Astronomers have known about the expansion of the universe for nearly a hundred years. Twenty years ago, when we set out to use exploding stars to measure gravity’s predicted effect, we discovered the expansion of the universe to be speeding up! This (un)discovery has been attributed to a “dark energy” that dominates the universe, whose nature is a deep mystery at the heart of physics. Robert P. Kirshner, Clowes Professor of Science, emeritus, Harvard Faculty of Arts and Sciences, will explain this phenomenon of cosmic acceleration.

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