Subpercent Accurate Fitting of Modified Gravity Growth

Abstract

Adding to our previous method for dealing with gravitational modifications at redshiftz&3through a single parameter, we investigate treatment of lower redshift modifications to linear growth observables. We establish sub percent accurate fits to the redshift space distortion observablefσ8(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.