Subpercent Accurate Fitting of Modified Gravity Growth
  Adding to our previous method for dealing with gravitational modifications at
redshift $z\gtrsim3$ through a single parameter, we investigate treatment of
lower redshift modifications to linear growth observables. We establish
subpercent accurate fits to the redshift space distortion observable
$f\sigma_8(a)$ 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 $\sim0.1\sigma$. The proposed 2--3 parameter modified gravity
description also can reveal physical characteristics of the underlying theory.
