Cosmo-Astro Seminar | Supernova Cousins: Leveraging Galaxy-group Information to Improve Type Ia Supernova Cosmology

Type Ia supernovae (SNe Ia) are a cornerstone of precision cosmology, but at low redshift their accuracy is increasingly limited by systematic uncertainties. This has gained urgency with recent results from the Dark Energy Spectroscopic Instrument suggesting possible cracks in the standard cosmological model, including evolving dark energy. Among the dominant systematics are peculiar velocities, which are tied to large-scale structure and environmental effects, which are linked to galaxy evolution. Peculiar velocities have been studied through models of the velocity field and galaxy groups and clusters, while environmental effects have been probed using rare "supernova siblings," multiple SNe Ia within the same host galaxy. These approaches, however, focus on either the motions of galaxies or the environment that these SNe go off in, but not both at the same time. In this talk, I introduce "supernova cousins": SNe Ia in different galaxies within the same group or cluster, thereby combining both effects. Using a sample of 1,086 low-z SNe Ia from ATLAS, we construct a sample of 93 SN Cousins. With this sample, I compare distance scatter across environments and test the limits of how well we can use them to improve SN cosmology.