By the end of the 20th century, physicists believed they had answered one of cosmology’s oldest questions: the universe would expand forever, its galaxies drifting into isolation until only our immediate neighborhood remained. Astrophysicist Ethan Siegel explains how that conclusion was reached. Now new measurements of galaxies across vast stretches of cosmic time suggest dark energy may not be constant after all.

It was a huge quest of 20th-century physics to discover what the fate of our universe will be. And at the end of the century, we thought we had it. We thought we knew our universe will just expand forever into oblivion with different bound structures getting farther and farther apart with only whatever they're bound to to hang on.…