Exploring the cosmic microwave background

Nearly forty years ago, two radio astronomers at Bell Laboratories in New Jersey were testing a telescope when they discovered unexplained static coming from every direction in the sky. After removing two nesting pigeons from the device, the noise was still there. The astronomers, Arno Penzias and Robert Wilson, soon learned they had stumbled on the earliest possible snapshot of the universe, when the cosmos was 300,000 years old.

At that time, the first atoms formed from a roiling soup of charged particles and atomic nuclei. The formation of atoms allowed radiation — which had previously not been able to travel far before being absorbed and re-emitted by the highly energetic particles — to escape. This light filled space and cooled as the universe continued to expand. By the time the Bell researchers detected it, the radiation had traveled about 14 billion years and had stretched into microwaves that corresponded to a temperature of about 3° Celsius above absolute zero. The radiation became known as the cosmic microwave background (CMB). It was extraordinarily uniform across the sky and showed that the universe at that young age was smooth, without the clumps of matter, such as stars and galaxies, that we see today.

This was somewhat disquieting — if the universe was so smooth, how and when did the seeds of the first structures form? Astronomers began work on instruments that would bring the snapshot into sharper focus. CMB radiation is so weak that Earth’s atmosphere muddies the signal. So in 1989, NASA launched a satellite to study it. In 1992, results from this mission, called the Cosmic Background Explorer (COBE), rocked the astronomical community.

COBE found tiny differences (on the order of ten parts per million) in the CMB’s temperature across the sky. They had found the smoking gun – evidence of the first seeds of structure. They believed that warmer regions eventually grew into stars, galaxies, and clusters of galaxies, while cooler regions emptied out into voids.

Other experiments to measure the CMB are conducted in environments that minimize the atmosphere’s obscuring effects, such as on balloons and in high and dry regions.

The BOOMERANG (Balloon Observations of Millimetric Extragalactic Radiation and Geophysics) experiment flew in a circle 120,000 feet (37 kilometers) above Antarctica from December 29, 1998, to January 9, 1999. An international collaboration, BOOMERANG created a map of the CMB at least 40 times more detailed than COBE’s, and witnessed harmonic-type peaks in the radiation predicted by inflation theory.

A second international balloon experiment called MAXIMA (Millimeter Anisotropy Experiment Imaging Array) completed two successful flights over Texas in 1998 and 1999. It studied the CMB at even smaller angular scales than BOOMERANG and confirmed the other experiment’s results.

The Degree Angular Scale Interferometer (DASI), a radio telescope composed of 13 connected detectors based near the South Pole, has been studying the CMB since late 1999. The National Science Foundation-funded telescope recently picked up extremely faint signals (less than one part per million) that the CMB is polarized. Most light is unpolarized, its waves arriving at our eyes and telescopes in a jumble of orientations. But when light bounces off a surface, its waves align in one plane. Since the CMB radiation last scattered from particles when the universe was 300,000 years old, theorists had predicted it would be polarized. Confirmation of this bolsters the big bang theory.

In 2001, NASA launched the Microwave Anisotropy Probe (MAP) to study the CMB in unprecedented detail. Data from this space mission is expected to be released in January 2003.

Other projects which have also used multiple, connected detectors, known as interferometers, to study the CMB include:

The Cosmic Anisotropy Telescope (CAT), based near Cambridge, England. The first interferometer to study the CMB, it began taking measurements in 1993 but was partially dismantled in 2000.

The ATCA Cosmic Microwave Background Radiation Anisotropy Experiment, based in Narrabri, Australia. Composed of five dishes which are each 22 meters across, it has been taking data since 1991 at a resolution of 2 arcminutes.

The Cosmic Background Imager (CBI), located in Chile’s Atacama desert. It is composed of 13 elements and began its measurements of the CMB in 1999.

The Very Small Array (VSA), located in Tenerife, Spain. Its 14 components can resolve objects between the angular sizes of 0.2° and 3° (the full moon is 0.5°). It began taking data in 2000.