Institute for Advanced Study/Princeton University Early Universe/Cosmology Lunch Discussion

Various cosmological parameters such as H₀ and S₈ measured using Type Ia Supernovae (SNe Ia) have been shown to be in tension with measurements from the early universe. Additionally, the recent DESI analysis made it clear that, depending on the analysis and treatment of systematics, similar SN samples (i.e., Pantheon+ and Union3) can result in different constraints on cosmological parameter measurements. In this talk, I will focus on improving these measurements and reducing scatter on the Hubble diagram by addressing both redshift scatter from leveraging galaxy groups and magnitude scatter from leveraging the near-infrared (NIR). Using the low-redshift sample from Pantheon+ combined with a new sample from the AAT spectrograph, we obtain and average the redshifts of galaxy groups to correct for peculiar velocities and smooth over small-scale motions of galaxies. With the DEHVILS sample, one of the largest uniform NIR samples of SN Ia light curves, we take advantage of the fact that NIR light is less affected by dust. With the impending arrival of both (i) the Rubin Observatory’s Legacy Survey of Space and Time, which we show will greatly benefit from accounting for peculiar velocities, and (ii) the Roman Space Telescope, which will observe in the NIR and need a well understood low-redshift anchor SN sample, we provide a motivating case for defining more galaxy groups using Subaru’s Prime Focus Spectrograph (PFS) and encourage further analysis on NIR SN Ia light curves.