Abstract
We investigate how to recognize and fit gravitational modifications to linear growth observables at redshifts z ≲3, complementing our previous work establishing that a single parameter could treat modifications at z ≳3. We establish subpercent accurate fits to the redshift space distortion observable f σ 8 ( a ) using two parameters binned in redshift, testing the results for modifications with time dependence that rises, falls, is nonmonotonic, is multipeaked, and corresponds to f ( R ) and braneworld gravity. The residuals are then propagated to cosmological parameter biases for DESI observations, and found to cause a shift in the dark energy joint confidence contour by less than the equivalent of ∼0.1σ. The proposed 2–3 parameter modified gravity description also can reveal physical characteristics of the underlying theory.