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 Δ G /G_N and range λ_C of a fifth force with Yukawa coupling arising from a chameleon- or symmetron-screened scalar field—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 survey of neutral atomic hydrogen (HI), identifying galaxies' gravitational environments with the maps of [H. Desmond, P. G. Ferreira, G. Lavaux, and J. Jasche, Mon. Not. R. Astron. Soc. 474, 3152 (2018)] and forward modeling with a Bayesian likelihood framework, we find, with screening included, 6.6 σ evidence for Δ G >0 at λ_C≃2 Mpc . The maximum-likelihood Δ G /G_N is 0.025. A similar fifth force model without screening gives no increase in likelihood over the case Δ G =0 for any λ_C. Although we validate this result by several methods, we do not claim screened modified gravity to provide the only possible explanation for the data: this conclusion would require knowing that the signal could not be produced by "galaxy formation" physics. We show also the results of a more conservative—though less well-motivated—noise model which yields only upper limits on Δ G /G_N, ranging from ∼10^-1 for λ_C≃0.5 Mpc to ∼few ×10^-4 at λ_C≃50 Mpc . Corresponding models without screening receive the somewhat stronger bounds ∼few ×10^-3 and ∼few ×10^-4 respectively. We show how these constraints may be improved by future galaxy surveys and identify the key features of an observational program for directly constraining fifth forces on scales beyond the Solar System. This paper provides a complete description of the analysis summarized in [H. Desmond, P. G. Ferreira, G. Lavaux, and J. Jasche, arXiv:1802.07206].