Large-scale cosmic surveys have measured the expansion rate of the universe as a function of time and found strong evidence that the rate is accelerating. Our knowledge of fundamental physics might be greatly enhanced by understanding the microscopic origin of this acceleration and the impact of the fundamental constituents of the universe on its evolution. The Rubin Observatory's Legacy Survey of Space and Time (LSST) is the latest cosmic survey that, over the course of a decade, will deliver the deepest, widest view of the optical-NIR universe ever. This will allow determination of different tracers of cosmic evolution to unprecedented precision. It will also create an inventory of our Solar System, map the Milky Way, and discover new phenomena -- optical transients -- which herald previously unknown phenomena.
Enabling this scientific revolution requires developing not just a new instrument, but a system that delivers science-ready data products, analysis tools, and computing resources to a world-spanning community. It requires the LSST Ecosystem, which is (i) the federally funded project that builds and operates the Rubin Observatory, (ii) a self-governed set of LSST science collaborations, and (iii) the LSST Discovery Alliance (LSST-DA) - a non-profit coalition of member institutions dedicated to enabling LSST science through support of those doing LSST science.