Reconstructing CMB fluctuations and the mean reionization optical depth

Abstract

The Thomson optical depth from reionization is a limiting factor in measuring the amplitude of primordial fluctuations, and hence in measuring physics that affects the low-redshift amplitude, such as the neutrino masses. Current constraints on the optical depth, based on directly measuring large-scale cosmic microwave background (CMB) polarization, are challenging due to foregrounds and systematic effects. Here, we consider an indirect measurement of large-scale polarization, using observed maps of small-scale polarization together with maps of fields that distort the CMB, such as CMB lensing and patchy reionization. We find that very futuristic CMB surveys will be able to reconstruct large-scale polarization, and thus the mean optical depth, using only measurements on small scales.

5 pages, 2 figs. Comments welcome

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• Beyond CMB cosmic variance limits on reionization with the polarized SZ effect
• Reconstructing large scales at cosmic dawn
• Reconstructing the Primary CMB Dipole
• Reconstructing Patchy Reionization with Deep Learning
• Cosmology at high redshift -- a probe of fundamental physics
• Cosmic variance mitigation in measurements of the integrated Sachs-Wolfe effect