Core Concept: Supernovae and the accelerating universe.

Astronomers Walter Baade and Fritz Zwicky were haunted by two celestial apparitions. It was the early 1930s, and the star-like specters had appeared—and disappeared— from the skies well before either astronomer had been born. But the brief, bright phenomena had transfixed onlookers. Veteran observer Tycho Brahe, for example, was moved to proclaim one a “miracle” when it materialized above his head in 1572; Ernst Hartwig reported another glowing in our neighboring galaxy, Andromeda, in 1885. Baade and Zwicky calculated that the object in Andromeda had poured out as much radiation in 25 days as our sun does in 10 million years. Such objects, which the researchers dubbed “super-novae,” presented “a very curious puzzle,” they wrote in one of two PNAS papers published in 1934 (1). Today, astronomers believe the 1572 and 1885 events, as well as others recorded hundreds of years earlier, were each a kind of stellar explosion known as a type Ia supernova (2). The physics behind such blasts is still somewhat puzzling, involving one or possibly two stellar corpses known as white dwarfs (3, 4). However, in the last two decades type Ia supernovae have illuminated an even more curious—and spookier—puzzle: the existence of a mysterious entity, dubbed “dark energy,” that is pushing space itself apart at ever faster rates. Identifying that entity—and in the process, the fate of the cosmos—may hinge on making ever more sensitive measurements of type Ia blasts from the ground and from space.