Bimetric gravity improves the fit to DESI BAO and eases the Hubble tension

Abstract: We investigate whether the latest combination of DESI DR2 baryon acoustic oscillation (BAO) measurements, cosmic microwave background (CMB) data (Planck 2018 + ACT), and Type Ia supernovae (SNe Ia) compilations (Pantheon+, Union3, and DES Y5) favor a dynamical dark energy component, and explore if such a scenario can simultaneously help resolve the Hubble tension. We contrast two frameworks: the widely used phenomenological $w_0 w_a$CDM model, and bimetric gravity, a fundamental modification of general relativity that naturally gives rise to phantom dark energy. The $w_0 w_a$CDM model is moderately preferred over $\Lambda$CDM, at the $2$-$4 \, \sigma$ level, when fitting DESI DR2 + CMB + SNe Ia, but it exacerbates the Hubble tension. By comparison, bimetric gravity provides a modest improvement in fit quality, at the $1 \, \sigma$ level, but, by inferring $H_0 = 69.0 \pm 0.4 \, \mathrm{km/s/Mpc}$, it partially eases the Hubble tension, from a $5 \,\sigma$ discrepancy to a $3.7 \, \sigma$ tension. Including locally calibrated SNe Ia brings the overall preference for the bimetric model over $\Lambda$CDM to the $2 \, \sigma$ level, comparable to that of the $w_0 w_a$CDM model when including the local SN Ia calibration.