Cosmic bandits: Exploration versus exploitation in CMB B-mode experiments |
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Institution: | 1. Theory Group, Department of Physics and Texas Cosmology Center, The University of Texas at Austin, TX 78712, USA;2. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA;1. NASRDA-Centre for Basic Space Science, Nsukka, Enugu State, Nigeria 410001, Nsukka, Nigeria;2. Department of Physics and Astronomy, University of Nigeria, Nsukka, Enugu State, Nigeria 410001, Nsukka, Nigeria;1. Division of Theoretical Astronomy, National Astronomical Observatory of Japan,2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan;2. Department of Natural Science, Kochi University, 2-5-1 Akebono-cho, Kochi 780-8520, Japan;3. Department of Human Informatics, Aichi Shukutoku University, 2-9 Katahira, Nagakute, Aichi 480-1197, Japan;1. Physics Department, University of Rijeka, Radmile Matejčić, 51000 Rijeka, Croatia;2. INAF Astronomical Observatory of Padova, via dell’Osservatorio 8, Asiago (VI) 36012, Italy;1. Sarojini Naidu College for Women, Kolkata 700028, India;2. Camellia Institute of Engineering, Kolkata 700129, India;3. St. Xavier’s College (Autonomous), Kolkata 700016, India;4. Harish Chandra Research Institute, Allahabad 211019, India |
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Abstract: | A preferred method to detect the curl-component, or B-mode, signature of inflationary gravitational waves (IGWs) in the cosmic microwave background (CMB) polarization, in the absence of foregrounds and lensing, is a prolonged integration over a single patch of sky of a few square degrees. In practice, however, foregrounds abound and the sensitivity to B modes can be improved considerably by finding the region of sky cleanest of foregrounds. The best strategy to detect B modes thus involves a tradeoff between exploration (to find lower-foreground patches) and exploitation (through prolonged integration). This problem is akin to the multi-armed bandit (MAB) problem in probability theory, wherein a gambler faces a series of slot machines with unknown winning odds and must develop a strategy to maximize his/her winnings with some finite number of pulls. While the optimal MAB strategy remains to be determined, a number of algorithms have been developed in an effort to maximize the winnings. Here, based on this resemblance, we tackle the search for IGW B modes with single frequency experiments in the presence of spatially varying foregrounds by developing adaptive survey strategies to optimize the sensitivity to IGW B modes. We demonstrate, using realistic foreground models and taking lensing-induced B modes into account, that adaptive experiments can substantially improve the upper bound on the tensor-to-scalar ratio (by factors of 2 and 3 in single frequency experiments, and possibly even more). Similar techniques can be applied to other surveys, including 21-cm measurements of signatures of the epoch of reionization, searches for a stochastic primordial gravitational wave background, deep-field imaging by the James Webb Space Telescope or various radio interferometers, and transient follow-up searches. |
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