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.