UC Santa Cruz

Observational determinations of the primordial light element abundances produced during Big Bang Nucleosynthesis (BBN) provide an important test on our current understanding of the Universe and the Standard Model due to their sensitivities to two parameters at the time of BBN: the baryon density and the expansion rate of the Universe. The primordial helium-4 abundance is particularly sensitive to the latter, which is partly driven by the number of effective neutrino species. In this thesis, I present a new observational survey to discover near-pristine environments in our local Universe whose properties can be used to determine the latest value of the primordial \fourHe\ abundance.

I first describe the details of our observational survey, which uses photometry from the Sloan Digital Sky Survey (SDSS) to identify candidate metal-poor galaxies. We use the Kast spectrograph on the Shane 3m telescope at Lick Observatory to obtain confirmation spectroscopy and follow up on a subset of these systems using LRIS and NIRSPEC/NIRES at Keck Observatory, which we name the Primordial Helium Legacy Experiment with Keck (PHLEK) survey. The high S/N optical and near-infrared (NIR) spectroscopy of the PHLEK sample enable a direct measurement of the electron temperature for the oxygen abundance and the detection of a suite of HeI lines for the helium abundance.

Our survey results include the discovery of the Little Cub, one of the lowest-metallicity star-forming galaxies currently known. The Little Cub has a gas phase oxygen abundance about a twentieth solar metallicity and is a testament to the success in picking out metal-poor systems from photometry alone.

Finally, I describe our code yMCMC, which uses the Markov Chain Monte Carlo (MCMC) technique to explore an 8-dimensional parameter space and solve for the parameters that best describe our observations. We supplement our PHLEK sample with SDSS spectroscopy and existing low-metallicity systems in the literature. Using systems well-modelled by yMCMC, we make an extrapolation to the primordial helium number abundance ratio, finding y_P=0.0805+/-0.0017. When combined with the existing primordial deuterium abundance, this places constraints on the baryon-to-photon ratio and effective number of neutrino species in agreement with the Standard Model.