In 1998, astronomers made a startling announcement. Space-time, the unified phenomenon that comprises our universe and that was previously understood to be expanding, was actually not just growing, but growing faster and faster as time went on. In other words, its expansion was accelerating. This was the birth of the cosmic acceleration problem: what was causing this acceleration? It seemed to be literally coming from nowhere – from the vacuum.

From the point of view of general relativity, cosmic acceleration could be explained by saying that empty space-time has energy that drives this expansion, that it isn’t completely empty. This energy…