Observations of the Cosmic Microwave Background (CMB) provide a unique window to the early Universe and are playing a key role in the transformation of Cosmology to a precise science. The CMB is a powerful tool for the study of the Universe that is still far from being fully exploited. This proposal seeks support for continued analysis of data from the Arcminute Cosmology Bolometer Array Receiver (ACBAR). ACBAR is a 16-element 230 microKelvin bolometer receiver that operated on the 2.1-m VIPER telescope at the Amundsen-Scott South Pole Station. Making full use of the excellent atmospheric conditions in the Austral winter at the South Pole, ACBAR has produced images of the CMB with sensitivity and resolution that exceed the capabilities of current balloon and satellite based telescopes. ACBAR accomplished it final season of observations and was decommissioned in December 2005. From the 2001-2003 observations, ACBAR produced high-resolution maps of the CMB fluctuations over a hundred square degrees of the lowest dust-contrast sky visible from the South Pole. However, during the 2004 and 2005 observational seasons, ACBAR tripled the sky coverage of earlier observations, including a deep image of same region of sky that was imaged in the long-duration balloon BOOMERanG 2003 flight. With these observations, it will be possible to cross-calibrate the BOOMERanG, ACBAR, and NASA's WMAP spacecraft experiments with an uncertainty of less than 2%. This uniform and precise calibration is necessary in order to realize the full potential of CMB observations to constrain cosmological parameters such as a running spectral index. The intellectual merit of the proposed work is multifaceted. This proposal will support the development and implementation of new algorithms for the analysis of large ground-based CMB datasets. These algorithms will be directly applicable to future ambitious experiments such as the South Pole 10-m Telescope. A full interpretation of the results requires a precise accounting of the effects of gravitational lensing and secondary anisotropies. The work outlined in this proposal will support the completion of PhD theses by three graduate students and will continue to involve undergraduate researchers in innovative research. The proposed work will have broad impact on the field of cosmology by producing measurements that are essential for the larger cosmology community engaged in the study of a diverse range of topics from inflation to the evolution of galaxy clusters. When combined with results from the WMAP experiment, ACBAR will provide precise constraints on cosmological parameters such as, dark matter density, running spectral index, and the size of matter density fluctuations. These measurements will play a key role in either supporting or refuting the emerging standard cosmological model. ACBAR will also place exciting new constraints on secondary anisotropies such as the Sunyaev-Zel'dovich effect from distant galaxy clusters, and should be able to make the first detection of gravitational lensing of the CMB. Through this effort, ACBAR will serve as a pathfinder for the analysis of the results form the next generation of experiments.
