  Two decades after its discovery, cosmic acceleration remains the most
profound mystery in cosmology and arguably in all of physics. Either the
Universe is dominated by a form of dark energy with exotic physical properties
not predicted by standard model physics, or General Relativity is not an
adequate description of gravity over cosmic distances. WFIRST emerged as a top
priority of Astro2010 in part because of its ability to address the mystery of
cosmic acceleration through both high precision measurements of the cosmic
expansion history and the growth of cosmic structures with multiple and
redundant probes. We illustrate in this white paper how mission design changes
since Astro2010 have made WFIRST an even more powerful dark energy facility and
have improved the ability of WFIRST to respond to changes in the experimental
landscape. WFIRST is the space-based probe of DE the community needs in the
mid-2020s.

Cosmology and Nongalactic Astrophysics;Instrumentation and Methods for Astrophysics