Subpercent Accurate Fitting of Modified Gravity Growth

TL;DR

This paper develops a highly accurate two-parameter model for modified gravity effects on cosmic growth, enabling precise fits to observational data and minimal bias in dark energy parameter estimation.

Contribution

It introduces a new two-parameter approach to model lower redshift modifications to linear growth, improving accuracy and interpretability over previous methods.

Findings

Achieves subpercent accuracy in fitting $f\sigma_8(a)$ across various modified gravity models.

Propagates residuals to cosmological parameters, causing less than 0.1σ bias in DESI forecasts.

The model can also help identify physical features of the underlying gravity theories.

Abstract

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.