This award supports the Princeton Gravity Group's continued exploration of gravitational physics and cosmology through the experimental and observational study of the cosmic microwave background (CMB) radiation. The specific experimental work includes: (1) Observations and data analysis for the Atacama Cosmology Telscope (ACT). ACT is fully functional and taking data. (2) The fielding and analysis of the Atacama B-mode Survey (ABS) experiment to measure the signature of gravitational waves from the early universe through their imprint on the polarization of the CMB. (3) The completion of the QUIET project, also targeting the CMB polarization, but at lower frequencies, which is just entering its observation and analysis phase. These experiments are complementary to both the existing WMAP satellite and the upcoming PLANCK satellite as well as to other ongoing work.
Measurements of the CMB are at the foundation of the standard cosmological model and probe gravitational physics at extreme length and temperature scales. The CMB is the cleanest probe of the physics of the nascent universe. In addition to unveiling fundamental physics during the first instants of the universe, the CMB provides information about the dark energy causing the present-day acceleration of the universe, is sensitive to the sum of the neutrino masses, can help locate the baryons that we know must exist but wich have not yet been found, and tells us how cosmic structure emerged in the early universe. This research program includes substantial educational development for students, continued public outreach, extensive international collaboration, and strong interagency collaboration (especially with NIST). The Princeton group will continue its vigorous program of hands-on research for undergraduates and graduate students.
The Princeton Gravity Group measures and analyzes the Cosmic Microwave Background (CMB). This faint radiation is the afterglow of the Big Bang and has now cooled to 3K, just a few degrees above absolute zero. The study of the CMB has led to the "standard model of cosmology." For example, we now know the composition and age of the universe at the few percent level. Our group has been centrally involved in establishing this model. There is much more to learn from the CMB and in the following we highlight just a couple of our outcomes. One may think of the CMB as coming to us and carrying information from a spherical shell at the edge of the observable universe. On its way to us, it weakly interacts with the cosmic structure that fills the volume of the universe. For example, when the CMB traverses a cluster of galaxies that contains hot electrons, the electrons give the CMB a small energy boost. This is called the Sunyaev-Zel’dovich effect. With our group’s dedicated telescope in northern Chile, the Atacama Cosmology Telescope, we have used this effect and discovered almost thirty new clusters. One of the clusters, nicknamed "El Gordo" by team member Felipe Menanteau, is the most massive, hottest, and X-ray luminous of any known cluster at seven billion light years distance or beyond. This amazing object made headlines at CNN (Jan 10, 2012) and around the world. It already has its own wiki page. On its way to us, the CMB can be deflected by intervening concentrations of matter through a process called gravitational lensing.One of the predictions of Einstein’s theory of General Relativity is that mass bends light. The effect we observe is from the net bending of the light from the edge of the universe by all of the intervening dark matter and clusters of galaxies. Our group was the first to detect the intrinsic signal and then to interpret it as a new handle on dark energy. This work was highlighted as one of the 2011 breakthrough of the year by a number of science magazines. The above are just two examples. In addition, we measure the polarization of the CMB to search for gravitational waves from the creation of the universe. We have also used the CMB to investigate and constrain the cosmological parameters that describe our universe. It has now become apparent that we can use the CMB to constrain or measure the sum of the neutrino masses. This is only possible because we know the basic model so well. It is likely that using the CMB to determine neutrino properties is the least expensive avenue for such investigations. The Princeton Gravity Group is heavily invested in training the next generation of scientists and sharing its finding with the general public. Over half a dozen former group members have gone on faculty positions in other universities. We run a vigorous "learn through research" program during the summer for undergraduates. Over the duration of this award, over sixty students have worked in our lab. Many have gone on to physics graduate schools around the country. We give numerous public talks in the US and around the world. We have been actively involved in, for example, the World Science Festival and the Girl Scouts.
