Roman’s High-Latitude Time-Domain Survey will find exploding stars that act as signposts to measure the universe’s expansion and the influence of dark energy. For thousands of years, humanity viewed the skies as unchanging, except for a few “wandering stars” (that we now know are planets). As we improved our ability to perceive the cosmos with light-gathering telescopes and electronic detectors, we realized that the universe is full of things that change in brightness, whether it be an exploding star or a matter-gulping black hole. NASA’s Nancy Grace Roman Space Telescope is poised to deliver an avalanche of such transients, including thousands of “standard candle” supernovae that allow us to measure the expansion history of the universe.