High fidelity. Raw microwave readings from the Cosmic Background Imager (left), minus instrument noise (center), reveal reverberations left over from the big bang.

Microwave Telescope Data Ring True

Scientists listening for the faint hiss of radiation left over from the big bang have just gotten an earful. Data from a telescope in Chile designed to hear the cosmic background radiation are providing strong support for theories about how the universe evolved during the first few hundred thousand years after the big bang. The result--which shows a signal that other experiments have missed--is the first from the Cosmic Background Imager (CBI) and the beginning of what scientists say could be a banner year for cosmology.

The CBI is catching the whispers of radiation born about 300,000 years after the big bang, when the universe was too hot for atoms to form. Light was constantly scattered in the monstrous plasma fireball, which reverberated with echoes of the great explosion. But as the universe cooled, electrons settled down with nuclei to form atoms. The opaque plasma became transparent, and the light that had been scattered and rescattered broke free.

That light, in the form of microwaves, now bombards Earth from all directions, allowing telescopes sensitive to that radiation to take pictures of the 300,000-year-old universe. Most recently, BOOMERANG, a balloon-borne experiment that circled the South Pole, made an exquisite map of the background radiation in a small region of the sky (Science, 28 April 2000, p. 595). But excited astronomers were puzzled when the data failed to show an expected pattern in the distribution of the radiation.

The early universe, scientists knew, rang like a bell after the big bang. Pressure waves rattled throughout the cosmos, causing variations in density that now show up as ripples in the amount of background radiation. And just as a bell's sound is made up of a fundamental frequency and a number of weaker higher-frequency overtones, the pressure waves in the universe had a "fundamental" of large-size peaks and dips in density and "overtones" of smaller and weaker peaks. BOOMERANG detected the fundamental's first peak but failed to detect the overtone second peak--as if theorists had predicted a bell but heard a horn instead. The missing second peak challenged observations of the amount of matter in the universe and threatened theories about how atomic nuclei formed.

Now, to cosmologists' relief, the latest observations suggest that the second peak is there after all. Anthony Readhead, an astronomer at the California Institute of Technology in Pasadena, and his colleagues will publish a paper describing the findings in Astrophysical Journal Letters.

Unlike the balloon experiments, which detect incoming radiation by converting it into heat, the CBI uses interferometry--detecting the phase and amplitude of incoming microwaves directly. Because interferometers have only recently become sensitive enough to measure cosmic background radiation, it's too early to say that the missing signal is definitely there, says Jeffrey Peterson, a cosmologist at Carnegie Mellon University in Pittsburgh. "But it takes a little of the sting out of the worries about the second peak."

Related sites

Anthony Readhead's home page at Caltech

The Cosmic Background Imager