Classifying Linearly Shielded Modified Gravity Models in Effective Field Theory

Abstract

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.

Unscreening Modified Gravity in the Matter Power Spectrum

“…1) of 70%, 60%, and 20% at screened scales of k ∼ 1.5 h/Mpc for models with σ 8 values equal to the F4, F5, and F6 f (R) scenarios, respectively. We also expect our method to be applicable to further screening mechanisms like the symmetron [4] or k-mouflage [3], however, not to linear shielding [5].…”

“…While alternatively, a modification of gravity may be responsible for cosmic acceleration, stringent limitations from experiments within our Solar System must be satisfied. A number of screening mechanisms [1][2][3][4][5] have been identified that can suppress modifications of gravity in high-density regions to recover GR, while still generating significant modifications within lower densities on larger, cosmological scales. However, this suppression effect, along with other nonlinear effects, also prevents strong anomalies from manifesting in the averaged large-scale structure of our Universe [6] and limits the constraints that can be inferred on these gravity models from cosmology.…”

Probing theories of gravity with phase space-inferred potentials of galaxy clusters

“…One such mechanism is the Chameleon mechanism whereby the additional fifth force active in low density regions is screened in regions of high density by shortening the range of interaction of the field [5,6]. A more general approach utilizes Effective Field Theory (EFT) of cosmic acceleration [7,8], where recent theoretical advances have shown that there exists a large model space that recovers an accelerated expansion on large scales, while reducing to Newtonian gravity on the small scales in linear theory [9]. Thus, there is a need for well-defined observational tests which can distinguish the many models, including their possible covariant nature, as well as ΛCDM.…”

Dark degeneracy I: Dynamical or interacting dark energy?

“…For scalar perturbations, the Einstein equations are still given by Eqs. (37), (38), (39) and (40) and energy-momentum conservation is respectively given by Eqs. (41) and (42).…”