My field of study is observational cosmology. Using large imaging and spectroscopic surveys of galaxies, my group attempts to address fundamental questions of the Universe: Why is the expansion of the Universe acceleration? How did structure grow during the expansion? What role did neutrinos play? Is General Relativity valid on cosmological scales? I am a member of the Dark Energy Survey (DES) and the Dark Energy Spectroscopic Instrument (DESI) collaborations. DES surveyed a 5,000 square degree area of the southern sky collecting images of 300 million galaxies using a new instrument, the Dark Energy Camera, we built for the survey. DECam is mounted on the Blanco 4-m telescope at CTIO, Chile. The 6 year survey program ended in 2019 and we are now analyzing the collected data. Using just a fraction of the final data sample, the DES Year 3 results placed some of the tightest constraints on the cosmological standard model to date. We are also engaged in DESI, the Dark Energy Spectroscopic Instrument. DESI uses 5,000 robotically controlled fiber positioners to record spectra of 35 million galaxies and quasars thereby enabling the most precise, 3-dimensional map spanning the nearby universe to more than 10 billion light years. My group contributed significantly to the DESI instrument and I serve as the DESI Instrument Scientist. DESI will test the cosmological standard model at unprecedented levels, probe alternatives to General Relativity, and provide insight into the inflationary period of the Universe.