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Dark energy

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". Dark energy may account for the accelerating universe as well as a significant portion of the mass in the universe. A similar concept is that of quintessence, which, unlike dark energy, is not a constant form of energy.

Table of contents
1 Proposal
2 Nature of phenomena
3 Speculation
4 Inflation


Dark energy was first proposed by Albert Einstein as a mechanism to balance gravitation and lead to a steady state universe. However this was quickly rejected when Hubble discovered that the universe was expanding. For a long time, dark energy was largely ignored as an historical curiosity, but recent measurements of distant supernova and the cosmic microwave background are most easily explained if some form of dark energy does exist.

Nature of phenomena

Because of its repulsive nature, dark energy tends to cause the expansion of the universe to accelerate, rather than slow down as would be expected in a purely matter dominated universe. An accelerating universe is exactly what was observed by looking at the most distant supernova.

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 is 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. See: shape of the universe). 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, i.e. 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 comes out exactly as needed to make the universe "flat".


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 some type of particle, referred to as quintessence. Some theories suggest such particles could have been created during the big bang in sufficient abundances to permeate all of space. However, if this were the case they might be expected to clump and vary in density as a function of time. No evidence of this is yet available, but neither can it be ruled out.


It should also be noted that some form of dark energy is the most likely explanation of cosmic inflation during the big bang. Such inflation is an essential feature of most current theories of cosmology and structure formation. It is unclear whether the dark energy present today is related to the dark energy that could have caused inflation.