The Pantheon+ Analysis: Improving the Redshifts and Peculiar Velocities of Type Ia Supernovae Used in Cosmological Analyses

Abstract

We examine the redshifts of a comprehensive set of published Type Ia supernovae, and provide a combined, improved catalog with updated redshifts. We improve on the original catalogs by using the most up-to-date heliocentric redshift data available; ensuring all redshifts have uncertainty estimates; using the exact formulae to convert heliocentric redshifts into the Cosmic Microwave Background (CMB) frame; and utilizing an improved peculiar velocity model that calculates local motions in redshift-space and more realistically accounts for the external bulk flow at high-redshifts. In total we reviewed 2821 supernova redshifts; 534 are comprised of repeat-observations of the same supernovae and 1764 pass the cosmology sample quality cuts. We found 5 cases of missing or incorrect heliocentric corrections, 44 incorrect or missing supernova coordinates, 230 missing heliocentric or CMB frame redshifts, and 1200 missing redshift uncertainties. Of the 2287 unique Type Ia supernovae in our sample (1594 of which satisfy cosmology-sample cuts) we updated 990 heliocentric redshifts. The absolute corrections range between 10 −8 ≤ ∆z ≤ 0.038, and RMS(∆z) ∼ 3 × 10 −3 . The sign of the correction was essentially random, so the mean and median corrections are small: 4×10 −4 and 4×10 −6 respectively. We examine the impact of these improvements for H 0 and the dark energy equation of state w and find that the cosmological results change by ∆H 0 = −0.11 km s −1 Mpc −1 and ∆w = −0.001, both significantly smaller than previously reported uncertainties for H 0 of 1.4 km s −1 Mpc −1 and w of 0.04 respectively.

The art of building a smooth cosmic distance ladder in a perturbed universe

“…To obtain the corresponding expression for an observer in the Heliocentric frame, we simply need to boost to the Heliocentric frame. Recent analysis of this effect for the SN1A shows that the impact of this on the Hubble rate is about a few percent which is much smaller than the effect we are interested here [17]. To define a surface of constant redshift, we expand the full expression for the redshift in perturbation theory…”

“…This is very crucial because the local distance ladder is calibrated using distance measurement to sources that have decoupled from the Hubble flow [9,10]. For those sources, it is well-known that the redshift is not a monotonic function of the Hubble flow [16][17][18]. Several works have studied the luminosity distance (distance modulus) in perturbation theory before [19][20][21][22][23][24][25][26], the result from these efforts show that the background FLRW spacetime is a good approximation of the area distance in the observed universe [27,28].…”

The Pantheon+ Analysis: Forward-Modeling the Dust and Intrinsic Colour Distributions of Type Ia Supernovae, and Quantifying their Impact on Cosmological Inferences

“…Cross-calibration for all surveys is done in . The redshifts for the entire Pantheon+ sample have been reevaluated in Carr et al (2021) and peculiar velocities are computed in Peterson et al (2021).…”

Constraining R$_V$ Variation Using Highly Reddened Type Ia Supernovae from the Pantheon+ Sample

“…Pantheon+ is released as a series of papers. The redshifts and peculiar velocities of the SNe Ia are described in Carr et al (2021). A comprehensive analysis of redshift systematics are presented in Peterson et al (2021).…”