The Detailed Science Case for the Maunakea Spectroscopic Explorer, 2019 edition

( Abridged) The Maunakea Spectroscopic Explorer (MSE) is an end-to-end science platform for the design, execution and scientific exploitation of spectroscopic surveys. It will unveil the composition and dynamics of the faintUniverse and impact nearly every field of astrophysics across all spatial scales, from individual stars to the largest scale structures in the Universe. Major pillars in the science program for MSE include (i) the ultimate Gaia follow-up facility for understanding the chemistry and dynamics of the distant Milky Way, including the outer disk and faintstellar halo at high spectral resolution (ii) galaxy formationand evolutionat cosmic noon, via the type of revolutionary surveys that have occurred in the nearby Universe, but now conducted at the peak of the star formation history of the Universe (iii) derivation of the mass of the neutrino and insights into inflationary physics through a cosmological redshift surveythat probes a large volume of the Universe with a high galaxy density. MSE is positioned to become a critical hub in the emerging international network of front-lineastronomical facilities, with scientific capabilities that naturally complement and extend the scientific power of Gaia, the Large Synoptic Survey Telescope, the Square KilometerArray, Euclid, WFIRST, the 30m telescopes and many more.