The Low-Frequency Frontier: Cosmology with Future 21cm Experiments

Abstract

This thesis presents theoretical and observational investigations in two areas of cosmology: the detection of inflationary gravitational waves using the circular polarization of the redshifted 21cm line from neutral hydrogen during the Dark Ages, and the study of galactic foregrounds at low-frequencies using the Owens Valley Radio Observatory Long Wavelength Array (OVRO LWA). In the theoretical part of this thesis, we propose a new method to measure the tensorto-scalar ratio r using the circular polarization of the 21 cm radiation from the Dark Ages. In Chapter II we discuss the basic principles of inflationary physics, which is now accepted as a standard paradigm for the generation of perturbations in the early universe. Along with density (scalar) perturbations, inflation also produces gravitational wave (tensor) modes. In Chapter IV we outline a novel, albeit futuristic method to detect inflationary gravitational waves. Our method relies on the splitting of the F = 1 hyperfine level of neutral hydrogen due to the quadrupole moment of the CMB during the Dark Ages. We show that unlike the Zeeman effect, where MF = ±1 have opposite energy shifts, the CMB quadrupole shifts MF = ±1 together relative to MF = 0. This splitting leads to a small circular polarization of the emitted 21cm photon, which is in principle observable. Further, we forecast the sensitivity of future radio experiments to measure the CMB quadrupole during the era of first cosmic light (z ~ 20). The tomographic measurement of 21 cm circular polarization allows us to construct a 3D remote quadrupole field. Measuring the B-mode component of this remote quadrupole field can be used to put bounds on the tensor-to-scalar ratio r. We make Fisher forecasts for a future Fast Fourier Transform Telescope (FFTT), consisting of an array of dipole antennas in a compact grid configuration, as a function of array size and observation time. The forecasts are dependent on the evolution of the Lyman-α flux in the pre-reionization era, that remains observationally unconstrained. Finally, we calculate the typical order of magnitudes for circular polarization foregrounds and comment on their mitigation strategies. We conclude that detection of primordial gravitational waves with 21 cm observations is in principle possible, so long as the primordial magnetic field amplitude is small, but would require a very futuristic experiment with corresponding advances in calibration and foreground suppression techniques. In the observational part of this thesis, we investigate the cross-correlation between low-frequency radio maps from the Owens Valley Radio Observatory Long Wave-length Array (OVRO LWA) and tracers of the ISM: dust, Hα, and HI. Our goal is to search for any anomalous radiative processes at low frequencies (20 − 80 MHz). In Chapter III we discuss the basic principles of 21cm cosmology and provide an overview of current and planned 21cm experiments. Broadband foreground sources pose the greatest challenge to 21cm tomography and need to be characterized carefully before the technique becomes a sensitive probe of the dark ages and the epoch of reionization. The foregrounds are expected to be predominantly galactic and approximately four orders of magnitude larger than the cosmological signal. In Chapter V, we investigate the nature of the diffuse Galactic radio emission in the 20 − 80MHz frequency range using data from the OVRO-LWA. We cross-correlate LWA maps with tracers of ISM (dust, Hα, HI) from a number of surveys , to investigate galactic foregrounds relevant to detection of 21cm signal from the Dark Ages. We describe a formalism to compute the cross-power spectra between LWA maps and ISM tracers. Our results are consistent with no correlation between tracers of the gas and dust in the ISM at high Galactic latitudes (b > 55°) and low-frequency maps from the LWA, at scales l ~ 10 − 600 at a 99.9% confidence level.