Classifying Linearly Shielded Modified Gravity Models in Effective Field Theory

TL;DR

This paper classifies modified gravity models within effective field theory that can mimic standard cosmology on linear scales, highlighting the challenges in distinguishing them observationally and proposing potential new tests.

Contribution

It identifies three classes of modified gravity models that replicate the concordance model on linear scales, including those within Horndeski theories, and discusses observational limitations.

Findings

Large model space mimics the concordance model on all linear scales.

Partially shielded models may address tensions between large and small scale data.

Distinguishing fully shielded models from standard cosmology is limited by cosmic variance.

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.