Classifying Linearly Shielded Modified Gravity Models in Effective Field
  Theory
  We study the model space generated by the time-dependent operator
coefficients in the effective field theory of the cosmological background
evolution and perturbations of modified gravity and dark energy models. We
identify three classes of modified gravity models that reduce to Newtonian
gravity on the small scales of linear theory. These general classes contain
enough freedom to simultaneously admit a matching of the concordance model
background expansion history. In particular, there exists a large model space
that mimics the concordance model on all linear quasistatic subhorizon scales
as well as in the background evolution. Such models also exist when restricting
the theory space to operators introduced in Horndeski scalar-tensor gravity. We
emphasize that whereas the partially shielded scenarios might be of interest to
study in connection with tensions between large and small scale data, with
conventional cosmological probes, the ability to distinguish the fully shielded
scenarios from the concordance model on near-horizon scales will remain limited
by cosmic variance. Novel tests of the large-scale structure remedying this
deficiency and accounting for the full covariant nature of the alternative
gravitational theories, however, might yield further insights on gravity in
this regime.
