But the story of the cosmological constant was far from over.

Fast forward to the 1990s, when we find two teams of astronomers undertaking painstakingly precise observations of distant supernovae — exploding stars so brilliant they can be seen clear across the cosmos — to determine how the expansion rate of space has changed over the history of the universe. These researchers anticipated that the gravitational attraction of matter dotting the night’s sky would slow the expansion, much as Earth’s gravity slows the speed of a ball tossed upward. By bearing witness to distant supernovae, cosmic beacons that trace the universe’s expansion rate at various moments in the past, the teams sought to make this quantitative. Shockingly, however, when the data were analyzed, the teams found that the expansion rate has not been slowing down. It’s been speeding up.

It’s as if that tossed ball shot away from your hand, racing upward faster and faster. You’d conclude that something must be driving the ball away. Similarly, the astronomers concluded that something in space must be pushing galaxies apart ever more quickly. And after scrutinizing the situation, they have found that the push is most likely the repulsive gravity produced by a cosmological constant.

When Einstein introduced the cosmological constant, he envisioned its value being finely adjusted to exactly balance ordinary attractive gravity. But for other values the cosmological constant’s repulsive gravity can beat out attractive gravity, and yield the observed accelerated spatial expansion, spot on. Were Einstein still with us, his discovery that repulsive gravity lies within nature’s repertoire would have likely garnered him another Nobel prize.

As remarkable as it is that even one of Einstein’s “bad” ideas has proven prophetic, many puzzles still surround the cosmological constant: If there is a diffuse, invisible energy permeating space, where did it come from? Is this dark energy (to use modern parlance) a permanent fixture of space, or might its strength change over time? Perhaps most perplexing of all is a question of quantitative detail. The most refined attempts to calculate the amount of dark energy suffusing space miss the measured value by a gargantuan factor of 10123 (that is, a 1 followed by 123 zeroes) — the single greatest mismatch between theory and observation in the history of science.

THESE are vital questions that rank among today’s deepest mysteries. But standing beside them is an unassailable conclusion, one that’s particularly unnerving. If the dark energy doesn’t degrade over time, then the accelerated expansion of space will continue unabated, dragging away distant galaxies ever farther and ever faster. A hundred billion years from now, any galaxy that’s not resident in our neighborhood will have been swept away by swelling space for so long that it will be racing from us at faster than the speed of light. (Although nothing can move through space faster than the speed of light, there’s no limit on how fast space itself can expand.)

Light emitted by such galaxies will therefore fight a losing battle to traverse the rapidly widening gulf that separates us. The light will never reach Earth and so the galaxies will slip permanently beyond our capacity to see, regardless of how powerful our telescopes may become.