This project will make a precision measurement of the cosmic expansion rate. This hybrid program has a target of opportunity follow-up component and a survey component. The survey will strategically cover the regions of interest of many black hole events. It will collect a sample of gravitational-wave triggered neutron star merger events and a sample of black hole mergers for which potential host galaxies have been cataloged. The rate of cosmic expansion is a key parameter in modern cosmology. There is currently tension between local measurements and the results from cosmic microwave background studies, which motivates the pursuit of improved independent observables: gravitational waves have long been proposed as one such. This work covers two such alternatives. Broadening the impact of research on society at large requires effective communication to audiences from diverse backgrounds, including pre-college teens currently under-represented in STEM fields. The researcher will engage with high schools and emphasize under-represented groups, using the infrastructure of the Brandeis Science Communication Laboratory.<br/><br/>This study uses the full range of available data from the Dark Energy Camera (DECam) on NSF's Blanco 4-meter telescope in Chile, including images taken as part of the Dark Energy Survey (DES), as well as non-DES DECam images. It will also use data from another NSF facility, the Laser Interferometer Gravitational-wave Observatory, LIGO. Methods to be used include a Hubble diagram fit to a sample of neutron star mergers and a statistical dark sirens analysis for black hole events. This is a new method independent of other cosmological probes, because standard sirens are distance indicators based solely on general relativity. Detectors with enough sensitivity to enable this approach became operational in 2015. This research will fully exploit the novel datasets as multi-messenger probes for cosmology using DES/DECam data, making a measurement of the expansion rate with 3-5% (statistical) uncertainty by 2022, and enabling percent-level precision with future data from facilities like the Large Synoptic Survey Telescope.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.