Dark energy

Dark energy is the unknown form of energy that is causing the expansion of the Universe to accelerate. It is the largest single ingredient of the universe: in the standard model of cosmology, dark energy makes up about 68% of the total mass–energy content, while dark matter accounts for about 27% and ordinary (baryonic) matter and energy for about 5%.

Dark energy differs from matter in two essential ways. It is spread almost uniformly through space, rather than being gathered into galaxies and clusters by gravity, and its density changes very slowly as the universe expands. Because matter and radiation are diluted by the expansion while dark energy is not, the fraction of the universe made of dark energy has grown over time; it has come to dominate the dynamics of the cosmos only in the recent cosmic past, and it is what now drives the expansion to accelerate.

History

The cosmological constant and the expanding universe

The first theoretical ancestor of dark energy was introduced by Albert Einstein in 1917: the cosmological constant, usually written Λ, a term in his field equations of general relativity representing an energy that is uniform in space and constant in time, which he added to allow a static universe. After the expansion of the universe was established, from Georges Lemaître's 1927 expanding-universe solution to Edwin Hubble's 1929 distance–velocity relation, the cosmological constant was no longer needed for its original purpose, but it remained available as an ingredient of cosmological models.

In the simplest Big Bang models, the expansion of a matter-filled universe should decelerate: gravity pulls every galaxy towards every other, slowing the expansion set going by the Big Bang. Measuring whether the expansion was slowing exactly as predicted required objects whose intrinsic brightness is known well enough that their apparent brightness reveals their distance, so-called standard candles.

The discovery of the accelerating universe

During the 1990s, two teams, the High-Z Supernova Search Team and the Supernova Cosmology Project, used type Ia supernovae as standard candles to trace the expansion history of the universe over the last several billion years.

In 1998, the High-Z team, led by Adam Riess and Brian Schmidt, reported the analysis of 16 high-redshift type Ia supernovae together with 34 nearby ones. The high-redshift supernovae were on average 10–15% farther away than expected in a low-density universe without a cosmological constant, and the data unanimously favoured eternally expanding models with a positive cosmological constant, ΩΛ > 0, and a currently accelerating expansion.

One year later, the Supernova Cosmology Project, led by Saul Perlmutter, published its analysis of 42 type Ia supernovae at redshifts between 0.18 and 0.83. The data indicated that the cosmological constant is non-zero and positive, with 99% confidence.

Saul Perlmutter, Brian Schmidt and Adam Riess shared the 2011 Nobel Prize in Physics for the discovery of the accelerating expansion of the universe through observations of distant supernovae.

The history of the universe, from the Big Bang to the present day. The expansion first slowed under the pull of matter and radiation, then accelerated as dark energy came to dominate. Credit: NASA/WMAP Science Team (public domain).

The term dark energy was coined in analogy with dark matter: like that component it is invisible, and the name is intended to leave open what the new ingredient actually is.

Observational evidence

Type Ia supernovae

Type Ia supernovae are the explosions of white dwarfs that have accreted matter beyond a critical mass, which makes their peak brightness remarkably uniform; after correction by the width–luminosity relation, they act as standardisable candles whose apparent brightness measures cosmic distances out to redshifts of the order of one. Comparing those distances with redshifts shows that distant supernovae are dimmer, and therefore farther, than the decelerating models of the time predicted, which is the direct evidence for accelerated expansion.

The cosmic microwave background

The cosmic microwave background (CMB) offers an independent measurement of the geometry and content of the universe. The final full-mission Planck measurements, published in 2020, are consistent with a spatially flat universe whose total density is close to the critical density, but in which ordinary and dark matter together contribute only about 31% of that density. The remaining roughly 68% must be a smooth component that does not cluster like matter, in agreement with the supernova result: dark energy.

