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Published July 26, 2016 | Published + Submitted
Journal Article Open

A Practical Theorem on Using Interferometry to Measure the Global 21-cm Signal

Abstract

The sky-averaged, or global, background of redshifted 21 cm radiation is expected to be a rich source of information on cosmological reheating and reionization. However, measuring the signal is technically challenging: one must extract a small, frequency-dependent signal from under much brighter spectrally smooth foregrounds. Traditional approaches to study the global signal have used single antennas, which require one to calibrate out the frequency-dependent structure in the overall system gain (due to internal reflections, for example) as well as remove the noise bias from auto-correlating a single amplifier output. This has motivated proposals to measure the signal using cross-correlations in interferometric setups, where additional calibration techniques are available. In this paper we focus on the general principles driving the sensitivity of the interferometric setups to the global signal. We prove that this sensitivity is directly related to two characteristics of the setup: the cross-talk between readout channels (i.e., the signal picked up at one antenna when the other one is driven) and the correlated noise due to thermal fluctuations of lossy elements (e.g., absorbers or the ground) radiating into both channels. Thus in an interferometric setup, one cannot suppress cross-talk and correlated thermal noise without reducing sensitivity to the global signal by the same factor—instead, the challenge is to characterize these effects and their frequency dependence. We illustrate our general theorem by explicit calculations within toy setups consisting of two short-dipole antennas in free space and above a perfectly reflecting ground surface, as well as two well-separated identical lossless antennas arranged to achieve zero cross-talk.

Additional Information

© 2016 The American Astronomical Society. Received 2016 January 5; revised 2016 May 16; accepted 2016 May 18; published 2016 July 26. We would like to thank Michael Eastwood, Shriharsh Tendulkar, and Kris Sigurdson for several helpful discussions, and Matias Zaldarriaga for comments on the manuscript. We also thank Aaron Parsons, Adrian Liu, and Morgan Presley for useful feedback at an early stage of this work. We thank the anonymous referee for useful comments, and in particular for asking the question that inspired Section 5. TV gratefully acknowledges support from the Schmidt Fellowship and the Fund for Memberships in Natural Sciences at the Institute for Advanced Study. CH is supported by the US Department of Energy, the David & Lucile Packard Foundation, and the Simons Foundation. T.-C. C. acknowledges support from MoST grant 103-2112-M-001-002-MY3. Part of the research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

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Published - apj_826_2_116.pdf

Submitted - 1512.05248v2.pdf

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August 20, 2023
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October 20, 2023