How do scientists know that dark energy is causing the expansion of the universe to accelerate?
Quick Answer
They compare distances and redshifts of distant Type Ia supernovae and found that faraway supernovae are dimmer than expected in a slowing universe. This suggests the expansion has been speeding up for billions of years.
The Full Story
In the late 1990s, two research teams studying distant Type Ia supernovae found that these explosions were fainter than expected for their redshifts. The result implied that the universe’s expansion is not slowing down under gravity alone; it is accelerating. This discovery was led by the Supernova Cosmology Project and the High-z Supernova Search Team, and it earned the 2011 Nobel Prize in Physics for Saul Perlmutter, Brian Schmidt, and Adam Riess. The acceleration is usually attributed to dark energy, though its physical nature remains unknown. Combined cosmological evidence suggests dark energy makes up about 68% of the universe.
Key Facts
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"The discovery that the universe is speeding up"
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