The Pantheon plus Analysis: Evaluating Peculiar Velocity Corrections in Cosmological Analyses with Nearby Type Ia Supernovae

Separating the components of redshift due to expansion and peculiar motion in the nearby universe (z < 0.1) is critical for using Type Ia Supernovae (SNe Ia) to measure the Hubble constant (H (0)) and the equation-of-state parameter of dark energy (w). Here, we study the two dominant motions contributing to nearby peculiar velocities: large-scale, coherent-flow (CF) motions and small-scale motions due to gravitationally associated galaxies deemed to be in a galaxy group. We use a set of 584 low-z SNe from the Pantheon+ sample, and evaluate the efficacy of corrections to these motions by measuring the improvement of SN distance residuals. We study multiple methods for modeling the large and small-scale motions and show that, while group assignments and CF corrections individually contribute to small improvements in Hubble residual scatter, the greatest improvement comes from the combination of the two (relative standard deviation of the Hubble residuals, Rel. SD, improves from 0.167 to 0.157 mag). We find the optimal flow corrections derived from various local density maps significantly reduce Hubble residuals while raising H (0) by similar to 0.4 km s(-1) Mpc(-1) as compared to using CMB redshifts, disfavoring the hypothesis that unrecognized local structure could resolve the Hubble tension. We estimate that the systematic uncertainties in cosmological parameters after optimally correcting redshifts are 0.06-0.11 km s(-1) Mpc(-1) in H (0) and 0.02-0.03 in w which are smaller than the statistical uncertainties for these measurements: 1.5 km s(-1) Mpc(-1) for H (0) and 0.04 for w.

Automated summary

Separating the redshift components caused by expansion and peculiar motion is crucial for measuring the Hubble constant and the equation-of-state parameter of dark energy. In this study, the researchers analyze the dominant motions in nearby peculiar velocities, including large-scale coherent-flow motions and small-scale motions within galaxy groups. Using a sample of 584 low-redshift supernovae, they evaluate the effectiveness of correcting these motions and find that combining group assignments and coherent-flow corrections yield the greatest improvement in Hubble residual scatter. The results also show that optimal flow corrections significantly reduce Hubble residuals, suggesting that unrecognized local structure is unlikely to resolve the Hubble tension.