Professor Pedro Ferreira

Abstract:

Intragalaxy signals contain a wealth of information on fundamental physics, both the dark sector and the nature of gravity. While so far largely unexplored, such probes are set to rise dramatically in importance as upcoming surveys provide data of unprecedented quantity and quality on galaxy structure and dynamics. In this paper, we use warping of stellar disks to test the chameleon- or symmetron-screened fifth forces which generically arise when new fields couple to matter. We take r -band images of mostly late-type galaxies from the Nasa Sloan Atlas and develop an automated algorithm to quantify the degree of U-shaped warping they exhibit. We then forward model the warp signal as a function of fifth-force strength, ΔG/GN, and range, λC, and the gravitational environments and internal properties of the galaxies, including full propagation of the non-Gaussian uncertainties. Convolving this fifth-force likelihood function with a Gaussian describing astrophysical and observational noise and then constraining ΔG/GN and λC by Markov chain Monte Carlo, we find the overall likelihood to be significantly increased (Δlog(L)≃20) by adding a screened fifth force with λC≃2 Mpc and ΔG/GN≃0.01. The variation of Δlog(L) with λC is quantitatively as expected from the correlation of the magnitude of the fifth-force field with the force’s range, and a similar model without screening achieves no increase in likelihood over the General Relativistic case ΔG=0. Although these results are in good agreement with a previous analysis of the same model using offsets between galaxies’ stellar and gas mass centroids [H. Desmond et al., Phys. Rev. D 98, 064015 (2018).], we caution that the effects of confounding baryonic and dark matter physics must be thoroughly investigated for the results of the inference to be unambiguous.

Abstract:

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 ΔG/GN and range λC 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, 6.6σ evidence for ΔG>0 at λC≃2Mpc. The maximum-likelihood ΔG/GN is 0.025. A similar fifth force model without screening gives no increase in likelihood over the case ΔG=0 for any λC. 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 ΔG/GN, ranging from ∼10−1 for λC ≃ 0.5 Mpc to ∼ few ×10−4 at λC ≃ 50 Mpc. Corresponding models without screening receive the somewhat stronger bounds ∼ few ×10−3 and ∼ few ×104 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].