Improving the Supernova Hubble Diagram on Both Axes

Abstract

Simply by plotting distances versus recessional velocities of astronomical objects on the original ‘Hubble diagram,’ Edwin Hubble provided evidence for the expansion of the universe and helped spark a movement that has led to what modern cosmology is today. More recently, measurements of distances and recessional velocities, or redshifts, were used to discover the fact that ‘dark energy’ dominates our universe. However, the nature of dark energy remains unknown. Accordingly, improving measurements of distances and redshifts continues to be one of the most important tasks for cosmological analyses. This dissertation serves as a comprehensive overview of the current state of supernova (SN) cosmology and a valuable resource for future researchers seeking to deepen their understanding of these cosmic phenomena.Type Ia supernovae (SNe Ia) are used on the modern-day Hubble diagram to characterize the expansion of the universe as well as constrain the amount of dark energy in our universe to high precision. However, recent findings using SNe Ia have given rise to various tensions, including the ‘Hubble tension,’ which point to potential cracks in the standard model for cosmology, ΛCDM, and open the door for potential new physics. To address these tensions, I have led multiple projects on some of the top systematics in SN cosmology today. These projects have improved the supernova Hubble diagram on both axes.For the x-axis – redshifts, I led the largest ever SN analysis correcting for peculiar velocities, which are physical motions distinct from the expansion of the universe that affect observed redshifts. I found that accounting for peculiar velocities at low redshift on both large and small scales reduces scatter on the Hubble diagram (~30% reduction in ?2 values) and improves precision on cosmological parameters. I further concluded that correcting for peculiar velocities alone could not solve the Hubble tension. I extended this analysis by leading a pilot program on a multi-object spectrograph targeting the galaxy groups of SNe Ia that had not previously had a galaxy group defined. I determined that more galaxies were found to be in galaxy groups than previously expected and confirmed that by taking this galaxy group information into account, small-scale motions could be averaged over and smoothed out. This work has provided motivation for both current and future spectrographs to obtain galaxy group information for thousands more SN host galaxies in order to improve the constraining power of the next generation of telescopes.For the y-axis – distances, works in the literature indicate that better distance precision is attainable in the near infrared (NIR). To study this, I ran my own SN survey in the NIR, called the Dark Energy, H0, and peculiar Velocities using Infrared Light from Supernovae (DEHVILS) survey. With this NIR SN analysis, I achieved improved distance precision (≲ 0.13 mag) as compared to the optical and contributed significantly (~100 light curves) to the sample of publicly available and cosmology-quality NIR light curves. Additionally, I investigated the details of the DEHVILS sample and challenged claims in the literature about the true cause of intrinsic variation observed in the brightnesses of SNe Ia.My work in peculiar velocities and in the NIR is especially relevant for the upcoming next-generation surveys such as the Rubin Observatory’s Legacy Survey of Space and Time (LSST), which will observe at least an order of magnitude more SNe Ia than ever before, and NASA’s next flagship mission, the Roman Space Telescope, which will observe in the NIR out to a redshift of z ≈ 3. Specifically, my work in peculiar velocities has prepared us to make the most of the low-redshift sample from LSST, and my work in the NIR with the DEHVILS sample has prepared us for the NIR analysis we will do with Roman over the next decade. With these groundbreaking surveys and projects, we have the opportunity to uncover the true cosmological model for our universe.