Supernova Cosmology Project (1999): 42 Supernovae and Dark Energy

The Supernova Cosmology Project, led by Saul Perlmutter, showed in this 1999 paper that the expansion of the universe is accelerating. Using 42 distant type Ia supernovae, the team found with 99% confidence that the cosmological constant is positive. It is one half of the discovery of dark energy, which won the 2011 Nobel Prize in Physics.

The question before the paper

Two numbers decide how the expansion of the universe changes with time: Ω, the density of matter, which pulls the expansion back, and Λ, the cosmological constant, a possible energy of empty space that would push it outward. Through the 1990s most cosmologists expected Λ to be zero and hoped supernovae would measure Ω. Distant supernovae, whose brightness reveals their distance, offered a way to measure both.

What the paper found

• A positive cosmological constant. The data showed that Λ is non-zero and positive with 99% confidence.
• Not enough matter to slow things down. For a flat universe the matter density came out at Ω_M = 0.28 (+0.09/-0.08), well below the value needed for gravity to dominate.
• Accelerating expansion. With that little matter and a positive Λ, the expansion of the universe is speeding up today.

How dark energy was discovered by the Supernova Cosmology Project

The team analyzed 42 type Ia supernovae at redshifts of 0.18 to 0.83, light that left them billions of years ago, and combined them with nearby supernovae. Type Ia explosions are not identical, but their peak brightness correlates with how quickly their light curve rises and fades: broader light curves belong to brighter explosions. Correcting for this width-luminosity relation turns each supernova into a standardized candle whose apparent brightness gives its distance.

Comparing distance with redshift for the whole sample showed that the distant supernovae are fainter, and so farther, than they would be in a universe where gravity has been slowing the expansion.

Why it mattered

The High-Z Supernova Search Team reached the same conclusion with a different sample in its 1998 paper on the accelerating universe. This paper was submitted to arXiv in December 1998, independently. Two competing teams finding the same surprising answer is what made the result credible so quickly.

The accelerating universe became the central problem of modern dark energy research. Saul Perlmutter received half of the 2011 Nobel Prize in Physics; Brian Schmidt and Adam Riess shared the other half.

What we know now

Supernova samples have grown from dozens to thousands. The Dark Energy Survey’s five-year sample of 1,635 supernovae, classified by machine learning, gave a matter density of 0.321 ± 0.007 combined with other data, consistent with Perlmutter’s value but far more precise. Combined with the galaxy maps of the Dark Energy Spectroscopic Instrument, those supernovae now hint that dark energy may not be a constant at all: see the DESI first-year results and is dark energy weakening?.

Who wrote it

Saul Perlmutter is first author, writing with G. Aldering, G. Goldhaber and the rest of the Supernova Cosmology Project. The paper is the project’s half of the discovery: a different team, a different sample of supernovae and a different analysis, arriving at the same conclusion as the High-Z team.

Read the original

Perlmutter, S., Aldering, G., Goldhaber, G., et al. (1999). “Measurements of Ω and Λ from 42 High-Redshift Supernovae.” The Astrophysical Journal. doi:10.1086/307221. Free on arXiv: astro-ph/9812133.

Questions people ask

How was dark energy discovered?

Dark energy was discovered by measuring distant type Ia supernovae, exploding white dwarfs of nearly standard brightness. In 1998 two teams, the Supernova Cosmology Project and the High-Z team, found that distant supernovae are fainter than expected, showing that the expansion of the universe is accelerating.

What did the accelerating universe Nobel Prize recognize?

The 2011 Nobel Prize in Physics recognized the discovery of the accelerating expansion of the universe through observations of distant supernovae. Saul Perlmutter of the Supernova Cosmology Project received half; Brian Schmidt and Adam Riess of the High-Z Supernova Search Team shared the other half.