The Effect of Weak Lensing on Supernova Ia Cosmology at Redshifts Greater Than One

The Nancy Grace Roman Space Telescope (Roman) will discover and measure large numbers of Type Ia supernovae (SNe Ia) to unprecedented high redshifts (z~3). The impact of weak-lensing magnification grows with redshift, and will affect considerations of survey strategy through higher uncertainties at high-z and also skewing the distribution of selected SNe Ia. We use simulations from the combined Dark Energy Survey, DESC DC2, and Roman work to determine magnifications for likely SNe discovered as part of a Roman survey. We find that the impact of lensing is less strong than the cannonical 0.055z regularly used in SNe Ia analyses and only reaches 0.05 mag at z=2. We find that because of selection effects, the distribution of brightnesses of SNe Ia is further skewed by magnification. This work can be used to predict constraints on σ₈ from Hubble residuals of SNe Ia.