Lyman Page, along with students and collaborators, measures the spatial temperature variations in the cosmic microwave background (CMB). The CMB, which pervades the universe, is the thermal afterglow of the big bang. Detailed knowledge of the magnitude and pattern of the fluctuations in temperature from spot to spot on the sky, or anisotropy, will help us understand how the universe evolved and how the observed structure, at sizes ranging from galaxies to superclusters of galaxies, was formed. From precise measurements of the CMB, one can also deduce many of the cosmological parameters and the physics of the very early universe. For example we have been able to determine the geometry and age of the universe, the cosmic density of baryons, the cosmic density of dark matter, and the Hubble constant.

Measuring the anisotropy is challenging, the variations in temperature are on the order of a few hundred thousandths of one degree Celsius. After nearly two decades of searching by groups at Princeton and elsewhere, NASA's Cosmic Background Explorer (COBE) satellite discovered the long sought after fluctuations in 1992. This discovery heralded a new era in comprehending the cosmos. Page's work centers on characterizing these fluctuations. The research program is focused on developing and building precision radiometers operable over a range of frequencies. Additionally, a considerable effort is spent on developing new techniques for analyzing anisotropy data.

This is an extremely exciting time for cosmology. The experimental tools and techniques, coupled with theory, have developed to the point were we can probe the physics of the infant universe in wonderful detail. There are many cosmologists at Princeton and a number work on CMB related projects. On the theoretical front, there are Uros Seljak, Paul Steinhardt, Jim Peebles, and David Spergel. On the experimental front, there are Joe Fowler, Norm Jarosik, Lyman Page, and Suzanne Staggs.

The Wilkinson Microwave Anisotropy Probe. Princeton is heavily involved in all aspects of the WMAP satellite. The project was a partnership with NASA/GSFC with collaborators at Chicago, UBC, Brown, and UCLA. Much of the instrument was designed and built at Princeton and the Princeton team played a large role in the data analysis. In the first data release (February 2003) we presented the most accurate and precise measurements of the CMB anisotropy to date. WMAP will operate through 2005. The analysis is in full swing. The satellite is named in honor of Prof. Wilkinson, a leader in experimental cosmology and a faculty member in the Physics Department until his death in 2002.

The Atacama Cosmology Telescope. WMAP continues to map the anisotropy with an angular resolution of 0.2 degrees. There is a wealth of cosmological information that may be obtained with measurements at finer angular scales. For example, we can understand how the first cosmic structures formed through the Ostriker-Vishniac and Sunyaev-Zel'dovich effects. From the growth of structure and the WMAP data, we can determine the neutrino mass and the possibly the equation of state of the dark energy or quintessence. The Princeton group is a leading member of a large collaboration to build a 6m telescope in Chile with thousands of detectors and an angular resolution of 0.03 degrees.