Abstract
Within the effective field theory approach to cosmic acceleration, the background expansion can be specified separately from the gravitational modifications. We explore the impact of modified gravity in a background different from a cosmological constant plus cold dark matter (ΛCDM) on the stability and cosmological observables, including covariance between gravity and expansion parameters. In No Slip Gravity the more general background allows more gravitational freedom, including both positive and negative Planck mass running. We examine the effects on cosmic structure growth, as well as showing that a viable positive integrated Sachs-Wolfe effect crosscorrelation easily arises from this modified gravity theory. Using current data we constrain parameters with a Monte Carlo analysis, finding a bound on the running |α M ,max |≲ 0.03 (95% CL) for the adopted form at all cosmic times. We provide the modified hi_class code publicly on GitHub, now enabling computation and inclusion of the redshift space distortion observable f σ 8 as well as the No Slip Gravity modifications.