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1998: exploding stars reveal faster expansion
1998· Discovery / experiment
ProvenTested many times; part of working physics.
Two rival teams found in 1998 that distant exploding stars were fainter than expected: the universe's expansion is speeding up.
See it on the Science map →How it works
Both used Type Ia supernovae: white dwarf stars that blow up at nearly the same peak brightness, so their dimness gives their distance. Mark Phillips showed in 1993 how to correct for slight differences (brighter ones fade more slowly). Saul Perlmutter's Supernova Cosmology Project and the High-Z team of Brian Schmidt and Adam Riess expected to measure how much gravity was slowing expansion. Instead, distant supernovae sat 10–15% farther away than expected even in a universe with little matter.
Why it matters
It overturned the expectation that gravity must be slowing the expansion, revealed that most of the universe is dark energy, and completed the standard model of cosmology.
How we know
Over 1,500 supernovae now (Pantheon+, 2022; Dark Energy Survey, 2024) confirm it, and the afterglow and galaxy surveys independently require acceleration. Science magazine named it 1998's Breakthrough of the Year.
Still a mystery
Whether Type Ia supernovae come from one white dwarf feeding on a companion or two merging is unsettled. If their brightness drifts slightly with cosmic age, precision results could shift.
Connections (3)
▸1998: exploding stars reveal faster expansion revealed Dark energyProven
The relationship
Acceleration means something is pushing space apart, yet ordinary matter, dark matter and light can only pull. The push requires a new ingredient with strongly negative pressure. The supernovae, combined with measurements of matter, implied it made up about 70% of the universe's energy, more than all matter combined. The name 'dark energy' came the same year.
How the link was found
Michael Turner coined 'dark energy' in 1998, by analogy with dark matter. The supernova papers came from Riess and colleagues (1998) and Perlmutter and colleagues (1999).
Why it's proven
The afterglow shows flat space, which needs about three times more energy than matter supplies; galaxy surveys weigh the matter independently. The gap matches the supernova amount.
Open questions and mysteries
Dark energy is a name for an effect, not an explanation. Is it energy of empty space, a new field, or a sign that general relativity needs correcting on cosmic scales?
▸1998: exploding stars reveal faster expansion completed, with the CMB, ΛCDM: the standard model of cosmologyWell supported
The relationship
Supernovae measure roughly the difference between dark energy and matter, since one speeds expansion and the other slows it. The afterglow measures their sum, through the flatness of space. Galaxy clusters weigh the matter. The three overlap at one answer: about 30% matter and 70% dark energy. That crossing turned scattered hints into the standard model.
How the link was found
The supernova teams (1998–99) showed the overlap with cluster masses; balloon measurements of flatness in 2000 and WMAP's first data in 2003 closed the triangle.
Is it proven?
Three independent methods, with different possible errors, agree. Galaxy-survey ruler measurements from 2005 onward added a fourth and landed in the same spot.
Open questions and mysteries
Why are matter and dark energy comparable in amount right now, when one dilutes and the other does not? Is that coincidence, or a clue?
▸1998: exploding stars reveal faster expansion showed acceleration in The universe is expandingProven
The relationship
Each supernova gives two readings: brightness, which gives how far its light traveled, and redshift, which gives how much space stretched meanwhile. If expansion had always been slowing, distant supernovae would look relatively close and bright. They were dimmer, so expansion was slower in the past and has sped up. It switched from slowing to speeding up roughly 5–6 billion years ago.
How the link was found
The High-Z team (Riess and colleagues, 1998, 16 distant supernovae) and the Supernova Cosmology Project (Perlmutter and colleagues, 1999, 42). The rivals' agreement made the result credible.
Why it's proven
Hubble Space Telescope supernovae from more than 8 billion years ago (Riess, 2004) look relatively brighter, catching the earlier slowing era. Dust or evolving stars could dim supernovae, but not create that turnaround.
Open questions and mysteries
Exactly when the switch happened depends on what dark energy is. If DESI is right that dark energy is fading, acceleration itself may weaken in the far future.