An introductory class on recent groundbreaking findings on dark energy (DESI)
M. Abdul Karim et al. (many authors), DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints, available at https://data.desi.lbl.gov/public/papers/y3/KP.DR2.BAO.pdf.
This is a synthetic introductory class (no expertise assumed!) generated using enTalkenator. Give it a quick listen! This and companion papers were widely reported, including here: https://www.nytimes.com/2025/03/19/science/space/astronomer-desi-dark-energy.html.
Abstract: We present baryon acoustic oscillation (BAO) measurements from more than 14 million galaxies and quasars drawn from the Dark Energy Spectroscopic Instrument (DESI) Data Release 2 (DR2), based on three years of operation. For cosmology inference, these galaxy measurements are combined with DESI Lyman-α forest BAO results presented in a companion paper. The DR2 BAO results are consistent with DESI DR1 and SDSS, and their distance-redshift relationship matches those from recent compilations of supernovae (SNe) over the same redshift range. The results are well described by a flat ΛCDM model, but the parameters preferred by BAO are in mild, 2.3σ tension with those determined from the cosmic microwave background (CMB), although the DESI results are consistent with the acoustic angular scale θ∗ that is well-measured by Planck. This tension is alleviated by dark energy with a time-evolving equation of state parametrized by w0 and wa, which provides a better fit to the data, with a favored solution in the quadrant with w0 >−1 and wa < 0. This solution is preferred over ΛCDM at 3.1σ for the combination of DESI BAO and CMB data. When also including SNe, the preference for a dynamical dark energy model over ΛCDM ranges from 2.8−4.2σ depending on which SNe sample is used. We present evidence from other data combinations which also favor the same behavior at high significance. From the combination of DESI and CMB we derive 95% upper limits on the sum of neutrino masses, finding mν < 0.064 eV assuming ΛCDM and mν < 0.16 eV in the w0wa model. Unless there is an unknown systematic error associated with one or more datasets, it is clear that ΛCDM is being challenged by the combination of DESI BAO with other measurements and that dynamical dark energy offers a possible solution.