Dark matter is a postulated form of matter that is not directly visible, but interacts via gravity . Their existence is postulated because this is the only way to explain the movement of visible matter in the standard model of cosmology , in particular the speed with which visible stars orbit the center of their galaxy . In the outer areas, this speed is significantly higher than one would expect based on the gravity of the stars, gas and dust clouds alone .
Dark matter is also postulated for the observed strength of the gravitational lensing effect . According to current knowledge, only about one sixth of the matter is visible and recorded in the standard model of elementary particle physics . The nature of dark matter is an important open question in cosmology .
Existence and meaning
According to Kepler's third law and the law of gravity, the speed of rotation of the stars should decrease with increasing distance from the galaxy center around which they rotate, since the visible matter is concentrated inside. Measurements of the Doppler shift show, however, that it remains constant or even increases, see rotation curve . This suggests that there is mass there that is not visible in the form of stars, dust or gas, i.e. dark matter.
Their existence has not yet been proven, but is suggested by other astronomical observations such as the dynamics of galaxy clusters and the gravitational lensing effect already mentioned , which cannot be explained by the visible matter alone, if one takes the recognized laws of gravitation as a basis.
Dark matter is assigned an important role in the formation of structures in the universe and in the formation of galaxies. Measurements within the framework of the standard model of cosmology suggest that the share of dark matter in the mass-energy density in the universe is about five times as high as that of ordinary (visible) matter. Photons and neutrinos also contribute to the energy density of the universe, but are evenly distributed and not significantly involved in the observed gravitational effects.
Evidence for the existence of dark matter
There are well-established indicators of dark matter at various size scales: galaxy superclusters, galaxy clusters, and galaxies. The range of scales between galaxies and galaxy clusters, in particular the cosmic neighborhood of the Milky Way , has only recently become the focus of the search for dark matter. A considerable part of the larger globular star clusters (over 1 million solar masses) of the galaxy NGC 5128 mainly contain dark matter.
In 1932 the Dutch astronomer Jan Hendrik Oort suspected the existence of dark matter in the area of the Milky Way disk based on his investigations into the number density and velocity distribution perpendicular to the disk of different star populations and for different distances to the disk. From this he determined a mass density in the disk (in the vicinity of the sun) of 0.092 solar masses per cubic parsec , which was far greater than the then known density of 0.038 in the form of stars. The current value of the density obtained with similar methods is 0.1 to 0.11 ; however, much of the discrepancy was identified as gas and dust, along with a stellar mass of 0.095 .
At about the same time, the Swiss physicist and astronomer Fritz Zwicky observed in 1933 that the Coma cluster (a galaxy cluster consisting of over 1000 individual galaxies, with a large dispersion of the individual speeds and an average distance speed of 7,500 km / s) was not affected by the gravitational effect of its visible components ( essentially the stars of the galaxies) is held together alone. He found that 400 times the visible mass is necessary to hold the pile together gravitationally. His hypothesis that this missing mass was in the form of dark matter met with widespread rejection among experts at the time.
The analysis of the orbital speeds of stars in spiral galaxies by Vera Rubin since 1960 again showed the problem: the orbital speed of the stars would have to be much lower than it actually is with increasing distance from the galaxy's center. Since then, dark matter has been taken seriously and, based on detailed observations, suspected it to be in almost all large astronomical systems.
With the implementation of large-scale surveys of galaxy clusters and galaxy superclusters , it also became clear that this concentration of matter could not be achieved by the visible matter alone. There is not enough of the visible matter to produce the density contrasts by gravity. See also Sloan Digital Sky Survey and Structure of the Cosmos .
Comparative observations of the gravitational lensing effect , the galaxy distribution and the X-ray emission in the bullet cluster in 2006 represent the strongest evidence to date of the existence of dark matter.
Models and simulations
The standard model of cosmology, the lambda CDM model , summarizes various results of the observational cosmology, results in the following composition of the universe according to mass fraction: Around 73 percent dark energy , 23 percent dark matter, around 4 percent "ordinary matter", for example atoms, and 0.3 percent neutrinos . The “ordinary matter” is divided roughly half into self-luminous (for example stars) and non-self-luminous components such as planets and above all cold gas.
