Observational Limits on Patchy Reionization: Implications for B-modes

Abstract

The recent detection of secondary CMB anisotropy by the South Pole Telescope places a conservative bound on temperature fluctuations from the optical depth-modulated Doppler effect of T_{3000} < sqrt{13} microK at multipoles l~3000. This bound is the first empirical constraint on reionization optical depth fluctuations at arcminute scales, tau_{3000} = 0.001 T_{3000}/microK, implying that these fluctuations are no more than a few percent of the mean. Optical depth modulation of the quadrupole source to polarization generates B-modes that are correspondingly bounded as B_{3000} = 0.003 T_{3000}. The maximal extrapolation to the l~100 gravitational wave regime yields B_{100} = 0.1 T_{3000} and remains in excess of gravitational lensing if the effective comoving size of the ionizing regions is R > 80 Mpc. If patchy reionization is responsible for much of the observed arcminute scale temperature fluctuations, current bounds on B_{100} already require R < 200 Mpc and can be expected to improve rapidly. Frequency separation of thermal Sunyaev-Zel'dovich contributions to the measured secondary anisotropy would also substantially improve the limits on optical depth fluctuations and B-modes from reionization.

4 pages, 3 figures; submitted to Phys. Rev. D (Brief Report)

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• Complete Reionization Constraints from Planck 2015 Polarization
• Constraints on Patchy Reionization from Planck CMB Temperature Trispectrum
• Measurements of the Cross Spectra of the Cosmic Infrared and Microwave Backgrounds from 95 to 1200 GHz
• Two-dimensional Topology of Cosmological Reionization
• A framework to mitigate patchy reionization contamination on the primordial gravitational wave signal