Large-scale structure and baryon acoustic oscillations

A third, independent class of evidence comes from the growth of large-scale structure. In the early universe, pressure waves travelled through the hot plasma until recombination, leaving a characteristic scale, the baryon acoustic oscillation (BAO) scale, imprinted in the clustering of galaxies; measuring this standard ruler at different cosmic epochs, together with the growth of structure itself, traces the expansion history and the competition between the pull of matter and the push of dark energy. Combined with the CMB, these measurements confirm the energy budget inferred from supernovae and pin down the equation of state of dark energy.

The physics of dark energy

Under the assumptions of homogeneity and isotropy on large scales, the expansion of the universe is described by the Friedmann equations, which follow from general relativity:

H^2 = \frac{8\pi G}{3}\rho - \frac{k c^2}{a^2} + \frac{\Lambda c^2}{3}

\frac{\ddot a}{a} = -\frac{4\pi G}{3}\left(\rho + \frac{3p}{c^2}\right) + \frac{\Lambda c^2}{3}

Here a is the scale factor (the Hubble parameter is H = ȧ/a), k the spatial curvature, ρ the total mass density, p the pressure, G the gravitational constant, c the speed of light, and Λ the cosmological constant.

The second equation shows why the 1998 result was so surprising: any fluid with a non-negative pressure, such as ordinary matter (p = 0) or radiation (p = ρc²/3), makes the right-hand side more negative, so it can only decelerate the expansion. Acceleration requires a dominant component with negative pressure satisfying ρ + 3p/c² < 0. Cosmologists characterise each component by its dimensionless equation-of-state parameter

w = \frac{p}{\rho c^2}

with w = 0 for non-relativistic matter, w = 1/3 for radiation, and w = −1 for a cosmological constant. Any component with w < −1/3 acts as dark energy; acceleration requires such a component, and for dark energy to dominate the energy budget it must also dilute more slowly than matter.

The simplest candidate is the cosmological constant itself. It behaves exactly as a fluid of constant energy density

\rho_{\Lambda} = \frac{\Lambda c^2}{8\pi G}

with pressure p = −ρc², that is, with an equation-of-state parameter w = −1 exactly. As the universe expands and matter is diluted, such a constant-density component becomes steadily more dominant, which naturally explains why the acceleration began only when the universe was already several billion years old and dark energy had overtaken matter.

What could dark energy be?

The cosmological constant and the vacuum

If dark energy is the cosmological constant, it can be interpreted as the energy of the vacuum: quantum field theory predicts that empty space should carry a vacuum energy that behaves exactly like a cosmological constant. The trouble is its magnitude: naive estimates of the vacuum energy exceed the observed value of Λ by some 120 orders of magnitude, one of the largest discrepancies between theory and observation in physics, a difficulty known as the cosmological constant problem. Because of this problem, many cosmologists suspect that dark energy is something more subtle than a bare constant.

Quintessence and modified gravity

Alternatives propose a dynamical dark energy, in which the acceleration is driven by a slowly-evolving scalar field, called quintessence, whose density declines with time, so that its equation-of-state parameter can differ from −1 and vary over cosmic history. Others argue that no new ingredient is needed at all, and that the acceleration signals a breakdown of general relativity on cosmological scales, which would require a modified theory of gravity.

Current and future probes

As of September 2026, distinguishing these possibilities is a major goal of observational cosmology. The Dark Energy Spectroscopic Instrument (DESI) and the European Space Agency's Euclid space telescope are measuring the expansion history and the growth of cosmic structure over most of cosmic history, and the Nancy Grace Roman Space Telescope, launched in August 2026, is extending the supernova and weak-lensing measurements to much larger samples. The key observable is the equation-of-state parameter w and its possible variation with time: any deviation from −1 would rule out a pure cosmological constant and point to one of the dynamical alternatives.

Further reading

• The Universe, NASA Science
• Planck, European Space Agency
• Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant, Riess et al. (1998)
• Measurements of Omega and Lambda from 42 High-Redshift Supernovae, Perlmutter et al. (1999)