Cosmological gravity on all scales IV: 3x2pt Fisher forecasts for pixelised phenomenological modified gravity

TL;DR

This paper presents Fisher forecast analyses for modified gravity parameters using 3x2pt observables from Stage IV large scale structure surveys, emphasizing non-linear modeling, scale cuts, and nulling techniques to improve constraints.

Contribution

It introduces a validated non-linear modeling approach for modified gravity with binned parameters and demonstrates the effectiveness of the BNT transformation in mitigating small-scale systematics.

Findings

High precision constraints on modified gravity parameters achievable.

Optimal redshift binning for $mbda$CDM growth identified.

BNT transformation effectively reduces small-scale systematic impacts.

Abstract

Stage IV large scale structure surveys are promising probes of gravity on cosmological scales. Due to the vast model-space in the modified gravity literature, model-independent parameterisations represent useful and scalable ways to test extensions of $\Lambda$CDM. In this work we use a recently validated approach of computing the non-linear $3\times 2$pt observables in modified gravity models with a time-varying effective gravitational constant $\mu$ and a gravitational slip $\eta$ that is binned in redshift to produce Fisher forecasts for an LSST Y10-like survey. We also include in our modelling an effective nulling scheme for weak-lensing by applying the BNT transformation that localises the weak-lensing kernel enabling well-informed scale cuts. We show that the combination of improved non-linear modelling and better control of the scales that are modelled/cut yields high precision constraints on the cosmological and modified gravity parameters. We find that 4 redshift bins for $\mu$ of width corresponding to equal incremental $\Lambda$CDM growth is optimal given the state-of-the-art modelling and show how the BNT transformation can be used to mitigate the impact of small-scale systematic effects, such as baryonic feedback.