Lyα as a cosmological probe of dark energy: assessing the impact of systematics

Abstract

In the late 1990s supernovae observations confirmed the late-time accelerated expansion of the Universe. This accelerated expansion is thought to result from the repulsive gravitational force of a mysterious substance called `dark energy', whose physical properties are completely unknown. One of the key science goals for the forthcoming decade is to obtain physical insight on the properties of dark energy through measurements of the large-scale clustering of matter in the Universe. Numerous dark energy experiments are planned employing a wide variety of techniques to probe the large-scale structure. Crucial to maximising the full potential of these experiments is assessing the impact of astrophysical systematics that can either diminish or destroy the cosmological signal. Within this thesis I focus specifically on assessing the impact of astrophysical systematics on large-scale structure measurements via the detection of Lyman-α (Lyα) radiation in both emission and absorption.

Firstly, the ongoing Baryon Oscillation Spectroscopic Survey (BOSS) will measure the baryon acoustic oscillation (BAO) scale from the three dimensional clustering of matter via the Lyα forest. The BAO scale is a characteristic length scale at which matter is observed to cluster in excess, imprinting a measurable signature on the large-scale structure in the Universe crucial to understanding dark energy. I develop calibrated semi-analytic Lyα forest simulations, equivalent in size and resolution to the largest N-body and hydrodynamical simulations, that can be performed on a single desktop computer in under a day. The synthetic Lyα forest spectra are shown to be in broad agreement with a range of observational measurements including the Lyα flux probability distribution and 1D line-of-sight flux power spectrum.

I demonstrate that the BAO scale can be correctly recovered from the 3D Lyα flux power spectrum measured from the simulated data. I estimate that a BOSS-like 10^4 deg^2 survey with ~15 background quasars per square degree and a signal-to-noise ratio of ~5 per pixel should achieve a measurement on the BAO scale to within ~1.4 per cent. Recently, BOSS published the first measurement of the BAO scale from the Lyα forest (Busca et al., 2013, Slosar et al., 2013) with a recovered accuracy of ~2.5 per cent. Using these simulations for an equivalent Lyα forest data sample I recover a fractional error of ~2.3 per cent. The speed and efficiency of this simulation approach is well suited for exploring the astrophysical systematics on the recovery of the BAO signature from large scale spectroscopic surveys such as BOSS.

Using these semi-analytic Lyα forest simulations, I assess the impact of HeII reionisation on the recovered accuracy of the BAO scale. Inhomogeneous HeII reionisation is driven by bright, rare quasars which can result in large-scale UV background and temperature fluctuations that could potentially affect the fractional precision to which the BAO scale can be recovered from the Lyα forest. I develop a semi-analytic model for HeII reionisation which agrees well both qualitatively and quantitatively with existing numerical cosmological HeII reionisation simulations. Investigating a variety of HeII reionisation models I assess their impact on the statistical measurements of the Lyα forest. I observe that HeII reionisation can produce a fractional increase of ~50 per cent in power at large-scales in the 3D Lyα flux power spectrum, reducing to ~5 per cent at k>0.03 Mpc^-1. However, despite the fractional increase in the large-scale power, I do not observe any change in the predicted accuracy to which the BAO scale can be recovered.

Secondly, the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) aims to measure the large-scale clustering of Lyα emitting (LAE) galaxies to explore the properties of dark energy. However, the observed clustering properties of LAE galaxies are sensitive to the radiative transfer of Lyα photons through the intergalactic medium (IGM) which can mimic gravitational effects, potentially reducing the precision of cosmological constraints. I assess the impact of these non-gravitational Lyα radiative transfer effects on the observed clustering of LAE galaxies, focusing in particular on the effects of the IGM velocity gradients, local density within the environment of an LAE galaxy and ionising background fluctuations. For example, linear redshift-space distortions on the LAE galaxy power spectrum are potentially degenerate with the Lyα radiative transfer effect owing to the dependence of Lyα flux on IGM velocity gradients.

Using Fisher matrices I assess the impact of these Lyα radiative transfer effects on recoverable cosmological constraints important for dark energy studies such as the growth rate of structure, f, the Hubble rate, H(z) and the angular diameter distance, D_A(z). At the power spectrum level, I observe a complete degeneracy between f and the Lyα radiative transfer effect associated with IGM velocity gradients, while D_A(z) and H(z) are independent of these degeneracies. Deriving next-to-leading order corrections for the clustering of LAE galaxies within the Eulerian perturbation theory framework I show that these degeneracies can be broken by considering higher order galaxy clustering statistics such as the three-point function (bispectrum). Therefore, making it possible to recover cosmological parameters from LAE galaxy surveys. Finally, I observe that by combining the LAE galaxy power spectrum and bispectrum measurements, the constraints on D_A(z) and H(z) can be further improved.