Accelerating galactic archaeology
Why most of the matter in the Universe appears to be dark, and constituted of some unknown particles, is one of the most important open questions in physics today. Here we propose to investigate the relative merits of different possibilities for the nature of this dark matter, as well as alternative proposals for the nature of the gravitational interaction, by comparing their predictions to new high spatial resolution measurements of the 3D acceleration field in the Milky Way.  These measurements will be achieved primarily by finding and analysing the many stellar streams that criss-cross our Galaxy. We have already built a stream-finding prototype that we have used with the Gaia mission’s second data release (DR2) to detect 13 new beautiful phase-space streams in the Milky Way. This project aims to develop analysis techniques that will allow us to combine data from all relevant sky surveys together with future Gaia releases to identify many other Galactic star streams. It is plausible that 100 or more streams may be identified by the end of our project. The conjoint analysis of these interwoven structures will provide us with the means to derive, for the very first time, the three-dimensional acceleration field on scales of 1-100 kpc. The streams will be used to probe the granularity of the dark matter distribution, testing whether their kinematics and sub-structure are consistent with interaction with the expected sub-halos of the standard Λ Cold Dark Matter (ΛCDM) paradigm. We will also simulate star streams in several alternative scenarios, including fuzzy dark matter, dipolar dark matter, superfluid dark matter, and modified Newtonian dynamics, and quantify their relative merits to simulations in ΛCDM. Together, these studies will test theories of gravity and dark matter theories, place the best constraints on the distribution of dark matter in our Galaxy, and probe the substructure of the dark matter, thereby setting the state of the art for the next decade.