UC Irvine

One of the most compelling challenges in cosmology is understanding the nature of dark energy and how it affects the expansion of the universe. The Dark Energy Spectroscopic Instrument (DESI) is attempting to improve our understanding by using baryon acoustic oscillations (BAO) to study the expansion history of the universe. In Chapter 1 I provide a brief introduction to the expansion history of the universe as well as the probes of large scale structure. In Chapter 2 I provide an in depth description of the DESI instrument, focal plane, and main survey. This thesis consists of two main projects focused on improving the DESI focal plane and analyses using quasars and the Lyman-alpha forest.

The first project, discussed in Chapter 3, focuses on improvements to the DESI focal plane. The focal plane uses 5,000 robotic fiber positioners, approximately 12% of which are unusable due to poor performance. Roughly half display linear phi behavior. This number is increasing at a rate of approximately 1 new positioner per week. If enough positioners become unusable, the main survey (and potential future DESI-ext and DESI-2 surveys) may not be able to finish on time. Many different mitigation strategies were studied, but I found that the best performance came by increasing the motor speed and by applying a scale factor correction. Each move also follows a new move strategy consisting of a back and forth move. Move tests were performed on the DESI focal plane in 2023 and in March and July of 2024. The July tests showed that 228 positioners could be recovered and returned to night operations. This is roughly 72% of the linear phi positioners.

The second project, discussed in Chapter 4, focuses on how quasar redshift errors impact the correlation functions used in the Lyman-alpha forest analyses. We measure systematic quasar redshift errors as an offset in the Lyman-alpha forest-quasar cross-correlation. Using data from the DESI Early Data Release (EDR) I found evidence for a redshift-dependent bias causing quasar redshifts to be underestimated with increasing redshift. This bias stems from the improper modeling of the Lyman-alpha optical depth in the high redshift quasar templates used for redshift estimation. New templates were derived for the year 1 quasar sample. The analysis was repeated and I found that the bias resolved and is now consistent with zero. The new templates were used to provide redshifts for the DESI Year 1 quasar sample and will be used for future data releases as well.