For most of the twentieth century, the central assumption about the cosmos was that gravity was steadily applying the brakes to its expansion. Astronomers expected to measure exactly how much this expansion was slowing down. Instead, observations of distant supernovae in the late 1990s revealed something astonishing: the universe is not slowing down at all. It is speeding up.

This episode traces the history of how we came to understand the expanding universe, from Edwin Hubble’s first measurements of the distance to Andromeda, to the modern network of observations that confirm cosmic acceleration. We examine the critical role of standardizable candles like Type Ia supernovae, the acoustic ripples frozen in the distribution of galaxies, and the faint afterglow of the cosmic microwave background. Together, these independent methods point to t ...