Cosmology’s oldest question: How the universe will end

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.

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