Q:How did scientists first discover that the universe's expansion is accelerating?
Scientists first discovered the universe's accelerating expansion in 1998 by observing distant type Ia supernovae—stellar explosions that serve as "standard candles" due to their consistent intrinsic brightness
56. Two independent teams, the Supernova Cosmology Project and the High-Z Supernova Search Team, found that these supernovae appeared dimmer and more redshifted than expected, indicating they were farther away than predicted if the universe's expansion was slowing down
356. This surprising result showed that the expansion rate was actually increasing, not decreasing as previously thought, leading to the conclusion that a mysterious force—later called dark energy—was driving the acceleration
356. The discovery earned the leaders of these teams the Nobel Prize in Physics
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