Abstract

In recent years, ‘scalar-tensor’ theories of gravity, in which a new scalar field couples to gravity, have been subject to increased attention. In these theories, the scalar field mediates ‘fifth forces’, which can be rendered undetectable in the laboratory or Solar System by screening mechanisms, which act to suppress the fifth force in regions of high density. Nonetheless, interesting signatures arise elsewhere, such as on galactic scales. This talk will divide roughly into two halves, describing two such astrophysical signatures: upturns in galaxy rotation curves, and asymmetries in stellar streams. In the first half, I will describe the results of a recent investigation, searching for the upturn signature in the high-quality rotation curves of the SPARC sample. This investigation was primarily limited by uncertainty about dark matter halo shapes: a fifth force in the outer regions of a galaxy with a cuspy halo can resemble the rotation curve of a galaxy with a cored halo and no fifth force, leading to a degeneracy. In the second half, I will turn to the stellar stream investigation, which remains in its ‘proof of concept’ stage. In particular, I will show results from restricted N-body simulations of stream formation, and discuss constraints obtainable from upcoming Gaia data releases.