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.
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 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 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.
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.
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. Structures which are not already gravitationally bound will ultimately fly apart with apparent speeds greater than the speed of light. Since our knowledge of the universe is limited to what we can see via 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 the "Big Rip", everything in the universe right down to the atoms themselves could ultimately be blown apart, leaving a universe totally empty and devoid of any structure whatever.
Finally, the dark energy might dissipate with time, or even reverse its force. 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.
- HubbleSite press release: New Clues About the Nature of Dark Energy: Einstein May Have Been Right After All