This proposal seeks continued support for observations with the Arcminute Cosmology Bolometer Array Receiver (ACBAR). ACBAR is a 16 element 230-mK bolometer receiver designed to produce high resolution images of the CMB in three mm wavelength bands. Mounted on the 2.1m Viper telescope at the South Pole, ACBAR has sensitivity that rivals balloon-borne experiments and angular resolution that they cannot hope to achieve. Making full use of the excellent atmospheric conditions in the Austral winter at the South Pole, ACBAR is producing images of the Cosmic Microwave Background (CMB) radiation with sensitivity and resolution that exceed the capabilities of even the proposed Planck satellite. Observations of the CMB provide a unique window to the early Universe; these data play a key role in the transformation of Cosmology to a precise science. In particular, small angular scale observations of the CMB are a new frontier about which comparatively little is known. On these angular scales, contributions from secondary anisotropies introduced by intervening structure are expected to become dominant. For example, the scattering of photons by hot gas bound to clusters of galaxies results in a spectral distortion of the CMB known as the Sunyeav-Zeldovich (SZ) effect. Observations of the SZ effect can provide important new constraints on theories of the growth of Universe's structure. The unique capabilities of ACBAR allow it to address a broad range of science focused on measuring the primary and secondary anisotropies of the CMB. ACBAR was deployed to the South Pole in December 2000 and has been operating since. The second half of the winter of 2003 and the entire 2004 season will be dedicated to observations with ACBAR. The requested 18 months of support for observations with ACBAR and analysis of the resulting data will help to realize the full potential of this uniquely powerful instrument for the study of cosmology. The four institutions supported by this proposal will continue to collaborate in the maintenance and operation of ACBAR and Viper, and participate in the analysis of the data. The results from ACBAR will serve as a vital complement to the large-scale MAP spacecraft dataset, and provide an essential check of the fine-scale excess power reported by the single frequency BIMA and CBI experiments. The novel instrumentation, observation techniques, and analysis developed for ACBAR are generally applicable to future ground based millimeter astronomy experiments. This project has provided hands on research experience to more than seven undergraduate students and will result in the theses of four graduate students.
