MITEoR: a HERA pathfinder instrument for cheaper 21 cm precision cosmology

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Mapping neutral hydrogen throughout our Universe via its redshifted 21 cm line offers a unique opportunity to probe the cosmic "dark ages" and the formation of the first luminous objects. Moreover, because it can map a much larger volume of our Universe, it has the potential to overtake the cosmic microwave background as the most sensitive cosmological probe of the epoch of reionization, inflation, dark matter, dark energy, and neutrino masses. This potential has stimulated growing community interest, reflected by the Hydrogen Epoch of Reionization Arrays (HERA) effort and the Astro2010 Decadal Review, with initial focus of technology development and demonstration.The goal of this research is to develop a digital radio interferometer back-end with a novel architecture, test it with a 64 dual-polarization antenna array, and generate calibrated foreground maps covering much of the sky at 110-190 MHz, all of which will help further HERA development. By exploiting a hierarchical antenna grid and 4-dimensional Fast Fourier Transforms (FFT), the proposed correlator cost scales as N log N with the number of independent elements rather than as N^2 like other HERA pathfinder experiments, allowing significant cost savings/collecting area increases down the road. Also, N independent sky beams are imaged simultaneously, improving mapping sensitivity. By exploiting the massive baseline redundancy in this antenna grid, gain and phase calibration for all antennas can be made significantly more accurate and fully automated. This can in turn produce more accurate modeling of the synthesized and primary beams, which has been shown to improve the quality of the foreground modeling and removal which is so crucial to 21 cm cosmology.The proposed MIT Epoch of Reionization (MITEoR) instrument will be built on the NSF-funded open-source CASPER FPGA hardware platform, ensuring that the technology and instrumentation that we develop can be used by the worldwide community and incorporated into future HERA instruments, regardless of their antenna design and frequency range. In particular, most current visions for massive future radio arrays (like HERA-II and SKA) involve large compact cores, for which the technology that we will develop arguably offers both the lowest correlator cost and the most accurate calibration.Along with strengthening the foundation for the emerging standard model of cosmology, this research should develop broadly useful technology, algorithms and tools. The hierarchical FFT imaging idea is also relevant to other areas, e.g., the search for radio transients and perhaps microwave background polarization.As in the past, such products of our work useful to the broader community (data, algorithms, software, etc.) will promptly be made publicly available. The proposed research will also provide valuable student training. Furthermore, since the origin, evolution and fate of the universe have captivated the imagination of the general public, our group will continue to be very engaged in both formal and informal public outreach activities on this topic.