An illuminating journey: Reading cosmic history from the big bang's radiation

Abstract

H olding his 3-month-old son, Joshua, in his arms, cosmologist David N. Spergel will proudly watch the launch next week of a NASA satellite that he helped father. The satellite will record the remnant glow from the infant universe in greater detail than ever before. Even before the Microwave Anisotropy Probe satellite gets off the ground, however, Spergel and his colleagues have hatched a plan to examine that early light in a dramatically different way. The satellite will continue the practice of treating the cosmic microwave background (CMB)-the glow left over from the Big Bang-as a snapshot of the early universe. Taking a new perspective, Spergel and his colleagues propose to use that radiation as a flashlight to illuminate the evolution of structure in the universe over its 13-billion-year history. That evolution began with the condensation of gas clouds into fledgling galaxies and continued with production of the first stars and the assembly of galaxies into mammoth clusters. To realize this ambitious idea, Spergel and several other astronomers intend to examine the subtle markings acquired by the CMB as it traversed billions of lightyears to reach Earth. Like a weary traveler who has picked up dust from each country he's visited, the microwave background has been marked by all the cosmic architecture that it has encountered. As it streams across the universe, the CMB lighting up the past between here and there, says Spergel, who is based at Princeton University. The photons in the CMB have been traveling freely through space ever since the universe was about 300,000 years old. And a lot of things happened to them along the way, Spergel notes You have lots of imprints on the microwave background from the emergence of structure. Those imprints show up on a much finer scale than that of the primordial features painted onto the CMB by the Big Bang itself. Those relatively large hot and cold spots in the CMB, which the spaceborne Microwave Anisotropy Probe will detect, represent the seeds from which galaxies and galaxy clusters ultimately arose. Galaxies and galaxy clusters buffet the CMB photons coursing through them. These interactions generate hot and cold spots that differ by only a millionth of a degree or so from the average temperature of the microwave background, a chilly 2.76 kelvins. That's one-tenth as small as the temperature fluctuations imprinted on the CMB by the tumultuous conditions in the nascent universe. The tinier, post-Big Bang variations also occur on spatial scales only one-third the size of the primordial hot and cold spots that the Microwave Anisotropy Probe can discern. The sensitive detectors required to study these tiny variations, which theo-