Why were distant Type Ia supernovas important evidence for accelerated expansion?

Type Ia supernovae are critical to cosmology because they serve as standard candles, meaning they possess a consistent, uniform intrinsic luminosity. Because they are exceptionally bright, they can be observed at vast cosmological distances, allowing astronomers to accurately measure the distances to their host galaxies.123

In the late 1990s, researchers used these supernovae to compare the expected brightness of distant objects with their actual observed brightness. They discovered that Type Ia supernovae from the distant past appeared 20–25 percent dimmer than they would have in a universe expanding at a constant rate or slowing down.21

This observation indicated that these supernovae were farther away than expected, implying that the expansion of the universe has been speeding up over time. This accelerated expansion is attributed to dark energy, a gravitationally repulsive force that dominates the mass-energy content of the universe.123