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Searching for the Cosmic Dawn with the Hyperfine Structure Transition of Hydrogen


Eastwood, Michael William (2019) Searching for the Cosmic Dawn with the Hyperfine Structure Transition of Hydrogen. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/DYPP-JP25.


The 21 cm hyperfine structure transition of neutral hydrogen promises to open a window into the first billion years of the Universe (z > 6). With the exception of rare lines of sight towards exceptionally distant and luminous galaxies, this period of the universe's history remains largely unexplored. During this time the 21 cm transition is expected to be detectable as a 10--100 mK perturbation in the thermal Cosmic Microwave Background (CMB) spectrum.

Due to the large field of view of low frequency radio telescopes (typically composed of dipole antennas) and the fact that the line of sight distance can be inferred from the measured frequency of the transition, the ultimate goal of 21 cm cosmology is to produce three dimensional tomographic maps of the 21 cm brightness temperature. In this way, the formation of the first stars and galaxies will be revealed through their influence on the neutral gas around them.

This thesis saw the construction of the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA), a new low frequency (27--85 MHz) radio telescope located near Bishop, California. Composed of 288 crossed-dipole antennas, the OVRO-LWA is capable of imaging the entire visible hemisphere in a single 13 s snapshot image with 8 arcmin angular resolution.

The primary challenges faced by efforts to detect the highly redshifted 21 cm transition are seeing past the blinding glow of foreground radio emission that is five orders of magnitude brighter than the cosmological emission, and calibrating the instrument to a level where it's possible to make the separation between foreground emission and the 21 cm signal. In this thesis I will present foundational work using the OVRO-LWA to place upper limits on spatial fluctuations of the 21 cm transition during the Cosmic Dawn---the period of first star formation.

In this thesis I present the highest angular resolution maps of the full sky below 100 MHz, and generated with a new widefield imaging technique that is specialized for drift scanning interferometers. These sky maps are a 10-fold improvement in angular resolution over existing maps at comparable frequencies, and are publicly available now for use in modeling and subtracting the contamination of foreground emission in 21 cm experiments.

Using a 28 hr integration with the OVRO-LWA, I place to-date the most constraining upper limits on the amplitude of the 21 cm spatial power spectrum at the Cosmic Dawn, and the first limits at z > 18. Although the current constraints Δ212 ≲ (104mK)2 do not meaningfully restrict the parameter space of models of early star formation, they do inform the design and calibrations necessary for future measurements to push towards a detection of the high-redshift 21 cm transition. In making this measurement I demonstrate the application of a new foreground filter that accounts for the full covariance of the foreground emission, and provide an updated measurement of the foreground angular covariance. Finally, I interpret the limiting factors in this measurement and determine the instrumental calibration and characterization requirements the OVRO-LWA will need to achieve in order to make a detection of the 21 cm power spectrum of the Cosmic Dawn.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:cosmology: observations – dark ages, reionization, first stars, radio continuum: galaxies, radio continuum: ISM
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Astrophysics
Awards:France A. Córdova Graduate Student Fund, 2016.
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Hallinan, Gregg W.
Group:Astronomy Department, Owens Valley Radio Observatory (OVRO)
Thesis Committee:
  • Phinney, E. Sterl (chair)
  • Hallinan, Gregg W.
  • Hirata, Christopher M.
  • Kulkarni, Shrinivas R.
  • Pearson, Timothy J.
  • Readhead, Anthony C. S.
Defense Date:3 September 2018
Non-Caltech Author Email:michael.w.eastwood (AT)
Record Number:CaltechTHESIS:05272019-174203596
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for Ch. 4
Eastwood, Michael William0000-0002-4731-6083
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:11547
Deposited By: Michael Eastwood
Deposited On:28 May 2019 23:46
Last Modified:12 Jan 2022 00:01

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