Sounds of the universe confirm Big Bang

The early universe rang like a bell. Although scientists have known that for many years, they’ve been deaf to the full richness of that ancient ringing.

Astronomers now report the results of two experiments that tuned in the Big Bang’s relic vibrations. They say that this newly detected primordial fanfare proclaims as never before that all the structures in the universe — from stars to galaxies to huge galaxy clusters — had their origins in random, unimaginably tiny fluctuations

in density during the earliest moments of the universe. Then, according to theorists, a brief but powerful period of hyperexpansion, called inflation, stretched these subatomic fluctuations to cosmic scales.

“This will go down in the textbooks as a key test of the basic framework” of the Big Bang, comments cosmologist Michael S. Turner of the University of Chicago.

The new studies also provide fresh evidence that more than 95 percent of the material in the universe is made of some invisible, exotic stuff that exerts a gravitational tug but doesn’t otherwise resemble ordinary matter.

Both experiments examine the cosmic microwave background, the remnant radiation from the Big Bang that cooled to microwave energies as it traveled through space for some 13 billion years. The two research groups observed the microwave background with instruments located at the South Pole, but the experiments had very different designs. That makes the agreement between the tests all the more compelling, notes Turner.

One of the instruments, known as BOOMERANG (Balloon Observations of Millimeter Extragalactic Radiation and Geophysics), is a single balloon-borne radio telescope (SN: 4/29/00, p. 276). It flew over Antarctica for 11 days in late 1998, says Andrew E. Lange of the California Institute of Technology in Pasadena, who heads the BOOMERANG collaboration.

In contrast, DASI (Degree Angular Scale Interferometer) is a ground-based instrument consisting of 13 small radio telescopes, which record radiation at frequencies significantly lower than BOOMERANG does. Various pairs of these detectors compare neighboring patches of sky to search for tiny variations in the temperature of the microwave background, explains team leader John E. Carlstrom of the University of Chicago.

The experiments provide the most detailed images to date of the universe when it was only about 300,000 years old, says Turner. Before that, the universe was so hot that atoms and electrons were separate. Matter and photons were tightly coupled because the photons, relentlessly bouncing back and forth between electrons, couldn’t travel freely.

It’s in this bizarre environment that the cosmos’ primal sounds were generated and left their imprints on the matter-bound photons. Whenever gravity caused matter to compress, the pressure exerted by the trapped photons offered resistance. The tug-of-war between gravity and radiation pressure generated acoustic oscillations, which are regions of higher and lower pressure that have the same form as sound waves. Inflation theory predicts that these sound waves would be generated at a variety of wavelengths.

Within these acoustic waves, regions of higher pressure raised the temperature of the photons ever so slightly, while expansion created lower-pressure regions that reduced it. The result was tiny hot and cold spots within a sea of otherwise uniform radiation. After 300,000 years, the cosmos cooled enough for electrons and protons to combine into atoms. Photons, which aren’t scattered as easily by atoms as by electrons, suddenly could stream freely into space. Billions of years later, they provide a freeze-frame snapshot of the early universe.

In 1992, the Cosmic Background Explorer (COBE) satellite became the first detector to observe the tiny temperature fluctuations in the microwave background. But COBE didn’t have sufficient resolution to discern the small patches of sky over which the temperature variations were greatest.

Last year, an analysis of a small amount of data collected during BOOMERANG’s 1998 flight — as well as data gathered by another balloon experiment, MAXIMA (Millimeter Anisotropy Experiment Imaging Array) — confirmed that the first peak in the early universe’s temperature variation occurred on the same spatial scale predicted by inflation theory (SN: 6/3/00, p. 363).

If that theory were correct, the peak in temperature fluctuations found by BOOMERANG and MAXIMA would be one of many. In particular, it would correspond to the longest sound wave that could fit within the universe when it was 300,000 years old. The data presented last year didn’t harbor clear signs of other peaks, however. Their discovery was like finding a lone musical note without a single overtone.

Now, the cosmic symphony is beginning to be heard. With most of the BOOMERANG data analyzed, Lange says that he and his colleagues, who include C. Barth Netterfield of the University of Toronto and John Ruhl of the University of California, Santa Barbara, see a second peak and even the hint of a third. The DASI team, which includes collaborators from the California Institute of Technology in Pasadena and the University of California, Berkeley, has found similar evidence.

At press time, the researchers were slated to present their findings on April 29 at a meeting of the American Physical Society in Washington, D.C.

Finding additional peaks not only supports the inflation model but also offers critical information on the composition of the universe. The ratio of the second peak to the first, for instance, yields the density of ordinary matter, or baryons, in the cosmos. The answer to that simple calculation is bolstering the case for a universe dominated by extraordinary forms of matter.

Carlstrom’s team estimates that the density of baryons can account for only about 4.5 percent of all matter. The rest must be dark, invisible material, as indicated by a variety of other studies. Moreover, says Carlstrom, the baryon density indicated by DASI is “bang on” with the number obtained by measuring the amount of the hydrogen isotope deuterium — a sensitive indicator of the overall density of ordinary matter — forged in the Big Bang. Lange says that his team’s results are “consistent” with the DASI team’s findings.

“We are in the midst of the most exciting period ever in cosmology,” says Turner. “And it’s only going to get better.”