For centuries, it was assumed that our universe was static. In the late 1920s, astronomers defeated this assumption with a startling new discovery. From Earth, the light of distant galaxies appeared to be red, meaning that those galaxies were receding from us. This led to the revolutionary realization that the universe is expanding. The Red Limit is the tale of this discovery, its ramifications, and the passionately competitive astronomers who charted the past, present, and future of the cosmos.