A Caltech Library Service

Detecting primordial gravitational waves with circular polarization of the redshifted 21 cm line. I. Formalism

Hirata, Christopher M. and Mishra, Abhilash and Venumadhav, Tejaswi (2018) Detecting primordial gravitational waves with circular polarization of the redshifted 21 cm line. I. Formalism. Physical Review D, 97 (10). Art. No. 103521. ISSN 2470-0010. doi:10.1103/PhysRevD.97.103521.

[img] PDF - Published Version
See Usage Policy.

[img] PDF - Accepted Version
See Usage Policy.


Use this Persistent URL to link to this item:


We propose a new method to measure the tensor-to-scalar ratio r using the circular polarization of the 21 cm radiation from the pre-reionization epoch. Our method relies on the splitting of the F = 1 hyperfine level of neutral hydrogen due to the quadrupole moment of the cosmic microwave background (CMB). We show that unlike the Zeeman effect, where M_F = ± 1 have opposite energy shifts, the CMB quadrupole shifts M_F = ± 1 together relative to M_F = 0. This splitting leads to a small circular polarization of the emitted 21 cm radiation. In this paper (Paper I in a series on this effect), we present calculations on the microphysics behind this effect, accounting for all processes that affect the hyperfine transition. We conclude with an analytic formula for the circular polarization from the Dark Ages as a function of pre-reionization parameters and the value of the remote quadrupole of the CMB. We also calculate the splitting of the F = 1 hyperfine level due to other anisotropic radiation sources and show that they are not dominant. In a companion paper (Paper II) we make forecasts for measuring the tensor-to-scalar ratio r using future radio arrays.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Hirata, Christopher M.0000-0002-2951-4932
Venumadhav, Tejaswi0000-0002-1661-2138
Additional Information:© 2018 American Physical Society. (Received 9 August 2017; published 21 May 2018) We thank Vera Gluscevic and Antonija Oklopčić for enlightening conversations and comments during the preparation of this work. C. H. and A. M. are supported by the U.S. Department of Energy and the David & Lucile Packard Foundation. C. H. is supported by the Simons Foundation and the National Aeronautics and Space Administration. T. V. gratefully acknowledges support from the Schmidt Fellowship and the Fund for Memberships in Natural Sciences at the Institute for Advanced Study.
Funding AgencyGrant Number
Department of Energy (DOE)UNSPECIFIED
David and Lucile Packard FoundationUNSPECIFIED
Simons FoundationUNSPECIFIED
Schmidt FellowshipUNSPECIFIED
Institute for Advanced StudyUNSPECIFIED
Issue or Number:10
Record Number:CaltechAUTHORS:20180521-153923564
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:86527
Deposited By: George Porter
Deposited On:21 May 2018 22:49
Last Modified:15 Nov 2021 20:39

Repository Staff Only: item control page