This model has also proven itself in large-scale cosmological simulations , for example in the Millennium simulation , since it leads to a structure formation that corresponds to the current observation situation. Based on this, local simulations of some dark matter halos , which are similar to that of the Milky Way, make statistical predictions about the density of dark matter in the area of the sun's orbit around the galactic center and the speed distribution of these particles. These parameters influence detector experiments on earth that want to detect dark matter directly and can therefore be tested. Due to the fact that structures in the distribution of dark matter are concentrated in the halos of galaxies, large-scale network structures of observed galaxy distributions can be compared with computer simulations of large-scale, but unfortunately non-observable networks of dark matter, with mathematical theorems about the statistical structure such networks are useful.
X-rays through pair annihilation of dark matter
According to current theory, dark matter must exist, otherwise the stars would not continue to rotate around the center of their galaxies as they actually do. Particularly preferred candidates for dark matter include so-called weakly interacting massive particles ( WIMPs ). Researchers are looking for this, for example, in the Italian underground laboratory Gran Sasso . However, recent scientific publications in the field of astroparticle physics are increasingly suggesting that WIMPs are not viable prospects for dark matter.
After investigations of the observations carried out in 2014 by several independent groups, the presence of a previously undiscovered spectral line with an energy of 3.5 k eV in the X-ray light of distant galaxies and galaxy clusters was reported. These unusual X-rays could provide an indication of the nature of dark matter. It has already been pointed out that dark matter particles could decay and emit X-rays in the process.
Joachim Kopp's team from the Mainz Cluster of Excellence for Precision Physics, Fundamental Interactions and Structure of Matter (PRISMA) takes a different approach. The PRISMA researchers propose a scenario in which two particles of dark matter collide and destroy each other. This is analogous to annihilation (when an electron hits its antiparticle, the positron ). After a closer review of this model and comparison with experimental data, there appears to be greater agreement than older models. Accordingly, dark matter particles would be fermions with a mass of only a few kiloelectron volts, which are often referred to as “sterile neutrinos”. Such light dark matter is usually viewed as problematic because it is difficult to explain how galaxies could have formed. The model by Joachim Kopp from the University of Mainz offers a way out by assuming that the destruction of dark matter takes place as a two-stage process. In the initial phase, an intermediate state would thus be formed which later dissolves into the observed X-ray photons. The results of the calculations show that the resulting X-ray signature correlates closely with the observations and thus represents a new possible explanation for this.
This new model is itself so general that it offers a new approach to the search for dark matter, even if it turns out that the spectral line discovered in 2014 has a different origin.
Possible forms of dark matter
In particle physics , various candidates are discussed as constituents of dark matter. Direct evidence in the laboratory has not yet been successful, so the composition of dark matter is considered unknown.
Baryonic dark matter
Ordinary matter is made up of protons , neutrons and electrons. The number of electrons is the same as that of protons. Electrons have a mass less than a factor of 1800 than protons and neutrons, which therefore determine the mass of ordinary matter to a good approximation. Since protons and neutrons belong to the baryons , ordinary matter is also called baryonic matter .
Since hot gases always emit radiation, the only option left for dark matter is cold gas . Against this hypothesis, the fact that cold gas can (under certain circumstances) warm up and even huge amounts of gas could not generate the required mass speaks against it.
Cold clouds of dust
A similar solution is the possible existence of cold dust clouds , which due to their low temperature do not radiate and would therefore be invisible. However, they would re-emit the light from stars and thus be visible in the infrared range. In addition, so large amounts of dust would be required that they would have had a significant impact on the formation of the stars.
Serious candidates were brown dwarfs , who are counted among the MACHOs (Massive astrophysical compact halo objects) . These are celestial bodies in which the pressure is so low that only deuterium fusion can take place instead of hydrogen, which means that they do not shine in the visible spectrum. However, if a MACHO stands exactly in front of a star, it acts as a gravitational lens and amplifies its radiation. Indeed, this has been observed sporadically between Earth and the Large Magellanic Cloud . Today, however, it is assumed that MACHOs only make up a small part of dark matter.
Non-baryonic dark matter
Anapole Majorana fermions
In May 2013, the theoretical physicists Robert Scherrer and Chiu Man Ho proposed anapole (non-polar) Majorana fermions as carriers of the dark matter of the universe. Anapole particles have a toroidal ( hoop- shaped) field, which causes an electric field to remain enclosed in this torus (hoop) and thus not be noticeable externally. This is in contrast to the known electrical monopoles and magnetic dipoles , whose fields radiate into the environment with decreasing intensity ( Coulomb's law ).
In the standard model of particle physics, none of the elementary particles is a Majorana fermion. Instead, all fermions are described here by Dirac spinors , including the neutrinos , which could thus be distinguished from antineutrinos. However, contrary to experimental results, the neutrinos in the Standard Model are massless. A popular explanation for the observed neutrino masses, the Seesaw mechanism, requires the description of the neutrinos by Majorana spinors and thus the equality of neutrinos and antineutrinos. This in turn would imply a violation of lepton number conservation, since particles and antiparticles are assigned the same lepton number.
