Modified Gravity Away from a $\Lambda$CDM Background
  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 ($\Lambda$CDM) 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 maximum
running $|\alpha_M|\lesssim 0.03$. We provide the modified {\tt hi\_class} code
publicly on GitHub, now enabling computation and inclusion of the redshift
space distortion observable $f\sigma_8$ as well as the No Slip Gravity
modifications.
