What did Type Ia supernova observations reveal about the expansion of the universe?

Observations of Type Ia supernovae were instrumental in revealing that the expansion of the universe is accelerating, rather than slowing down as previously expected.12

Because Type Ia supernovae possess a remarkably uniform luminosity, astronomers use them as "standard candles" to measure distances to remote galaxies and determine the expansion rate of the universe over time. In 1998, two independent research teams—the Supernova Cosmology Project and the High-Z Supernova Search Team—discovered that distant Type Ia supernovae were 20–25 percent dimmer than they would be in a universe without a repulsive force.1234

This indicated that these supernovae were farther away than expected, implying that the expansion of the universe is faster today than it was in the past. This acceleration is attributed to dark energy, a gravitationally repulsive force that acts as the dominant component of the universe's mass-energy. This discovery fundamentally transformed modern cosmology.4312