In physical cosmology, dark energy is a form of matter or energy that would be present in all space, producing a pressure that tends to accelerate the expansion of the Universe, resulting in a repulsive gravitational force. Considering the existence of dark energy is the most frequent way to explain recent observations that the Universe appears to be in accelerated expansion. In the standard model of cosmology, dark energy contributes almost three-quarters of the total mass-energy of the Universe.

Nature of Dark energy
The exact nature of dark energy is a matter of debate. It is known to be very homogeneous, not very dense, but its interaction with any of the fundamental forces other than gravity is not known. As it is not very dense, about 10-29 g / cm³, it is difficult to perform experiments to detect it. Dark energy has a great influence on the Universe, since it is 70% of all energy and because it uniformly occupies interstellar space. The two main models are the quintessence and the cosmological constant.

History
The cosmological constant was first proposed by Albert Einstein as a means of obtaining a stable solution to the Einstein field equation that would lead to a static Universe, using it to compensate for gravity. Not only was the mechanism an inelegant example of “fine tuning,” it was soon shown that Einstein’s static Universe would be unstable because local heterogeneities would eventually lead to uncontrolled expansion or contraction of the Universe. The equilibrium is unstable: if the Universe is expanding slightly, then the expansion releases the energy of the vacuum, which causes even more expansion. In the same way, a Universe that contracts slightly will continue to contract.

These types of disturbances are unavoidable, due to the uneven distribution of matter in the Universe. Observations by Edwin Hubble showed that the Universe is expanding and that it is not static at all. Einstein referred to his failure to predict a dynamic Universe, in contrast to a static Universe, as “his big mistake” of him. After this statement, the cosmological constant was long ignored as a historical curiosity.

Alan Guth proposed in the 1970s that a negative pressure field, similar in concept to dark energy, could lead to cosmic inflation in the pre-primal Universe. Inflation postulates that some repulsive forces, qualitatively similar to dark energy, result in a huge and exponential expansion of the Universe shortly after the Big Bang. Such expansion is an essential feature of many current Big Bang models. However, inflation must have occurred at a much higher energy than the dark energy we observe today and is thought to have ended completely when the Universe was only a fraction of a second away.

It is unclear what relationship, if any, exists between dark energy and inflation. Even after inflationary models have been accepted, the cosmological constant is thought to be irrelevant in the current Universe.

The term “dark energy” was coined by Michael Turner in 1998. At that time, the problem of the lost mass of primordial nucleosynthesis and the large-scale structure of the Universe was established and some cosmologists had begun to theorize that there was an additional component. in our Universe. The first direct evidence of dark energy came from observations of the expansion acceleration of supernovae, by Adam Riess and later confirmed by Saul Perlmutter.

This resulted in the Lambda-CDM model, which until 2006 was consistent with a series of rigorously increasing cosmological observations, the latest in 2005 from the Supernova Legacy Survey. The first SNLS results revealed that the mean behavior of dark energy behaves like Einstein’s cosmological constant with an accuracy of 10%. Results from the Hubble Space Telescope Higher-Z Team indicate that dark energy has been present for at least 9 billion years and during the period preceding cosmic acceleration.