIMA results presented at the meeting did not have nearly the statistical weight of the other two groups, but were generally consistent; the three-way agreement brought a great round of applause from the audience of astronomers eager to unravel the mysteries of the early universe.

The MAXIMA team is now analyzing data from a second balloon flight in June 1999, which they hope will strengthen the findings from the earlier flight. Further data will come from DASI and its sister instrument, CalTech's Cosmic Background Imager, as well as the University of Chicago's TopHat experiment and NASA's Microwave Anisotropy Probe.

MAXIMA also plans to launch an experiment to study polarization of the microwave radiation, described as "a vibration of photons as they travel through space." Studying the patterns of variations in polarizations will be useful in sorting out variants of the inflation model.

Turner observed that last year's discovery of the first microwave peak constituted the first great vindication for the Inflation model and that this new discovery of secondary peaks was the second great vindication. The third type of evidence, Turner said, would be the detection of gravity waves from before the time of the CMB. "This is just the beginning," he said. "Not only will we be able to test inflation, but we will be able to learn about its underlying physical cause."

-Phillip F. Schewe and Ben Stein of AIP Public Information contributed to this article


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