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Light from exploding stars halfway across the universe reveals an astonishing fact: the expansion of the universe is speeding up! Astronomers attribute this to a mysterious "dark energy" that drives cosmic acceleration. And we need a lot of it—dark energy accounts for 2/3 of the matter and energy in the universe today. Curiously, when Albert Einstein first thought about gravity in the universe, in 1917, he introduced a repulsive "cosmological constant" that he thought would match a static, unchanging universe. When, in 1929, astronomical observations showed the universe was not static, but expanding, he stopped talking about the cosmological constant. It has dubbed his "greatest blunder." But today's observations show that we need something that acts just like the cosmological constant to produce cosmic acceleration.
Robert P. Kirshner, the Harvard College Professor of Astronomy and Clowes Professor of Science at Harvard University and president of the American Astronomical Society from 2003 to 2005, describes how we use observations of supernovi—exploding stars—to trace cosmic history and to learn more about the nature of the dark energy, one of the deepest mysteries of the physical world.
The Bethe Lecture Series, established in 1977 by the Cornell Department of Physics and the College of Arts and Sciences, honors Hans A. Bethe who joined Cornell's faculty in 1936, and whose research extended across fields as diverse as the quantum theory of solids and the nuclear processes that power the sun, receiving the Nobel Prize for the later work in 1967. Bethe continued to make significant scientific contributions until his death in 2005.