This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). The Cosmic Microwave Background (CMB) observations hold a remarkable wealth of information about the early Universe and they recently transformed cosmology into a precision science. While the basic notion and the ingredients of an expanding Universe are well established, fundamental questions on the Universe's origin still remain unanswered. Measuring the CMB polarization provides a powerful tool to address the most important of these questions. The polarization pattern in the CMB can be decomposed into two distinct components, E-mode and B-mode, and the latter (yet to be detected) can provide two remarkable insights. With the detected B-mode polarization, one can study the large-scale structure of the Universe and the parameters that affect this structure, such as absolute neutrino mass and Dark Energy evolution. The degree-scale B-mode polarization also should carry imprints from the Cosmic Gravitational-wave Background (CGB), giving us a glimpse of the Universe during its tiniest (10^-35) fraction of the very first second of life. This CGB signal is a direct measurement of the energy scale at which the inflationary Big Bang occurs. The proposal requests funds for development of an enhanced large-format focal plane polarimeter POLAR-1 with 4,608 polarization-sensitive superconducting bolometers operating at 150 GHz and integrated with a 1.5-m reflective radio telescope. The proposed combination of angular resolution and bolometer array size make it ideally suited to study both the lensing-induced and the CGB-generated B-polarization. POLAR-1 will detect lensing induced B-polarization with extremely high significance and search for CGB down to 2% of the initial perturbation in power. The project's broader impacts would be in its path-finding nature of building an instrument ~10x more sensitive to the CMB polarization. As cosmology captures the public imagination, it is a remarkably effective vehicle for stimulating students' interest in science. The project will continue contributing significantly to the training of next generation of scientists by integrating graduate and undergraduate education with technology and instrumentation development, astronomical field observations, and scientific analysis.
The program set out to develop high-throughput systems for next generation cosmic microwave background (CMB) polarization experiments that are 10x more sensitive than current generation projects. We studied the most economical designs that can be cheaply duplicated, and developed required sub-systems. The main areas of research include (1) modular focal plane architectures with optimal coupling; (2) large cryogenic optical components - vacuum windows, infrared-blocking filters, low-loss lenses, and their antireflection-coating; (3) high-throughput (telescope) optics. The team members from four participating institutes have obtained major achievements in all three areas. We developed a planar antenna-coupled polarimeter with tapered illumination and improved coupling. These detectors are packaged in high-density modules with SQUID current sensors on the back side. Both developments significantly enhance the coupling efficiency for this type of detectors, which are used widely in CMB polarization experiments. In the area of cold optics, we developed a complete set of components appropriate for next generation mm-wave systems with large (>~60cm) windows. These components include a low-loss HDPE window, a series of metal mesh filters (patterned by high-power lasers), absorptive filters based on ceramic alumina, and image forming alumina lenses. This optical train has been demonstrated to work in conjunction with a standard multi-staged Helium-3 refrigerator and a pulse tube cooler. Finally, in the area of CMB optics, we investigated two compact optical designs with high-optical throughput (etendue) - a 1.6m crossed-Dragone reflector and a 60-cm two-lens refractor (2x larger than BICEP and BICEP2). The pros and cons of both systems have been studied and compared. For the reflector, we devised and tested a novel scheme to terminate the spillover coupling onto the cold sky using reflective random scatters. We concluded that refractors still present an extremely cost-effective option for next generation experiments. We pursued the refractor approach further in the follow-up project BICEP3, which essentially utilized all the technologies developed by this MRI award. BICEP3 is scheduled to start observation at South Pole in 2015. Broader impact: in addition to being directly relevant to BICEP3, the technologies and approaches can be used in CMB-S4, a stage-IV CMB polarization experiment aiming to survey a large fraction (~50%) of the sky with arcminute resolution. The widely-endorsed CMB-S4 will be able to map matter distribution at high redshift without bias, enabling a vast range of cross-cutting research in astrophysics and precision cosmology. The activities associated with this MRI award, including instrument design, testing, and optimization for future scientific projects (BICEP3, CMB-S4), have provided ample opportunities for the training of graduate students, postdoctoral scholars, and other young investigators.
