Major New Cosmic Microwave Background Measurements

Major new cosmic 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. 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 (about twice the angular size of the Moon), 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, etc.

A year ago the Boomerang collaboration, which used a balloon-based detector floating over Antarctica, provided a detailed map (Update 481) 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 earlier in the week at the American Physical Society (APS) 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 (John Carlstrom, University of Chicago, 773-834-0269) were largely in concert with Boomerang's presentation at the meeting (Barth Netterfield, Univ Toronto, 416-946-5465); 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 (see figures here, and here) as were the values of various cosmological parameters. For example, the position of the first peak yields the total energy of the universe (a parameter, denoted by the letter omega, expressed as a fraction of the critical density needed for halting the cosmological expansion). Boomerang and DASI found values of 1.03 and 1.04, respectively, with about a 6% uncertainty. 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, again in keeping with recent expectations.

New Maxima results (Shaul Hanany, Univ Minnesota, 612-626-8929) 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.

Noted cosmologist Michael Turner (Univ Chicago, 773-702-7974) 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.

(Recently posted preprints on the Los Alamos server include the following: Maxima astro-ph/014459; Boomerang astro-ph/0104460; and DASI astro-ph/0104488, 89, and 90.)