Until the age of about 300,000 years, the young universe consisted of no more than a cloud of photons tightly coupled with ionized precursors of matter. This was a "dark" time--no light could escape the burgeoning universe since all the photons were trapped and scattered within the plasma of ionized matter. As the universe cooled, however, conditions finally began to allow stable atoms to form, releasing the photons from matter¿s grip and creating the beginnings of what researchers have termed the Cosmic Microwave Background (CMB). The CMB is a uniform fabric of radiation that covers the entire universe at a constant temperature of 2.7 kelvins. The cooled remnants of the Big Bang, it presents a snapshot of the universe as matter first began to form and release light. Studying the CMB is the best way for researchers to understand the beginnings of the universe and the origins of matter. Subtle variations in the CMB¿s temperature have long been thought to represent the origins of galaxies, but until now, the variations that cosmologists actually saw were much too large to pertain directly to galaxies or galaxy clusters. The structures detected by the Cosmic Background Imager are about 10 15 times the mass of the sun¿large by human standards, but the smallest ones yet seen in the CMB.
"We have seen, for the first time, the seeds that give rise to clusters of galaxies, thus putting theories of galaxy formation on firm observational footing," says team leader Anthony Readhead of California Institute of Technology. "This is basic research at its finest and most exciting," adds NSF director Rita Colwell. "Each new image of the early universe refines our model of how it all began. Just as the universe grows and spreads, humankind¿s knowledge of our own origins continues to expand."