Kyle Dawson - The University of Utah

Kyle S. Dawson

Professor, Physics and Astronomy

328 INSCC • 801-581-4785 • kdawson@astro

Cosmology with Spectroscopic Surveys

Given the initial variations in density following the end of inflation, the cosmological model describes the expansion of the Universe and the growth of those density fluctuations due to gravitational collapse. The model that best describes the observed expansion and growth relies on Einstein's theory of General Relativity (GR) as the model for gravity, and an expansion history driven by a Universe composed of matter, photons, neutrinos, and a dark energy component. There is no clear explanation for the dark energy component, but a cosmological constant that is compatible with General Relativity describes all observations to date. Whether explained by a cosmological constant or some other mechanism, the dark energy component leads to a cosmic expansion rate that is now increasing with time.

New observations are required to better understand if cosmic acceleration is caused by deviations from GR on large scales, a cosmological constant, or a new form of energy. It is possible to distinguish scenarios of acceleration that require dark energy from those that require modifications to GR by independently probing both cosmic expansion history and the structure growth rate. Of equal importance to dark energy, the fundamental properties of neutrinos and the global curvature of space are recorded in cosmic expansion history and the structure growth rate. Wide-field optical spectroscopy figures prominently in the effort to constrain fundamental physics; the three dimensional maps of galaxies and quasars obtained by such surveys provide an atlas of large-scale structure in which the physics of the expansion history, neutrino masses, curvature, and GR are embedded.

In 2014, we completed a spectroscopic survey of 10,000 square degrees using a 1000-fiber spectrograph mounted to the 2.5-meter telescope at Apache Point Observatory in New Mexico. Designed to study Baryon Acoustic Oscillations (BAO), we observed more than one million galaxies and 200,000 distant quasars over a period of five years. The data were used to constrain the cosmic expansion history to 1-2% precision in three distinct periods of cosmic time, the most distant of which dates back more than 10 billion years. This project, known as the Baryon Oscillation Spectroscopic Survey (BOSS), placed tighter constraints on cosmic expansion history than any other spectroscopic survey. Please see the list of my BOSS publications for more details on the scientific results from this project.

As a followup to BOSS, I became the Principal Investigator on the Extended Baryon Oscillation Spectroscopic Survey (eBOSS). eBOSS constrained cosmology through spectroscopic observations of an entirely new sample of galaxies and quasars from that used in BOSS. eBOSS ran from 2014 until 2019 using the same spectrograph and telescope as that used in BOSS. We obtained roughly 350,000 galaxy redshifts and roughly 350,000 quasar redshifts to explore cosmology in epochs never studied from the perspective of spectroscopic clustering. My research group led the effort in running eBOSS and made significant contributions to both core cosmology measurements and to measuring the signatures of astrophysical processes behind the distant galaxies and quasars we observe.

The eBOSS collaboration released the final measurements of BAO and growth and the cosmological interpretation of those measurements in July 2020. The final SDSS, BOSS, and eBOSS measurements include BAO measurements of angular-diameter distances and Hubble distances from eight different samples and six measurements of the growth rate parameter from redshift-space distortions (RSD). These measurements allow the most precise measurements to date on the nature of dark energy, the curvature of the Universe, the neutrino mass, the Hubble constant, and the current amplitude of matter fluctuations. When assessing the gain in precision across these cosmological parameters, we find that the SDSS BAO and RSD data reduce the total posterior volume by a factor of 40 relative to the previous generation of experiments. Adding again the Planck, Dark Energy Survey, and Pantheon Supernova Ia samples leads to an overall contraction in the five-dimensional posterior volume of three orders of magnitude. While the combined analyses allow 1% constraints on key cosmological parameters, they also add several layers of robustness to the estimates of the current cosmic expansion rate that are in strong tension with other techniques. As the estimates of the Hubble constant from Planck and SDSS rely on different physical assumptions than estimates using nearby galaxies, it is possible that the tension is due to a previously unknown particle or field that affects the expansion rate in the early Universe. If future measurements reveal this to be the case, it will be one of the most exciting physics discoveries of the decade. Please see the list of all eBOSS publications for the results from this project.

Finally, as of September 1, 2020, I am the co-spokesperson along with Nathalie Palanque-Delabrouille for the Dark Energy Spectroscopic Instrument (DESI). DESI completed commissioning in 2020 and will likely begin scientific operations around New Year 2021. When complete, DESI will be the largest spectroscopic survey ever conducted. DESI will use four different classes of tracer to measure BAO and RSD over all redshifts to z=3.5. In total, more than 30 million galaxy and quasar redshifts will used to measure BAO, RSD, and inflationary parameters. A summary of the scientific program for DESI can be found in the final design report.

Prior Research

As a graduate student, I used the Berkeley-Illinois-Maryland Array (BIMA) and the Owens Valley Radio Observatory (OVRO) to perform radio/microwave observations of the sky at 30 GHz. We used these interferometers to study small-scale anisotropy in the Cosmic Microwave Background (CMB) induced by galaxy clusters through a process known as the Sunyaev-Zel'dovich (SZ) effect. A blind survey for arcminute-scale anisotropy in 18 independent fields constituted my thesis project. With these data, I placed the best limits at the time on secondary CMB anisotropy at angular scales optimized for the SZ signature. We also used these instruments to conduct a survey of known galaxy clusters. These data were used in combination with archival X-ray data to place new constraints on the cosmic distance scale using projections of the cluster gas profile as a distance indicator and to measure the gas mass fraction in galaxy clusters. Please see the list of my CMB publications for more details on my work with CMB observations.

As a postdoctoral researcher, I used the Hubble Space Telescope to study Type Ia supernovae (SNe Ia) and galaxy clusters. I continued this work in my first five years at Utah and expanded it to include studies of SNe in the ultraviolet with the Swift satellite and in the optical from the Sloan Digital Sky Supernova Survey. The results of my work in the field of SN cosmology can be found in the list of SN Publications. The results from our studies of galaxy clusters can be found in the list of Cluster Publications.

As a postdoctoral researcher, I also developed new charge-coupled devices (CCDs) for a proposed space-based telescope. These CCDs were designed for significantly improved quantum efficiency up to 1000 nm and for more resilience against the harsh environment of space radiation. I conducted table-top experiments by exploring the damage incurred by 12.5 MeV protons from the LBNL 88-Inch Cyclotron onto these detectors. We then modelled the degradation in weak lensing signal expected from radiation damage. This experience led to other work in calibration and instrumentation that can be found in my list of Instrument Publications.