### arXiv:1607.03113 [astro-ph.CO]AbstractReferencesReviewsResources

#### How Do You Solve a Problem Like Modified Gravity? Challenges in Connecting Theory and Observations

Published 2016-07-11Version 1

Cosmic acceleration may be due to modifications of cosmic gravity. To test this we need robust connections between theory and observations. However, in a model independent approach like effective field theory or a broad class like Horndeski gravity, several free functions of time enter the theory. We demonstrate that no simple parametrizations of these functions are valid, with first approximations such as $\alpha_i(t)\propto\Omega_{\rm de}(t)$ drastically misestimating the observables. Quantifying the bias, we find that even a few percent inaccuracy on the property functions $\alpha_i(t)$ give many expected-sigma bias on cosmic survey observables (often missing the signature of modified gravity). We also address the challenge of generally valid consistency relations and the role of tensor (gravitational wave) perturbations.

**Comments:**8 pages, 3 figures