A Baby Picture That's Worth a Nobel Prize

The 2006 Nobel Prize for Physics will be awarded to John Mather of NASA/Goddard Space Flight Center and George Smoot of the University of California, Berkeley and Lawrence Berkeley National Laboratory. They are cited for the study of the early universe. They were instrumental in developing the Cosmic Background Explorer (COBE) experiment. This orbiting spacecraft was the first to detect faint temperature variations in the cosmic microwave background (CMB), the bath of radiation representing the first light able to move freely through the universe after the big bang. COBE's map of these temperature variations across the whole sky has been called the earliest "baby picture" we have of our universe.

The CMB was initially observed in the 1960s by Arno Penzias and Robert Wilson at Bell Labs, in New Jersey, for which they would later receive the Nobel Prize. It was thought at the time that the CMB must have been at least somewhat inhomogeneous (it couldn't have been absolutely uniform across the sky) since the subsequent galaxies we now see would have to form from slight imbalances of matter in the pervasive hot plasma that constituted the substance of the universe (as far as we know) just before the first atoms formed. But how big those clumps of matter were, showing up as slight temperature variations in the map of the CMB across the sky, was unknown.

At a press conference at the American Physical Society April meeting in 1992 COBE speakers, including Smoot and Mather, announced the discovery of variations at the level of parts per hundred thousand against an overall average temperature of 2.7 degrees Kelvin (see PNU 77).

The microwave background is in effect the biggest thing we can see (indeed it spreads out across the whole sky), the farthest-out thing we can map, and the furthest-back in time. COBE was the first to measure the variations and the first to provide a really precise average temperature for the universe, 2.726 degrees Kelvin (PNU 109). At the American Astronomical Society meeting where the this temperature was reported, an audible gasp was heard from the audience as the set of accumulated data points was placed on top of the expected blackbody spectrum -- the fit between data and theory was that good. The COBE work represented a feat of great experimental science since the faint variations in the temperature of the distant CMB had to be measured against a foreground cloud of microwave radiation coming from our solar system, our galaxy, and other celestial objects. Furthermore, the motion of Earth around the sun, the sun around the Milky Way, and the Milky Way within our local cluster of galaxies also had to be taken into account.

Later CMB detectors, including the balloon-borne Boomerang and the land-based Degree Angular Scale Interferometer (DASI), added more and more detail to the microwave background (PNU 537). The broad map of the microwave sky, showing splotches of slightly higher or cooler temperatures, grew ever sharper. But physicists more often presented their data chiefly in the form of a graph of multipole moments, corresponding to the microwave contributions from different angular scales, as if the CMB were composed of cosmic dipole, quadrupole, octupole, etc. components.

The most recent and best microwave measurements have been presented by the WMAP detector, which provides the clearest multipole curve yet as well as supplying the best values for important cosmological parameters such as the age of the universe, the overall curvature of spacetime, and the time when the first atoms formed and the first stars (PNU 769).

Pertinent background information on the Nobel prize include several fine articles in Scientific American: January 1990 on COBE itself, May 1978 on the big bang and the discovery of the CMB, May 1984 on the inflationary model, and March 2005 on big bang misconceptions.

The official Nobel Web site
The Lawrence Berkeley National Laboratory
Image at Physics News Graphics