My research focus is on experimental/observational cosmology and instrumentation, including space flight instrumentation.

I became the Principal Investigator (P.I.) of the Wilkinson Microwave Anisotropy Probe (WMAP) mission in 1996. WMAP was competitively selected as a NASA medium-class Explorer (MIDEX) mission. WMAP was launched June 2001 and its first scientific results were made public in February 2003. WMAP quantified the age, content, history, and other key properties of the universe with unprecedented accuracy and precision. This was recognized by Science magazine as the 2003 "Breakthrough of the Year."

Previously I was the Deputy P.I. of the Differential Microwave Radiometers (DMR) instrument and a member of the Science Team of the Cosmic Background Explorer (COBE) mission. The COBE Differential Microwave Radiometers instrument was used to make the first detection of variations across the sky (anisotropy) of the temperature of the afterglow radiation from the Big Bang, the cosmic microwave background radiation.

I participate in NASA's Legacy Archive for Microwave Background Data Analysis (LAMBDA) data center. LAMBDA provides COBE, WMAP, and other cosmological data to the community.

In 2010 I began work on a new experiment funded by the National Science Foundation (NSF) to test and characterize "inflation" theory. It is widely believed that the infant universe underwent a rapid period of exponential expansion called "inflation". Many of the predictions of inflation theory have been verified by WMAP and other measurements. An important test remains. Gravitational waves are generically produced by inflation. Some day in the distant future those gravitational waves may be detected. In the meantime, the gravitational waves can be sought by their imprint on the polarization of the cosmic microwave background radiation. Several research groups are seeking this unique polarization signature of Inflation. Our group is building the Cosmology Large Angular Scale Surveyor (CLASS) instrument to search for this faint polarization pattern. We plan to situate CLASS in the Atacama desert in northern Chile.

Some kind of "dark energy'' is driving the accelerated expansion of the universe. The determination of the nature of the dark energy is a major goal of physics and cosmological research. Several groups are building and using instruments to probe the dark energy. My research group at Johns Hopkins is actively engaged in dark energy research and I have long played a leading role in studying the possibilities for a space mission. The Wide Field InfraRed Space Telescope (WFIRST) mission would be capable of several kinds of dark energy observations as well as other valuable astronomical research. The European mission called Euclid is also planning to conduct dark energy research. I am a member of the Euclid Consortium and I am planning to participate in the analysis of Euclid data. I earlier served on a National Research Council committee that recommended to NASA that it provide hardware for Euclid. I am also a member of the Subaru Prime Focus Spectrograph (PFS) instrument team. It also has the goal of studying the distribution of dark matter in the universe and to use it as a proble of the dark energy. We have started to build part of the PFS instrument at Johsn Hopkins University. Among other applications, this spectrometer will help to further constrain the geometry of the universe beyond the WMAP constraints.