Precise cosmological analysis of Type Ia supernovae: the Hubble constant and dark energy

Abstract

The field of physical cosmology has advanced greatly in the last few decades. In this time, a holistic `concordance' model of our Universe has been pieced together: an isotropic and homogeneous Universe which started very small, went through a brief inflationary period, and obeys General Relativity. Approximately a third of its energy content can be attributed to matter (mostly cold dark matter, with some baryonic), and the remainder to `dark' energy, which appears to be a cosmological constant, tied to spacetime itself. Together, these components are referred to as the LambdaCDM model.

Type Ia supernovae (SNe Ia) have been instrumental in reaching this understanding, playing a pivotal role in the late 1990s by signalling the Universe's accelerated expansion. Since then, SNe Ia and the other probes (baryon acoustic oscillations, weak lensing, and clustering) have been employed to improve constraints on the quantities that parametrise LambdaCDM, and the equation-of-state parameter w for dark energy. These efforts have converged in the Dark Energy Survey (2013-2018), which coordinates these probes. The paradigm for cosmology now is to diminish systematic errors to obtain precise measurements of cosmological parameters through observations far into the distant past.

More locally, the Hubble constant H_0 determines the expansion rate of the Universe at present; this normalises its distance scale. In the past several years, the precise value of H_0 has come into contention, with a significant discrepancy between results determined from a local distance ladder (with SNe Ia at the top), and those inferred from observations of the early Universe assuming the cosmological models that drive its expansion history. Taken at face value, this discrepancy could signal systematic errors in either measurement, or inaccurate assumptions about the model.

The efforts in this thesis use SNe Ia to address questions at both ends of the Universe's expansion history - the Hubble constant locally, and dark energy at higher redshifts - using the current best methods to account for the statistical and systematic terms which affect supernovae. These methods, were developed by the Supernova Legacy Survey (SNLS) and subsequent Joint Lightcurve Analysis (JLA), rely on covariance matrices to encapsulate correlated uncertainties in all lightcurve parameters, which are then propagated through probabilistic Bayesian parameter estimation methods to uncertainties in cosmological parameters.

In the works contained within this thesis, I have developed a framework for performing precise cosmological analysis of SNe Ia samples, including using covariance matrices to quantify systematic terms. I have applied to the aforementioned pertinent questions: the value of the Hubble constant and the nature of dark energy, using the datasets in SH0ES (Riess et al. 2011) and intermediate Dark Energy Survey supernova data.