Since the dawn of the universe, the biggest stars have ended their lives with a bang, blowing out their outer layers in bright, fiery bursts that can be seen many light-years away. Astronomers use these supernova explosions like marks on an expanding balloon to measure how fast the universe is growing.
Based on studies of dozens of supernova explosions, astronomers in the late 1990s realized that the universe's expansion seems to be accelerating. They hypothesized that some unseen "energy," which works the opposite of gravity, was pushing everything outward. The concept of so-called dark energy quickly became popular, and ultimately, scientists' consensus view. It earned three physicists the 2011 Nobel Prize.
Recently, however, some scientists have been poking at this foundation of dark energy research.
