Roman is a survey telescope rather than a pointing telescope. Hubble and the James Webb Space Telescope look deeply at small patches of sky; Roman gives up none of Hubble's mirror size but sees a patch at least a hundred times wider, so it can build statistical samples large enough to answer questions that single deep images cannot. Three science areas define the mission. For dark energy, it measures the shapes of vast numbers of galaxies and observes Type Ia supernovae to track how the expansion of the universe has changed over cosmic time. For exoplanets, it runs a microlensing survey of the inner Milky Way - watching for the brief brightening that occurs when a foreground star and its planets bend the light of a background star - and NASA expects this to reveal more than a thousand planets, including cold and free-floating worlds that transit surveys systematically miss. For infrared astrophysics, it will measure light from around a billion galaxies over the mission and deliver one of the deepest views ever obtained of the heart of our galaxy, through the Galactic Bulge Time-Domain Survey. The Coronagraph Instrument is carried as a technology demonstration: it blocks the overwhelming glare of a host star so that the far fainter light of an orbiting planet can be imaged and its spectrum taken, a capability future missions will need in order to characterise Earth-like worlds.

Dark energy accounts for the accelerating expansion of the universe and is still unexplained; dark matter is inferred only from its gravitational effects. Roman is the first observatory designed from the start to attack both statistically, with samples large enough to distinguish between competing explanations rather than merely illustrate them. The exoplanet side matters for a different reason: microlensing is sensitive to planets far from their stars and to worlds drifting free of any star, precisely the population that transit surveys cannot see, so Roman fills in the part of the planetary census that is currently blank. For exam purposes, the mission is also a clean example of the division of labour among modern space telescopes - Hubble in visible and ultraviolet, Webb for deep infrared, Roman for wide-field infrared survey.