Research

Observational Cosmology

My main focus is on observational cosmology at high redshift, mostly with the Lyman Alpha Forest. I conduct this research with data the Dark Energy Spectrosocpic Instrument (DESI), which is in the midst of an eight-year survey of over 60 million galaxies and quasars.

One of my recent roles in DESI was co-coordinator of the DESI DR2 BAO results, which were released in March 2025. The exciting outcome of this work is that models with dynamic dark energy match the observations better than a cosmological constant. The DESI collaboration has a Guide to the DESI DR2 BAO Results that includes links to the papers, a press release, graphics, and more information.

I have worked on various other topics in observational cosmology with DESI including the 1D power spectrum of the Lyman Alpha Forest, machine learning methods to predict the intrinsic spectra of quasars in the Lyman Alpha Forest region, the transverse proximity effect, quasar clustering and high redshift, and the the identification of astrophysical and instrumental systematics. Nearly all of this work was led by a great group of students and postdocs (see my ADS library on the left menu).

I am grateful for financial support of my research in observational cosmology from the U.S. Department of Energy, Office of Science.

Astronomical Instrumentation

My largest, relatively recent instrumentation work was the construction, integration, and commissioning of DESI. I served as Instrument Scientist from 2016-2021, which corresponded to much of the construction phase through the start of operations. This Overview of the Instrumentation for the Dark Energy Spectroscopic Instrument provides a detailed description of the science requirements, how those drove the technical requirements, details of the technical design decisions, and the acceptance, integration, and commissioning stages.

I am presently working on research and development for a future Stage 5 Spectroscopic Instrument, which would convert the 4-m Mayall and Blanco telescopes into 6-m telescopes that could each simultaneously observe 13,000 spectra.

My previous instrumentation projects include membership on the Science Verification Squad for the Dark Energy Camera (DECam), construction of the KOSMOS and COSMOS multi-object spectrograophs for the 4-m Mayall and Blanco telescopes, the OSMOS for the 2-4m Hilter Telescope at MDM Observatory, and earlier work on the MMIRS and PANIC infrared instruments.

Previous Research

I have worked extensively on a range of topics in the past, including the formation and evolution of galaxies and the evolution of supermassive black holes, particularly quasars. My main research in this area combined data from the Dark Energy Survey and the OzDES Project to measure the masses of supermassive black holes via the reverberation mapping technique. We recently published a paper with our OzDES final data release, black hole mass results, and scaling relations. I also worked on related topics that include how AGN are fueled, the lifetime of AGN, and the evolution of AGN in clusters of galaxies. My work on some of these topics are summarized in a pair of review articles on QSO Lifetimes and Why does Low-Luminosity AGN Fueling Remain an Unsolved Problem?