QSO pairs and the Lyman-alpha forest: Observations, simulations, and cosmological implications

This dissertation addresses two cosmological applications of the Lyman-alpha (Lyalpha) forest observed in QSO pairs separated by several arcminutes or less. The Lyalpha flux autocorrelation and cross-correlation provide a measurement of cosmic geometry at z > 2, via a variant of the Alcock-Paczynski test. I present the results of an observing campaign to obtain moderate resolution spectroscopy of the Lyalpha forest in QSO pairs with small redshift differences (Deltaz < 0.25) and arcminute separations (theta < 5'). This new sample includes 29 pairs and one triplet suitable for measuring the cross-correlation and 78 individual QSO spectra for determining the autocorrelation. Continuum fits are provided, as are seven revisions for previously published QSO identifications and/or redshifts.; Using a suite of hydrodynamic simulations, anisotropies in the Lyalpha flux correlation function due to redshift-space distortions and spectral smoothing are investigated for 1:8 ≤ z ≤ 3, further enabling future applications of the Alcock-Paczynski test with Lyalpha correlation measurements. Sources of systematic error including limitations in mass-resolution and simulation volume, prescriptions for galactic outflow, and the observationally uncertain mean flux decrement are considered. The latter is found to be dominant. An approximate solution for obtaining the zero-lag cross-correlation for arbitrary spectral resolution is presented, as is a method for implementing the resulting anisotropy corrections while mitigating systematic uncertainty.; Finally, I establish a new test using the Lyalpha forest for distinguishing binary QSOs at the same redshift from wide-separation (theta > 7") gravitationally lensed QSOs. The latter phenomena only result from lensing by clusters of galaxies and, therefore, probe the abundance and evolution of the most massive, collapsed structures in the universe at z ≲ 3. The lensing hypothesis can be difficult to confirm or refute; however, the rms of the spectral difference minus the crosscorrellation is found to be effective for this purpose, and the dependance of this statistic on various observational parameters is investigated. The results are applied to the spectra of two confirmed binary QSO systems as well as the only known wide-separation gravitationally lensed QSO at z > 2, SDSS J1029+2623. The nature of the latter object is called into question and discussed.