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In cosmology, dark energy is a hypothetical form of energy which permeates all of space and has negative pressure resulting in an effective "repulsive gravitational force". (Michael Turner coined the term dark energy.) Adding dark energy to the standard theory of cosmology (i.e. FLRW metric) is currently the most popular method of accounting for the apparent observations of an accelerating universe as well as a significant portion of the missing mass in the universe. This new standard model of cosmology is named Lambda-CDM model. Two proposed forms of dark energy are the cosmological constant and quintessence, where the former is static and the latter is dynamic. Distinguishing between the two would require high precision measurements of the expansion of the universe to see how the speed of the expansion changes over time. Making such measurements is a topic of current research.
Proposal
The cosmological constant was first proposed by Albert Einstein as a mechanism to obtain a stable solution of his field equation that would lead to a static universe (effectively using dark energy to balance gravity). However, it was later recognized that the Einstein static universe would actually be unstable because the existence of local inhomogeneities would ultimately lead to either runaway expansion or contraction on a universal scale. More importantly, observations made by Edwin Hubble showed that the universe appears to be expanding and not static at all. After this realization, the cosmological constant was largely ignored as a historical curiosity.
In the 1970s Alan Guth proposed that a cosmological constant could drive cosmic inflation in the very early universe. Even after inflationary models became widely accepted, the cosmological constant was believed to be irrelevant to the current universe. However, in the late 1990s, satellites and the golden age of telescopes allowed high precision measurements of distant supernovae and the cosmic microwave background to be made. Several surprising features of these measurements are most easily explained if some form of dark energy does exist in our modern universe.
Nature of phenomena
Because of its repulsive nature, dark energy would tend to cause the expansion of the universe to accelerate, rather than slow down as would be expected in the traditional view of a purely matter dominated universe. An accelerating universe is what appears to be observed by looking at the most distant supernovae.
Another argument comes from studies of the total energy density of the universe. It has long been known from theoretical and observational arguments that the total energy density of the universe seems to be very near the critical density needed to make the universe "flat" (i.e. the curvature of space-time, defined in general relativity, goes to zero on large scales). Since energy is equivalent to mass (special relativity: E = mc2), this is usually expressed in terms of a critical mass density needed to make the universe flat. Observations of the luminous matter only account for 2-5% of the necessary mass density. Dark matter, matter which doesn't emit enough light to be seen, has long been hypothesized to make up this missing mass, but observations of galaxies and clusters made during the 1990s strongly suggest that dark matter can't account for more than ~25% of the critical mass density. If dark energy makes up the remaining ~70% of the critical energy density, the total energy density is consistent with what is needed to make the universe flat.
Speculation
The exact nature of this dark energy is largely a matter of speculation. Some believe that dark energy might be "vacuum energy", represented by the "cosmological constant" (Λ) in general relativity. The simplest explanation is to posit a "cosmological constant", meaning a constant uniform density of dark energy throughout all of space that is independent of time or the universe's expansion. This is the form of dark energy introduced by Einstein, and is consistent with our limited observations to date. If dark energy takes this form, it suggests that it is a fundamental property of the universe. Alternatively, dark energy might arise out of the particle-like excitations some type of dynamical field, referred to as quintessence. Quintessence differs in from the cosmological constant in that it can vary in space and time. In order for it not to clump and form structure like matter, it must be very light (so that it has a large Compton wavelength). No evidence of quintessence is yet available, but it cannot yet be ruled out.
Inflation
It should also be noted that some form of dark energy is closely related to the theory of cosmic inflation. Inflation postulates that some repulsive force, qualitatively similar to dark energy, resulted in an enormous and exponential expansion of the universe slightly after the Big Bang. Such expansion is an essential feature of most current theories of cosmology and structure formation. However, inflation must have happened at a much higher energy density than the dark energy we observe today and is believed to have completely ended early in the universe's life. It is unclear what, if any, relation dark energy and inflation have.
Future implications of dark energy
If the hypothetical dark energy continues to dominate the universe's energy balance, then the current expansion of space will continue to accelerate, ultimately becoming exponential in character, which is known as de Sitter space.
Structures which are not already gravitationally bound will fly apart with apparent speeds greater than the speed of light. Since our knowledge of the universe is limited to signals that can travel no faster than the speed of light, the acceleration will ultimately prevent us from even seeing distant portions of the universe which are now visible. However, it should be noted that if the dark energy density is non-increasing then any structures, such as galaxies and solar systems, which are currently gravitationally bound will remain so. Hence the Earth and the Milky Way may remain virtually undisturbed while the rest of the universe appears to run away from us.
Alternatively, dark energy might not be constant, but rather growing with time. In such a scenario, referred to as Phantom energy everything in the universe right down to the atoms themselves could ultimately be blown apart in a "Big Rip", leaving a universe empty and devoid of structure.
Finally, the dark energy might dissipate with time, or even become attractive. Such uncertainties leave open the possibility that gravity might yet rule the day and lead to a universe that contracts in on itself in a "Big Crunch". This is generally considered to be the least likely scenario.
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