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Why were distant Type Ia supernovas fainter than expected in a universe without dark energy?

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Type Ia supernovae are used as "standard candles" because they possess a remarkably uniform intrinsic brightness, allowing astronomers to calculate their distances by measuring how bright they appear from Earth.12

In the late 1990s, observations of distant Type Ia supernovae revealed that they appeared 20–25 percent dimmer than expected. Because these supernovae are standard candles, their fainter-than-expected appearance indicated that they were actually farther away than they would have been in a universe where the expansion rate was constant or decelerating.123

This discovery provided direct evidence that the expansion of the universe is accelerating. This acceleration is driven by dark energy, a gravitationally repulsive force that dominates the mass-energy budget of the universe. Because dark energy causes the expansion rate to increase over time, the light from these distant supernovae—emitted when the universe was younger and smaller—traveled a greater distance to reach us than it would have in a universe without this repulsive force.132