The “GreatDigInTheSky” project aimed to use the relic structures in the nearby universe to measure the acceleration field around our Galaxy to try to understand the nature of gravity and the properties of dark matter. One of the great puzzles in physics today is that our direct observations seem to be “missing” most of the universe’s mass. The leading theory is that this material is composed of some, as yet unidentified, elementary particle, that interacts only very weakly (or not at all) with normal matter. While this so-called “cold dark matter” theory is in good agreement with observations on cosmological scales, some important tensions appear on the physical scale of galaxies. Many competing theories have been developed to address these tensions, positing different physical properties for the dark matter particle. Yet others propose that dark matter does not exist, and that it is our theories of gravity that are at fault.
The recent revolution in the quality of astronomical data, especially for the Milky Way, now permits a careful reassessment of dark matter and gravity theories. The data of particular importance are the astrometric observations from the European Space Agency's Gaia mission. Our team has developed methods to use these Gaia data to identify a large number of star streams in the Milky Way. These structures appear as long bands of stars on the sky that share share similar distances and velocity. These star streams are relics from ancient dwarf galaxies and star clusters that were slowly wrecked over cosmic time due to the tidal forces of the Milky Way. Their importance is that the stars that they are composed of have very similar orbits, and as such they can be used to put stringent constraints on the acceleration field (i.e. on the force field) of our Galaxy.
Since theories of gravity and dark matter are effectively recipes for the force field, we worked to build methods that would allow us to distinguish which model works best, and which ones can be ruled out. Needless to say, finding the solution to this very fundamental problem of physics will help improve our understanding of the universe, of mass, and of the force of gravity.
Our project was successful in finding a large sample of 87 stellar stream structures and in developing a state-of-the-art toolset to analyse them. We were able to put stringent constraints on the distribution of mass in the Milky Way. We showed that wide binary stars pose a strong challenge to Modified Newtonian Dynamics. However, during the course of the project we realized that that the dynamics of stellar streams could be more complicated than we initially envisaged due to their possible partial dissolution within the dark sub-halo within which they were formed, before they were incorporated into the Milky Way. This complexity, while making the modeling even more challenging, actually provides new opportunities to probe the hierarchical assembly of our Galaxy. We are currently developing data-driven approaches to account for these effects in our dynamical models, with final analyses forthcoming that will exploit this added layer of information about galaxy formation.
