The Cosmic Twilight Polarimeter: a Model-Independent Approach to Constrain the Synchrotron Foreground Spectrum for Global 21-cm Cosmology

Nhan, Bang Dinh

Graduate Thesis Or Dissertation

The Cosmic Twilight Polarimeter: a Model-Independent Approach to Constrain the Synchrotron Foreground Spectrum for Global 21-cm Cosmology Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/d791sg234

Abstract

Detecting the cosmological sky-averaged (global) 21-cm spectrum as a function of observed frequency will provide a powerful tool to study the ionization and thermal history of intergalactic medium (IGM) in the high-redshift Universe (∼ 400 million years after the Big Bang). The biggest challenge in conventional ground-based total-power global 21-cm experiments is the removal of the Galactic and extragalactic synchrotron foreground (~ 10,000-100,000 K) to uncover the weak cosmological signal (∼ 10-100 mK) due to corruption on the spectral smoothness of foreground spectrum by instrumental effects. Although an absorption profile has been reported recently at 78 MHz in the sky-averaged spectrum by the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) experiment, it is necessary to confirm that the proposed observation is indeed the global 21-cm signal with an independent approach. In this thesis, we propose a new polarimetry-based observational approach that relies on the dynamic characteristics of the foreground emission at the circumpolar region to track and remove the foreground spectrum directly, without relying on any parametric foreground models as in conventional approaches. Due to asymmetry and the Earth's rotation, the projection of the anisotropic foreground sources onto a wide-view antenna pointing at the North Celestial Pole (NCP) can induce a net polarization which varies with time with a unique twice-diurnal periodicity. Different from the zenith-pointing global 21-cm experiments, by using this twice-diurnal signature, the Cosmic Twilight Polarimeter (CTP) is designed to measure and separate the varying foreground from the isotropic cosmological background simultaneously in the same observation. By combining preliminary results of the proof-of-concept instrument with numerical simulations, we present a detailed evaluation for this technique and its feasibility in conducting an independent global 21-cm measurement in the near future.

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