In cosmology, dark energy is a hypothetical form of energy . Dark energy was introduced as a generalization of cosmological constants to explain the observed accelerated expansion of the universe . The term was coined in 1998 by Michael S. Turner .
The physical interpretation of dark energy is largely unclear and its existence has not been directly proven experimentally. The most common models associate them with vacuum fluctuations . The physical properties of dark energy can be investigated through large-scale mapping of the structures in the universe, for example through the distribution of galaxies and galaxy clusters . Corresponding major astronomical projects are in preparation.
After the expansion of the universe was established by observing the redshift of the galaxies, more detailed measurements were taken to determine the rate of expansion and its change over the lifetime of the universe. Traditional models have said that the expansion is slowed down by matter and the gravity acting through it ; Measurements should quantify this slowdown.
The measurements, which were essentially based on distance determinations of distant supernovae of type Ia (SN IA), showed, contrary to the predictions that could be derived from the assumptions valid up to that point, an increase in the rate of expansion ( High-Z Supernova Search Team around Brian P. Schmidt , Supernova Cosmology Project by Saul Perlmutter , Adam Riess , both in the late 1990s). Schmidt, Riess and Perlmutter received the 2011 Nobel Prize in Physics for this . This unexpected observation has since been attributed to an indefinite dark energy . In the models, the universe at the present time, approx. 13.8 billion years after the Big Bang , consists of 68.3% dark energy, 26.8% dark matter and 4.9% visible, baryonic matter . The Planck mission corrected the WMAP data from 2012 somewhat in 2019. In the early days of the universe, when the matter was decoupled from the background radiation , the composition was still significantly different (see diagram on the right ). However, the data from the Planck mission result in consistent data on the proportion of dark energy with the SN IA, regardless of the distance determination project.
The existence of a dark energy could also explain the flatness of the universe. It is known that normal matter is insufficient to give the universe a flat, that is to say essentially Euclidean , geometry; it represents only 2–5% of the necessary mass. However, observations of the gravitational attraction between the galaxies show that dark matter can be a maximum of 30% of the required matter.
Dark energy is also an important parameter in models of structure formation in the universe.
The accepted theory for the large-scale development of the cosmos is the general theory of relativity (GTR). In the discussion about the expansion or contraction of the universe, matter causes the expansion to slow down through its gravitational effect; the cosmological constant (if it is positive), on the other hand, describes an accelerated expansion and, if it dominates the curvature on large scales , a flat universe.
The observed acceleration of the expansion movement means that a description by the cosmological constant makes sense. Previously it was only an ad hoc construction that could not be ruled out in the heuristic derivation of Einstein's field equations .
One of the first cosmological models that goes back to Albert Einstein describes a static, non-expanding universe. In the context of this model, the cosmological constant has a value other than zero. The cosmological constant corresponds to an energy of the vacuum that counteracts the gravity of the matter contained in the universe. After it was discovered that the universe is not static but is expanding, Einstein also went over to setting the cosmological constant equal to zero. Nevertheless, models in which the cosmological constant has a value other than zero have continued to be discussed in the literature, for example in the Lemaître universe (inflexion model).
Another problem was that the assumption of vacuum energy in quantum field theory made contributions to the energy-momentum tensor that corresponded to an extremely high value of the cosmological constant, which was not observed (problem of the cosmological constant).
Attempts to explain
The exact nature of dark energy can currently only be speculated. The simplest solution is to postulate a suitable value of a cosmological constant and accept it as a given and fundamental property of the universe.
One suggestion is to understand dark energy as the vacuum energy of "empty space" that occurs in quantum field theory . Since space increases with the expansion of the universe, the vacuum energy also increases and accelerates the expansion. This is the currently preferred explanation. However, so far (2020) there are no convincing quantitative derivations.
Alternatively, dark energy is seen as the effect of a scalar field that changes over time , called a quintessence . The fluctuations in such a field typically propagate at almost the speed of light . For this reason, such a field does not tend to form gravitational clumps: the fluctuations in overdense regions flow very quickly into underdense regions and thus lead to a practically homogeneous distribution.
The elementary particles that are ascribed to such a scalar field would be extremely light (about 10 −82 electron masses ) and, apart from gravitation, should practically not interact with normal (baryonic) matter.
Independent of the dark energy there is the dark matter , which is not to be confused with this. Their origin is also unknown. For example, it ensures the stability of galaxies when they collide with one another or when they rotate.
Dark energy and the associated fields are also a possible cause of inflation in the early days of the cosmos. However, it is unclear whether such dark energy is related to that proposed for the currently observed expansion.
Current research projects
More recent research programs are carried out with the Hyper Suprime-Cam of the Subaru telescope and, as part of the Dark Energy Survey, with the DECam of the Victor M. Blanco telescope . The launch of the Euclid space telescope was scheduled for 2019, but has been postponed to 2020. The main instrument is the eROSITA developed by the Max Planck Institute for Extraterrestrial Physics , which was launched with a Proton rocket on July 13, 2019 in the Russian Spektr-RG satellite . With the planned experiments, the researchers hope to track down the nature of dark energy.
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