MEET-U Project I: The key drivers of the preference for dynamic dark energy

Joint analysis of the baryon acoustic oscillations (BAO) measurement by the Dark Energy Spectroscopic Instrument (DESI) first data release, Type Ia supernovae (SNe) of the Dark Energy Survey Year 5 (DES5YR) release and cosmic microwave background (CMB) data favors a quintom-like dynamic dark energy model over the standard Lambda cold dark matter (ΛCDM) model at 3.9σ level (Adame et al. 2024). We demonstrate that the preference for dynamic dark energy does not rely on the detailed modeling of CMB physics and remains at 3.2σ level when the full CMB likelihood is replaced by a CMB acoustic-oscillation angle (θ_) prior and a baryon abundance (Ωbh²) prior. By comparing the data with over 10⁴ ΛCDM-based simulations, we find that both the SNe and BAO data contribute significantly to the preference for dynamic dark energy. The preference for dynamic dark energy is unlikely (probability 0.02) due to unknown systematics in DES5YR SNe and statistical fluctuations in DESI BAO, or vice versa.