The Accelerating Universe (1998): Supernovae Reveal Dark Energy

Dark energy was discovered in 1998 by two supernova teams. In this paper, Adam Riess and the High-Z Supernova Search Team showed that distant exploding stars look fainter than expected, meaning the expansion of the universe is speeding up. Something with repulsive gravity, now called dark energy, must be driving it.

The question before the paper

Since Hubble’s 1929 discovery, astronomers knew the universe is expanding. Gravity from all the matter in it should slow that expansion down, so the natural goal of the 1990s was to measure the slowdown, and with it how much matter the universe contains and whether it will expand forever. The tool was the type Ia supernova: the explosion of a white dwarf star, which reaches a nearly uniform peak brightness and can be seen across billions of light-years. Compare how bright distant ones look with how bright they really are, and you get their distance, and the expansion history along the way.

What the paper found

• Distant supernovae were too faint. Compared with expectations for a low-density universe without a cosmological constant, the distant explosions were on average 10% to 15% farther away.
• Acceleration, not deceleration. The data favored a positive cosmological constant (Ω_Λ > 0) at 3.0 and 4.0 sigma for two fitting methods, and a universe whose expansion is accelerating today.
• Much stronger with a flat universe. Assuming space is flat, a positive cosmological constant was required at 7 to 9 sigma.
• Matter alone cannot close the universe. A universe made “closed” by ordinary matter was ruled out at 7 to 8 sigma.

How dark energy was discovered

The High-Z team compared 16 high-redshift supernovae with 34 nearby ones. For each, they measured how its brightness rose and fell over weeks, which lets astronomers correct for small differences between explosions and treat them as “standard candles”. They then compared each supernova’s distance, from its brightness, with its redshift, which tells how much the universe has expanded since the light left.

If the expansion had been slowing, distant supernovae would appear slightly brighter than in an empty universe. Instead they appeared dimmer. The simplest explanation is that the expansion has been speeding up for the past several billion years, which in general relativity requires a positive cosmological constant or another form of energy with negative pressure.

Why it mattered

The result changed the inventory of the universe. Dark energy makes up about 70% of its energy budget and controls its far future: if it behaves like a cosmological constant, galaxies beyond our local group will eventually recede out of sight, a future explored in Deep Time, in Krauss and Scherrer’s end of cosmology and in our guide to how the universe will end. If dark energy instead grew stronger, it could end in a Big Rip.

The rival Supernova Cosmology Project reached the same conclusion independently, in a paper measuring Ω and Λ from 42 supernovae. Saul Perlmutter received half of the 2011 Nobel Prize in Physics; Adam Riess and Brian Schmidt shared the other half.

What we know now

Independent methods confirmed the acceleration: the cosmic microwave background measured by WMAP and Planck, baryon acoustic oscillations in galaxy maps, and much larger supernova samples such as the Dark Energy Survey’s 1,635 supernovae.

What dark energy is remains unknown. In 2025 the Dark Energy Spectroscopic Instrument reported a 2.8 to 4.2 sigma preference for dark energy that weakens over time, short of the 5-sigma discovery threshold. See is dark energy weakening? for the latest, and the dark energy guide for the full picture.

Who wrote it

Adam Riess led the analysis as first author, with Alexei Filippenko, Peter Challis and the rest of the High-Z Supernova Search Team, including Brian Schmidt. Two competing teams reaching the same surprising answer with different supernova samples is what made the result credible so quickly.

Read the original

Riess, A. G., Filippenko, A. V., Challis, P., et al. (1998). “Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant.” The Astronomical Journal 116, 1009. doi:10.1086/300499. Free on arXiv: astro-ph/9805201. Semantic Scholar lists about 4,300 citations (October 2026).

Questions people ask

Who discovered dark energy?

Two teams discovered it in 1998: the High-Z Supernova Search Team, with Adam Riess and Brian Schmidt, and the Supernova Cosmology Project, led by Saul Perlmutter. Both found distant supernovae fainter than expected. The three shared the 2011 Nobel Prize in Physics for discovering the accelerating expansion.

Why did Perlmutter, Schmidt and Riess win the Nobel Prize?

They won the 2011 Nobel Prize in Physics for discovering that the expansion of the universe is accelerating, using observations of distant type Ia supernovae. Perlmutter received half of the prize; Riess and Schmidt shared the other half. The unknown cause of the acceleration is called dark energy.

How do supernovae show the universe is accelerating?

Type Ia supernovae have nearly the same true brightness, so how bright they look reveals their distance. In 1998 distant ones looked 10% to 15% farther away than expected for a slowing universe. Light from them traveled farther than gravity alone allows, so the expansion must be speeding up.