It was a mere 20 years ago that our picture of the Universe got a stunning revision. We all knew our Universe was expanding, that it was full of matter and radiation, and that most of the matter out there couldn't be made of the same, normal stuff (atoms) that we were most familiar with. We were trying to determine, based on how the Universe was expanding, what our fate was: would we recollapse, expand forever, or be right on the border between the two?
Distant supernovae of a specific type were the tool we would use to decide. In 1998, enough data had come in that two independent teams released the surprising results: the Universe would not only expand forever, but the expansion was accelerating.
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