In 1997, physicist Saul Perlmutter was studying supernova explosions in order to get a sense of how fast the universe was expanding. But what he and his team discovered was unexpected. Rather than slowing down, our growing universe seemed to be gaining momentum, as if something was putting energy into the whole system. Here, he explains just how he came to the conclusion that won him the 2011 Nobel Prize in physics.
This program is part of the Big Ideas series.
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