Galaxy morphology rules out astrophysically relevant Hu-Sawicki $f(R)$ gravity

TL;DR

This study uses galaxy morphology data to place the tightest constraints yet on a modified gravity theory, specifically ruling out the Hu-Sawicki $f(R)$ model as relevant to astrophysics or cosmology.

Contribution

The paper develops a comprehensive Bayesian model to analyze galaxy morphology, providing the most stringent bounds to date on the Hu-Sawicki $f(R)$ gravity model.

Findings

No evidence for a screened fifth force within the tested range.

Set upper bounds on the strength of the fifth force, $ ext{Δ}G/G_N<0.8$ at 0.3 Mpc and $<3 imes 10^{-5}$ at 8 Mpc.

Constrain the $f_{R0}$ parameter of the Hu-Sawicki model to be less than $1.4 imes 10^{-8}$.

Abstract

$f(R)$ is a paradigmatic modified gravity theory that typifies extensions to General Relativity with new light degrees of freedom and hence screened fifth forces between masses. These forces produce observable signatures in galaxy morphology, caused by a violation of the weak equivalence principle due to a differential impact of screening among galaxies' mass components. We compile statistical datasets of two morphological indicators -- offsets between stars and gas in galaxies and warping of stellar disks -- and use them to constrain the strength and range of a thin-shell-screened fifth force. This is achieved by applying a comprehensive set of upgrades to past work (Desmond et al 2018a,b): we construct a robust galaxy-by-galaxy Bayesian forward model for the morphological signals, including full propagation of uncertainties in the input quantities and marginalisation over an empirical model describing astrophysical noise. Employing more stringent data quality cuts than previously we find no evidence for a screened fifth force of any strength $\Delta G/G_\text{N}$ in the Compton wavelength range $0.3-8$ Mpc, setting a $1\sigma$ bound of $\Delta G/G_\text{N}<0.8$ at $\lambda_C=0.3$ Mpc that strengthens to $\Delta G/G_\text{N}<3\times10^{-5}$ at $\lambda_C=8$ Mpc. These are the tightest bounds to date beyond the Solar System by over an order of magnitude. For the Hu-Sawicki model of $f(R)$ with $n=1$ we require a background scalar field value $f_{R0} < 1.4 \times 10^{-8}$, forcing practically all astrophysical objects to be screened. We conclude that this model can have no relevance to astrophysics or cosmology.