BAO vs. SN evidence for evolving dark energy

TL;DR

This paper reviews evidence for evolving dark energy using BAO and SN data, highlighting the influence of supernova datasets and potential systematic offsets that affect the significance of the findings.

Contribution

It critically assesses the robustness of evidence for time-varying dark energy, emphasizing dataset differences and systematic uncertainties impacting the results.

Findings

Evidence for evolving dark energy at 3σ level from BAO and SN data.

Supernova dataset choices significantly influence the dark energy evolution evidence.

Systematic offsets in supernova magnitudes can weaken the evidence for dark energy evolution.

Abstract

We critically review the evidence for time-varying dark energy from recent Baryon Acoustic Oscillations (BAO) and Supernova (SN) observations. First, we show that such evidence is present at the 3$\sigma$ level, even without the new BAO data from the dark energy Spectroscopic Instrument (DESI), by instead using BAO data from the dark energy Survey (DES), combined with the DES5Y supernovae and Planck CMB data. Next, we examine the role of the DES5Y supernova dataset, showing that the preference for time-varying dark energy is driven by the low redshift supernovae common to both the DES5Y and Pantheon+ compilations. We find that combining Pantheon+ and DES5Y supernovae by removing the common supernovae leads to two different results, depending on whether they are removed from the DES5Y or the Pantheon+ catalog, leading to stronger or weaker exclusion of $\Lambda$CDM, at the (3.8$\sigma$) and (2.5$\sigma$) level, respectively. These common supernovae have smaller error bars in DES5Y compared to Pantheon+, and, as recently pointed out, there is an offset in magnitude in DES5Y between supernovae at ($z > 0.1$), where almost all the measurements taken during the full five years of DES are, and the low-redshift ones ($z < 0.1$), where all the historical set of nearby supernovae lies. We show that marginalizing over such an offset in DES5Y would lead to significantly weaker evidence for evolving dark energy.