  We investigate the observational consequences of scalar instabilities in
bimetric theory, under the assumption that the Vainshtein mechanism restores
general relativity within a certain distance from gravitational sources. We
argue that early time instabilities have a negligible impact on observed
structures. Assuming that the instabilities affect sub-horizon density
fluctuations, we constrain the redshift, z_i, below which instabilities are
ruled out. For the "minimal" beta_1-model, observational constraints are close
to the theoretical expectations of z_i = 0.5, potentially allowing the model to
be ruled in or out with a more detailed study, possibly including secondary
cosmic microwave background constraints.
