Desi Dr2 Results. Ii. Measurements of Baryon Acoustic Oscillations and Cosmological Constraints

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-a forest BAO results presented in a companion paper (M. Abdul-Karim et al., companion paper, Phys. Rev. D 112, 083514 2025.). The DR2 BAO results are consistent with DESI DR1 and the Sloan Digital Sky Survey, 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. cold dark matter (.CDM) model, but the parameters preferred by BAO are in mild, 2.3s tension with those determined from the cosmic microwave background (CMB), although the DESI results are consistent with the acoustic angular scale theta(*) that is well measured by Planck. This tension is alleviated by dark energy with a timeevolving equation of state parametrized by w(0) and w(a), which provides a better fit to the data, with a favored solution in the quadrant with w(0) > -1 and w(a) < 0. This solution is preferred over.CDM at 3.1s for the combination of DESI BAO and CMB data. When also including SNe, the preference for a dynamical dark energy model over.CDM ranges from 2.8 - 4.2 sigma 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 Sigma m(nu) < 0.064 eV assuming.CDMand Sigma m(nu) < 0.16 eV in the w(0)w(a) model. Unless there is an unknown systematic error associated with one or more datasets, it is clear that.CDM is being challenged by the combination of DESI BAO with other measurements and that dynamical dark energy offers a possible solution.