On the detection of Spectral Distortions in the CMB: Recombination to Reionization

Abstract

The LCDM model of cosmology predicts inevitable, weak distortions in the spectrum of the Cosmic Microwave Background (CMB) from that of a blackbody. However, no such deviations have been measured to date. This thesis focuses on CMB spectral distortions arising from the cosmological epochs of recombination and cosmic dawn & reionization. A detection and measurement of these CMB spectral distortions will enable a better understanding of the thermal and ionization history of the Universe and help us probe redshifts that have never been directly observed thus far. I present a feasibility study for a ground-based detection of extremely weak, ripple-like additive features in the CMB spectrum created by photons emitted during cosmological recombination (900 < z < 7000). I identify an octave band in the frequency range 2–6 GHz to be optimal for a detection of this CMB spectral distortion. This band maximizes signal-to-noise ratio and has sufficient spectral structure in the signal to aid foreground separation. I introduce the Maximally Smooth (MS) function, an algorithm to distinguish smooth foregrounds from the ripple like signal. Using synthetic spectra, I demonstrate the efficacy of using MS functions over polynomials to separate foregrounds from the cosmological recombination signal. Using Bayesian tests I estimate that using an array of 128 cryogenically cooled, ideal radio-telescopes, spectral ripples from the recombination epoch can be detected with 90% confidence in 255 observing days. Thus, it is in principle possible to detect these cosmological recombination signals in realistic observing times. Among others, astronomical foregrounds pose challenges to the detection of CMB spectral distortions. It is thus necessary to have a realistic expectation of the Galactic and extragalactic foreground spectra towards any given direction in the sky. I present GMOSS: Global Model for the Radio Sky Spectrum, a physically motivated model of the radio sky over 22 MHz–23 GHz. GMOSS describes foreground spectra towards all sky directions over 5 pixels using processes including synchrotron emission with possible spectral break, emission from composite source populations, free-free emission and thermal absorption.

Using GMOSS I investigate the spectral complexity expected in foregrounds and the effect of the same on the detection of the global redshifted 21-cm signal from cosmic dawn & reionization (6 < z < 150). I find that over large beamwidths foregrounds are spectrally smooth and describable using MS functions for various samplings of sky- coverage. However, it is more computationally challenging to describe foreground spectra towards the galactic plane, which is best avoided by experiments seeking to detect CMB spectral distortions from cosmic dawn & reionization (collectively EoR). Once again, I demonstrate the advantage of using MS functions over polynomials to separate foregrounds from the global EoR signal in mock-sky spectra. I find that using MS functions to separate foregrounds, the global signal from EoR can be detected using an ideal instrument with 95% confidence in 10 minutes observing time.

I conclude the thesis with a brief discussion on design criteria for radio-telescopes seeking to detect distortions in the CMB spectrum arising from the epochs of recombination through reionization.