Fifth force constraints from the separation of galaxy mass components
  One of the most common consequences of extensions to the standard models of
particle physics or cosmology is the emergence of a fifth force. While generic
fifth forces are tightly constrained at Solar System scales and below, they may
escape detection by means of a screening mechanism which effectively removes
them in dense environments. We constrain the strength $\Delta G/G_N$ and range
$\lambda_C$ of a chameleon- or symmetron-screened fifth force with Yukawa
coupling -- as well as an unscreened fifth force with differential coupling to
galactic mass components -- by searching for the displacement it predicts
between galaxies' stellar and gas mass centroids. Taking data from the Alfalfa
HI survey, identifying galaxies' gravitational environments with the maps of
Desmond et al. (2018a) and forward-modelling with a Bayesian likelihood
framework, we find $6.6\sigma$ evidence for $\Delta G>0$ at $\lambda_C \simeq
2$ Mpc, with $\Delta G/G_N = 0.025$ at maximum-likelihood. A similar
fifth-force model without screening gives no increase in likelihood over the
case $\Delta G = 0$ for any $\lambda_C$. Although we validate these results by
several methods, we do not claim screened modified gravity to provide the only
possible explanation for the signal: this would require knowing that "galaxy
formation" physics could not be responsible. We show also the results of a more
conservative -- though less well motivated -- noise model which yields only
upper limits on $\Delta G/G_N$, ranging from $\sim10^{-1}$ for $\lambda_C
\simeq 0.5$ Mpc to $\sim \: \text{few} \times 10^{-4}$ at $\lambda_C \simeq 50$
Mpc. We show how these constraints may be improved by future galaxy surveys and
identify the key features of an observational programme for directly
constraining fifth forces on galactic scales. This paper provides a complete
description of the analysis summarised in Desmond et al. (2018b).