Whether a distinction can be made between neutrinos and antineutrinos is currently still open. One possibility for experimental clarification is the neutrino-free double beta decay , which is only possible if neutrinos are Majorana particles. This mode of decay is sought in experiments such as GERDA or EXO .
Hot Dark Matter (HDM)
Neutrinos have long been considered an obvious candidate for hot dark matter because their existence is already established, unlike other dark matter candidates. However, according to recent findings, the maximum mass of the neutrinos is not sufficient to explain the phenomenon. If dark matter consisted largely of fast, light particles, i. H. hot dark matter, this would result in a top-down scenario for the structuring process in the universe . Density fluctuations would first have collapsed on large scales, first galaxy clusters, then galaxies, stars, etc. would have formed. However, observations teach the opposite. Age determinations of galaxies have shown that they are predominantly old, while some galaxy clusters are currently in the process of formation. A bottom-up scenario, a hierarchical structure development, is considered proven. Therefore, hot dark matter can only make up a small part of the total dark matter.
Another candidate from the neutrino sector is a heavy sterile neutrino , the existence of which is unclear. Because of the "sterility" it could be much more massive than the standard model neutrinos.
Cold Dark Matter (CDM)
This variant includes still unobserved elementary particles, which are only subject to gravity and weak interaction , the so-called WIMPs ( English Weakly Interacting Massive Particles , German weakly interacting massive particles ). WIMPs can be reconciled with a hierarchical creation of the universe.
Candidates arise from the theory of supersymmetry , which doubles the number of elementary particles compared to the standard model. The hypothetical particles are mostly unstable and decay into the lightest of them (LSP, lightest supersymmetrical particle ). The LSP could be the lightest of the four neutralinos .
An international study published in 2010 under the leadership of Pavel Kroupa revealed considerable deviations in the astronomical observations from the predictions of the CDM model . For example, the luminosity and distribution of satellite galaxies in the Local Group do not meet expectations. Kroupa sees such a strong collision with the CDM theory in the data collected that "it no longer seems to hold".
On the other hand, researchers with deep-frozen semiconductor detectors (CDMS, Cryogenic Dark Matter Search ) in the Soudan Underground Laboratory want to have observed three collision events of WIMPs with atomic nuclei - with an estimated 0.7 background events.
Another indication comes from the composition of cosmic rays: For particle energies above 10 GeV , unexpectedly many positrons are found (antiparticles of the electron). The first such measurements came from the PAMELA experiment on the Russian Resurs-DK1 satellite and from the Fermi Gamma-ray Space Telescope . More precise data, especially a lower upper limit for the anisotropy, has been provided by the Alpha Magnetic Spectrometer on board the ISS since May 2011 . One explanation for the excess of positrons would be the pair annihilation of colliding dark matter particles. However, the measured positron distribution is also compatible with pulsars as the positron source or with special effects during the propagation of the particles. It is hoped that, after a longer measurement time, sufficient data will be available so that clarity can be gained about the cause of the positron excess.
Alternatives to dark matter
All of the above explanations as well as the existence of dark matter itself implicitly assume that gravity follows Newton's law of gravity or the general theory of relativity . But there are also considerations to explain the observations by a modification of the law of gravity instead of the introduction of an additional matter component.
Renowned astrophysicist as Jacob Bekenstein and John Moffat have the controversial MOON hypothesis ( Mo difizierte N ewtonsche D ynamik) developed by which the equivalence of inertial and gravitational mass with extremely small accelerations not apply more.
The TeVeS ( Te nsor- Ve ktor- S kalar-Gravitationstheorie ) was first formulated in 2004 by Jacob Bekenstein . The main difference to the general theory of relativity lies in the formulation of the dependence of the gravitational strength on the distance to the mass, which causes the gravitation. At TeVeS, this is defined by means of a scalar , a tensor and a vector field , while the general theory of relativity represents the spatial geometry by means of a single tensor field.
The S kalar- T ensor- V ektor- G ravitationstheorie (STVG) was 2014 by John Moffat designed not to be confused with the TeVeS. The STVG has been successfully used to calculate the rotation of galaxies, the mass distribution of galaxy clusters and the gravitational lensing effect of the bullet cluster without the need to postulate dark matter. The theory also provides an explanation for the origin of the principle of inertia .
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