  Ghost-free bimetric gravity is a theory of two interacting spin-2 fields, one
massless and one massive, in addition to the standard matter particles and
fields, thereby generalizing Einstein's theory of general relativity. To
parameterize the theory, we use five observables with specific physical
interpretations. We present, for the first time, observational constraints on
these parameters that: (i) apply to the full theory, (ii) are consistent with a
working screening mechanism (i.e., restoring general relativity locally), (iii)
exhibit a continuous, real-valued background cosmology (without the Higuchi
ghost). For the cosmological constraints, we use data sets from the cosmic
microwave background, baryon acoustic oscillations, and type Ia supernovae.
Bimetric cosmology provides a good fit to data even for large values of the
mixing angle between the massless and massive gravitons. Interestingly, the
best-fit model is a self-accelerating solution where the accelerated expansion
is due to the dynamical massive spin-2 field, without a cosmological constant.
Due to the screening mechanism, the models are consistent with local tests of
gravity such as solar system tests and gravitational lensing by galaxies. We
also comment on the possibility of alleviating the Hubble tension with this
theory.
