Dark energy is a hypothetical form of energy with a negative pressure that is thought to permeate spacetime.
According to Einstien’s theory of relativity, the effect of negative pressure is in opposition to gravitational force on massive and strong scales. It is believed by some that such a force maybe responsible for the universes evident accelerated expansion; which is ultimately indicating that our universe is dying!
Dark energy would make up the lions share of missing matter, making up about 74% of all matter in the vast universe. Two forms of dark energy are proposed as the 'cosmological constant,' a constant energy density filling spacetime homogeneously and there is 'quintessence', a dynamic field which has an energy density that can vary in space and time. It requires high-precision measurements to determine the exact speed of expansion.
Space and Time Warps
During the 1990's, observations of type 'la supernovae' by the 'Supernova Cosmology Project' and the 'High-Z Supernova Search Team' have provided the suggestion that the universe is indeed accelerating - being backed up with several pieces of observable evidences which include measurements of the 'background microwaves', 'gravitational lensing' and improved measurements on observable supernovae have been consistent with something called the 'Lambda-Chromodynamics Model,' (but i will not explain this as it is not germane to the argument.) The most direct evidence we have for dark energy comes from the la supernovae - the Hubble red-shift, a way of measuring 'time warps' from the light of receding objects - measuring how distant a thing is, is more difficult to say the least. Thus, it is necessary to find 'standard candles' - these universal candles actually help us in determining certain distances and magnitudes with good accuracy. Without standard candles, it is relatively impossible for us to measure red-shift distances without us making a wild guess, which is really not acceptable in the realms of science. Type la supernovae are the best known candles in our cosmos for our observations - simply because they are extremely bright and thus visible at distances of billions of light years. The la supernovae is explained by the favored example of an old 'white dwarf,' which gains mass from a counterpart star and grows until it reaches the 'Chandrasekhar limit' - at this limit, the white dwarf is unstable to thermonuclear runaway and the result is that it explodes with a tremendous brightness, flaring ever outward with energy. It is this brightness that makes observing the Hubble red-shift possible - and it is our observations that indicate the universe isn't decelerating, but is in fact accelerating, at an alarming rate... and it is these observations that the acceleration in universal expansion might be caused by dark energy with negative pressure.
A Required Substence
The presence of dark energy is really needed in physics, to reconcile with the measured geometry of spacetime with the total matter in our universe. Measurements of the cosmic background radiation made recently the 'Wilkinson Microwave Anisotropy Probe,' indicates that the universe is nearly flat - a big leap i must say from Einstien’s highly curved universe. For a universe to be flat, the mass-energy density of the universe must be proportional to the 'critical density.' 'Baryons' and 'dark matter' is thought to cover only about 30% of the universes critical density matter (more on dark matter later.) Thus, it implies the existence of an additional form of energy to account for the missing 70%. Before we continue, baryons are in the family of 'hadrons.' We have seen two types of baryons in this book - they being protons and neutrons that make up the nucleus of atoms. The very nature of dark energy is a matter of speculation. It is thought to be extremely homogeneous, not very dense and doesn't interact with electromagnetic, strong or weak forces; only gravity. Since it is not very dense, around 10^-29 grams per cubic centimeter, it is very hard to comprehend dark energy being detected in the laboratory - with it making at least 70-74% of universal matter would have as you can imagine a drastic impact on expansion. Two leading models as we have seen, concerning dark energy are quintessence and the cosmological constant. The simplest explanation for dark matter is that it is the 'cost of having space.' Thus a volume of spacetime has some kind of intrinsic, fundamental energy. This is the cosmological constant. It is sometimes called 'Lambda' after the Greek letter, which is used as a mathematical symbol to represent this quantity. The cosmological constant is estimated to be on the order of 10^-29g/cm3 or 10^120 in reduced 'Plank Units.' The cosmological constant might be well known by the reader. It was first proposed by Einstein himself as a mechanism to proclaim a solution to the gravitational field equation, that would lead to Einstien’s strange dream; a static universe. Though, his calculations proved to be quite unstable, and that a static universe would be unpredictable due to local inhomogeneties which would result in a universe that contracts or accelerates - this means that the universe, if it expands only a tiny fraction will continue to expand due to a release of energy from the vacuum, and if it decelerates a tiny fraction, again it will continue to contract. Thus, we now know that the universe is accelerating, and Hubble red shift proved this fact - Einstien’s mistake went past history, as nothing but a curious flaw, as he himself said, it was his biggest blunder.
Dark energy might arise from the particle-like excitations in a dynamical field called quintessence. Quintessence can vary in spacetime, so it differs from the cosmological constant. Some scientists believe quintessence might be found in violation of Einstien’s 'Equivalence Principle.' The 'cosmic coincidence,' theory asks the problem to why the universe accelerated when it did. If acceleration began earlier than it did, planets stars and galaxies would not have had time to form life - we ultimately would not have been here if it had started earlier. Promoters of the 'anthropic principle,' use this to support evidence of the universes means to create us - possibly by superintelligence, like God.
However, the 'tracker behavior' of quintessence solves this paradox. The tracker behavior model of quintessence has a density which closely tracks (but less than) the radiation density until the matter-radiation is equal, which ultimately triggers the 'quintessence field' to act as dark energy to dominate the universe. One case of quintessence which is thought to exist is called 'phantom energy,' where the energy density can increase in time. As weird and wonderful phantom energy sounds, it might actually result the universe in a 'Big Rip'.