In the 1990s, scientists studying exploding stars called supernovae in far-flung galaxies discovered that the Universe's expansion is accelerating, not slowing as theorists predicted. This discovery led them to the conclusion that some unknown process was causing the Universe to speed up, and they named it dark energy.
One of the biggest goals in science is to explain this mysterious energy, which is thought to make up about 70% of the energy density of the Universe.
Image: A Chandra X-ray Observatory image of the distant galaxy cluster Abell 2029. Astronomers use images like this to better understand dark energy's effects. (credit: NASA/CXC/IoA/S.Allen et al.)
A mysterious process speeds up the Universe's expansion.
A shocking result puzzles astronomers.
Astronomers were amazed when they discovered that the expansion of the Universe is speeding up. This finding led to the proposal of an unknown and as-yet unexplained force called dark energy.
Astrophysicist Saul Perlmutter uncovers a mystery in the Universe.
Astrophysicist Saul Perlmutter and his colleagues discovered that the expansion of the Universe is speeding up - a finding that goes against what was predicted. It was thought that the gravitational pull of all the matter in the Universe would slow down its expansion. This led scientists to propose a new, as-yet unexplained process called dark energy.
In physical cosmology and astronomy, dark energy is a hypothetical form of energy which permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted hypothesis to explain the observations since the 1990s indicating that the universe is expanding at an accelerating rate. According to the Planck mission team, and based on the standard model of cosmology, on a mass–energy equivalence basis, the observable universe contains 26.8% dark matter, 68.3% dark energy (for a total of 95.1%) and 4.9% ordinary matter. Again on a mass–energy equivalence basis, the density of dark energy (1.67 × 10−27 kg/m3) is very low: in the solar system, it is estimated only 6 tons of dark energy would be found within the radius of Pluto's orbit. However, it comes to dominate the mass–energy of the universe because it is uniform across space.
Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to vacuum energy. Scalar fields that do change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.
High-precision measurements of the expansion of the universe are required to understand how the expansion rate changes over time. In general relativity, the evolution of the expansion rate is parameterized by the cosmological equation of state (the relationship between temperature, pressure, and combined matter, energy, and vacuum energy density for any region of space). Measuring the equation of state for dark energy is one of the biggest efforts in observational cosmology today.
Adding the cosmological constant to cosmology's standard FLRW metric leads to the Lambda-CDM model, which has been referred to as the "standard model" of cosmology because of its precise agreement with observations. Dark energy has been used as a crucial ingredient in a recent attempt to formulate a cyclic model for the universe.