Dark energy

As dark energy is referred to in cosmology, a hypothetical form of energy. Dark energy was introduced as a generalization of the cosmological constant to explain the observed accelerated expansion of the universe. The term was coined by Michael S. Turner 1998.

The physical interpretation of dark energy is largely unknown and its existence is not experimentally proven. The most common models they bring with vacuum fluctuations, in conjunction, but there are also a number of other models discussed. The physical properties of the dark energy can be investigated by large-scale mapping of the structures in the universe, such as the distribution of galaxies and clusters of galaxies; corresponding large astronomical projects are in preparation.

Observation

After the expansion of the universe by observing the redshift of the galaxies was considered established, detailed measurements were carried out to determine the rate of expansion and its change over the lifetime of the universe. Traditional models said that the expansion is slowed down due to the matter and the gravitational attraction acting through them; Measurements should quantify this slowdown.

The measurements, which were essentially based on distance measurements of distant supernovae of type 1a showed, contrary to this doctrine, an increase in the rate of expansion. This unexpected observation has since been attributed to an indeterminate dark energy. In the models the universe is at the present time, about 13.8 billion years after the Big Bang, about 68.3 % of dark energy, 26.8 % dark matter and 4.9% of the visible, baryonic matter. ( In the early days of the universe, at the time of decoupling of matter from the background radiation, the composition was not significantly different. )

The existence of dark energy could also be an explanation for the " flatness " of the universe. It is known that the normal matter is insufficient to the universe, a flat (i.e., substantially Euclidean ) to give geometry; it represents only 2-5 % of the necessary mass. From observations of the gravitational attraction between the galaxies reveals that the dark matter may be a maximum of 30 % of the required material. Dark energy (due to Einstein's formula E = mc2 has a mass equivalent ) would provide the missing mass straight.

Dark energy is also an important parameter in models of structure formation in the universe.

Theoretical background

The now accepted theory for large-scale development of the cosmos is the General Theory of Relativity. In the discussion about the expansion or contraction of the universe, matter caused by its gravitational effect of a slowdown in the expansion; the cosmological constant (if it is positive ), however, describes an accelerated expansion, and that they dominated on large scales compared to the curvature, a flat universe.

The observed acceleration of the expansion movement means that a description of the cosmological constant is useful. Some scientists have so far been of the opinion that this constant is an ad hoc construct that provides no deeper foundation for the underlying cause. It seems, however, that the Einstein field equations were derived by integration. Any properly executed mathematical integration calls for the presence of a constant number, the so-called constants of integration. The constant of integration, which occurs correctly in the derivation of Einstein's field equation is called the cosmological constant. One of the first cosmological models, which goes back to Einstein, describes a static, non- expanding universe. Under this model, the cosmological constant has a nonzero value; the cosmological constant corresponds to an energy of the vacuum, which counteracts the gravity of the matter contained in the universe. After it was discovered that the universe is not static, but expanding, Einstein went on to set the cosmological constant equal to zero. However, models have been discussed in the literature continue, in which the cosmological constant has a nonzero value, such as in - Lemaître universe ( Inflexionsmodell ).

Attempts to explain

The exact nature of dark energy can currently only be speculated. The simplest solution is to postulate an appropriate value of a cosmological constant and to accept as a given and fundamental property of the universe.

One suggestion is to understand the dark energy as a vacuum energy that occurs in quantum field theory. However, there is as yet no convincing quantitative derivations.

Alternatively, dark energy is referred to as the effect of a scalar field, " quintessence " viewed. The fluctuations of such a field spread typically made with fast speed of light. For this reason, such a field also tends not to lump gravitativem: The fluctuations in overdense regions flow very quickly in under dense regions and thus lead to a practically homogeneous distribution. The elementary particles, which is ascribed to such a scalar field would be very easy (about 1082- of the mass of an electron ) and should - interact virtually impossible with normal ( baryonic ) matter - apart from gravity.

Inflation

Dark energy and the related fields are also a possible cause of inflation in the early days of the universe. However, it is unclear whether there is a relationship between such dark energy, and those proposed for the currently observed expansion.

Current Research Projects

Recent research programs are carried out, inter alia, with the Hyper Suprime - Cam of the Subaru telescope and under the Dark Energy Survey with the DECam the Victor M. Blanco telescope. The launch of the space telescope Euclid was planned for 2019 but was postponed to 2020.