Even 15 years ago, "everyone agreed that the universe was
expanding," he explains. Evidence for that expansion came via
observations dating to the 1920s that no matter where
astronomers turned their telescopes, galaxies were receding. The
more distant the galaxy, the faster the pace. Like Magic Marker
dots on an expanding balloon, galaxies were being carried along
by the expansion of space-time itself. Early on, this led to the
idea that the universe began in an enormous release of energy,
dubbed the Big Bang. Over the years, the idea has been refined,
but the general outline has been upheld by increasingly
sophisticated observations, which put the event at some 13.8
billion years ago. As scientists filled in the picture of the
universe's beginning, however, its future was still open to
debate, which centered on two possible paths. If the universe
held just the right density of matter and energy, the combined
gravity of everything in the cosmos would allow the universe to
expand forever, but at pace that woul