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Dark energy
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Physical cosmology
Universe · Big Bang
Age of the universe
Timeline of the Big Bang
Ultimate fate of the universe
Inflation · Nucleosynthesis
GWB · Neutrino background
Cosmic microwave background
Redshift · Hubble's law
Metric expansion of space
Friedmann equations
FLRW metric
Shape of the universe
Structure formation
Reionization
Galaxy formation
Large-scale structure
Galaxy filament
Lambda-CDM model
Dark energy · Dark matter
Dark fluid · Dark flow
Timeline of cosmological theories
Future of an expanding universe
Observational cosmology
2dF · SDSS
COBE · BOOMERanG · WMAP · Planck
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In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain observations over recent decades that the universe appears to be expanding at an accelerating rate. In the standard model of cosmology, dark energy currently accounts for 73% of the total mass-energy of the universe.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 is physically equivalent to vacuum energy. Scalar fields which 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.A 2011 survey of more than 200,000 galaxies appears to confirm the existence of dark energy, although the exact physics behind it remains unknown.
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