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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". 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. 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.1 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 global 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 inflationInflation is the idea—first proposed by Alan Guth (1981)—that the nascent universe passed through a phase of exponential expansion (the inflationary epoch) that was driven by a negative vacuum energy density (positive vacuum pressure). This expansion can 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, satellitesA space observatory is any object in outer space which is used for observation of distant planets, galaxies, and other outer space objects. Introduction A large number of observatories have been launched into orbit, and most of them have greatly enhanced and the golden age of telescopes allowed high precision measurements of distant supernovaRemnant of Kepler's Supernova, SN 1604. A supernova is a type of stellar explosion which appears to result in the creation of a new star upon the celestial sphere. Nova" is Latin for "new"). The "super" prefix distinguishes this from a nova, which also ine 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.
2 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 " flatThe term shape of the universe can most usefully refer either to the geometry ( curvature and topology) of a comoving spatial section of the universe (a loose term for this is the shape of space or more generally, to the shape of the whole of space-time." (i.e. the curvature of space-time, defined in general relativityGeneral relativity (GR or general relativity theory (GRT is the theory of gravitation published by Albert Einstein in 1915. The conceptual core of general relativity, from which its other consequences largely follow, is the Principle of Equivalence which, goes to zero on large scales). Since energy is equivalent to mass ( special relativitySpecial relativity (SR or the special theory of relativity is the physical theory published in 1905 by Albert Einstein. It replaced Newtonian notions of space and time, and incorporated electromagnetism as represented by Maxwell's equations. The theory is: 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 argued that dark matter couldn't account for more than ~25% of the critical mass density. Remarkably, the supernova observations predict that dark energy should make up ~70% of the critical energy density, thus when added to the mass-energy of matter, the total energy density is consistent with what is needed to make the universe flat.
