In 1998, two independent teams — the High-Z Supernova Search Team led by Brian Schmidt and Adam Riess, and the Supernova Cosmology Project led by Saul Perlmutter — published a result they had not been looking for. They had been trying to measure the deceleration of cosmic expansion, expecting that gravity would slow it down over time. The data showed the opposite: the universe's expansion is accelerating. Type Ia supernovae at high redshift were fainter — and therefore more distant — than they should have been if expansion were slowing. Some unknown component — soon labeled dark energy — was driving the universe's expansion to accelerate. The discovery won the 2011 Nobel Prize in Physics.
The universe is expanding: Hubble's law (1929) shows that distant galaxies recede with velocity proportional to distance, v = H₀·d. The expansion is intrinsic to spacetime — galaxies are not moving through space; space itself is stretching between them. General relativity relates the rate and acceleration of expansion to the energy content of the universe: matter and radiation produce deceleration, while a cosmological constant (Einstein's Λ) or a more general dark energy with negative pressure produces acceleration. The cosmic acceleration discovered in 1998 implies that most of the universe's energy density is in a form that behaves like a cosmological constant: roughly 70% dark energy, 25% dark matter, 5% ordinary matter. Type Ia supernovae — explosions of white dwarfs that exceed the Chandrasekhar mass — are standardizable candles: their intrinsic luminosity can be calibrated, so observed brightness gives distance. Combined with redshift, they yield the expansion history. The acceleration is now cross-confirmed by multiple independent observational probes: the cosmic microwave background, baryon acoustic oscillations, galaxy weak gravitational lensing, galaxy cluster counts, large-scale structure formation timing. All point to roughly the same ΛCDM parameters. The physical nature of dark energy is not understood. The simplest model is Einstein's cosmological constant — a constant energy density of empty space. In quantum field theory, the vacuum should have an energy density, but naive calculations give a value about 10¹²⁰ times the observed dark-energy density. This cosmological constant problem is one of the largest discrepancies between theory and observation in physics, and remains unresolved. Alternatives include dynamical dark energy (a slowly-rolling scalar field, quintessence), modified gravity theories, or anthropic selection.
Cosmological surveys (DESI, Euclid, Vera C. Rubin Observatory, the upcoming Roman Space Telescope) are measuring dark energy's equation of state with increasing precision. The early DESI results (2024) suggest possible time variation in the dark-energy density — a result whose statistical significance is currently debated, but which has reignited the question of whether dark energy is really constant. Gravitational-wave observations of binary neutron-star mergers provide an independent standard-siren method for measuring H₀. The discovery of cosmic acceleration transformed cosmology from a primarily descriptive field into a precision physics discipline, and the question of what dark energy is is now considered one of the largest unsolved problems in physics.