The 2006 Nobel Prize in physics honors two astrophysicists who first mapped the afterglow of the big bang--the so-called cosmic microwave background (CMB). Using NASA's Cosmic Background Explorer (COBE) satellite, John Mather, 60, of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and colleagues measured the precise spectrum of the microwaves. Using another instrument, George Smoot, 61, of the Lawrence Berkeley National Laboratory and the University of California, Berkeley, and colleagues detected slight variations across the sky in the temperature of the CMB, signs of the clumping of matter that would lead to the formation of galaxies.
The results confirmed that the universe was born in an enormous explosion and sowed the seeds for today's studies of exactly how the infant cosmos evolved, says George Efstathiou of the University of Cambridge in the U.K. The spectrum measurement "verified beyond any reasonable doubt that the cosmic microwave background radiation was created very early in the universe's history," he says. "The discovery of temperature ripples gave us the first probe of the exotic physics that occurred within 10-35 seconds after the big bang."
According to the big bang theory, the universe sprang into existence in an immense explosion and has been expanding and cooling ever since. Some 380,000 years after the cosmos was born, the protons and electrons that filled space combined to form neutral hydrogen atoms, which suddenly allowed the intense light that accompanied the protons and electrons to pass unimpeded. Thirteen billion years later, that radiation lingers everywhere, although it has cooled from a toasty 3000 kelvin to a frigid 2.725 kelvin (degrees above absolute zero).
The CMB was first predicted in 1948. It was discovered by accident in 1965, but key properties remained uncertain when NASA launched COBE in 1989.
Mather's team measured the spectrum of the radiation to great precision, demonstrating that it was released quickly and that the early universe was very hot and dense, as predicted by the big bang theory. "In the beginning, I was just trying to get the job done and I didn't think about how important it was," Mather says. "But since then, I've realized that it is an essential piece of the history of the universe."
Smoot's team found that the temperature of the radiation varied from place to place in the sky. Those variations reflect the early clumping of matter, through which galaxies and clusters of galaxies formed. Measuring about 1 part in 100,000, the variations were smaller than many theories had predicted and just big enough to be detected, Smoot recalls. "It was close," he says. In fact, the small size of the variations indicated that the gravity from ordinary matter would not suffice to explain the structure of the universe. That meant that some form of unobserved dark matter had to have started clumping earlier, laying the seeds for the galaxies and clusters, Smoot says.
Since the first COBE results were announced in 1992, studies of the CMB have yielded even more information. In 2003, researchers working with NASA's Wilkinson Microwave Anisotropy Probe satellite obtained a more-detailed map of the CMB, which allowed them to precisely determine the age and composition of the universe (ScienceNOW, 11 February 2003). And the European satellite Planck, which is scheduled to launch next year, will try to map the polarization of the microwaves, which could reveal the effects of gravitational waves in the primordial universe.