On the degeneracies between baryons, massive neutrinos and f(R) gravity in Stage IV cosmic shear analyses

TL;DR

This paper develops a fast emulator for nonlinear matter power spectra in $f(R)$ gravity with massive neutrinos, assessing the impact of nonlinear scales and baryonic physics on cosmological parameter constraints in Stage IV cosmic shear analyses.

Contribution

It introduces REACTEMU-FR, a novel emulator combining halo model reaction formalism and machine learning for accurate nonlinear spectra in modified gravity and neutrino models.

Findings

Including nonlinear scales only mildly improves parameter constraints.

Baryonic physics modeling dampens constraints on neutrino masses and gravity modifications.

Approximate baryonic models bias baryonic parameter estimates but not cosmological parameters.

Abstract

Modelling nonlinear structure formation is essential for current and forthcoming cosmic shear experiments. We combine the halo model reaction formalism, implemented in the REACT code, with the COSMOPOWER machine learning emulation platform, to develop and publicly release REACTEMU-FR, a fast and accurate nonlinear matter power spectrum emulator for $f(R)$ gravity with massive neutrinos. Coupled with the state-of-the-art baryon feedback emulator BCEMU, we use REACTEMU-FR to produce Markov Chain Monte Carlo forecasts for a cosmic shear experiment with typical Stage IV specifications. We find that the inclusion of highly nonlinear scales (multipoles between $1500\leq \ell \leq 5000$) only mildly improves constraints on most standard cosmological parameters (less than a factor of 2). In particular, the necessary modelling of baryonic physics effectively damps most constraining power on the sum of the neutrino masses and modified gravity at $\ell \gtrsim 1500$. Using an approximate baryonic physics model produces mildly improved constraints on cosmological parameters which remain unbiased at the $1\sigma$-level, but significantly biases constraints on baryonic parameters at the $> 2\sigma$-level.