Fifth Force Searches in Galaxies
Harry Desmond (Oxford)
July 23, 2018
Abstract: Extra (“fifth”) forces generically follow from new dynamical fields, and are therefore ubiqituous in extensions to the standard models of particle physics and cosmology. Universally-coupled fifth forces are sufficiently strongly constrained by lab and Solar System tests as to be virtually irrelevant for cosmology. However, broad classes of Lagrangians exhibit “screening mechanisms” which hide the fifth force in high-density environments such as the Milky Way while allowing it to remain operative on larger scales. I will describe the search for screened fifth forces on the scale of galaxies and their environments. This is achieved in two stages. First, we model the gravitational environments of the local ($z \lesssim 0.05$) universe to determine the screening properties of real galaxies and the strength of the fifth-force field over space. Second, we use this information to forward-model two signals — displacement between stellar and gas mass centroids and warping of stellar disks — and hence constrain the fifth-force strength and range with a Bayesian likelihood formalism. Taking $\sim$ 10,000 HI detections from the ALFALFA survey and $\sim$ 4,000 images from the Nasa Sloan Atlas, we demonstrate sensitivity to new forces of strength $~\% that of gravity for ranges O(1-10 Mpc). Both signals give evidence for a chameleon- or symmetron-screened fifth force of range ~1.8 Mpc and strength ~0.02 $G_N$, but we caution that unmodelled systematics such as baryonic physics may impact the inference at this level.