Dark matter

Dark matter

Dark matter is an imaginary form of matter, which is considered to be approximately 85% of the matter in the universe and about a quarter of its total energy density. The majority of dark matter is considered non-baryonic in nature, possibly some of which are still made of undiscovered sub-atomic particles. Its presence is contained in different types of gravitational effects, including gravitational effects which can not be explained unless there is more than this case. For this reason, most experts consider the dark matter omnipresent in the universe and have a strong influence on its structure and evolution. Dark matter is called dark because it does not interact with observable electromagnetic radiation as light, and thus invisible to the entire electromagnetic spectrum, making it extremely difficult to detect using normal astronomical devices.

The primary proof for dark matter is that the calculation shows that many galaxies will fly individually, or not, or will not run, instead of roaming, as they do, if they do not contain large amounts of unseen substances . Other lines of evidence include observations in gravitational lensing, from the cosmic microwave background, from astronomical observations of the present structure of the observable universe, by the creation and development of galaxies, galactic galaxies within galactic clusters At the speed of . In the standard lambda-CDM model of cosmology, 5% of the total mass-energy in the universe is the form of simple matter and energy, 27% dark matter and 68% of unknown energy called dark energy. Thus, dark matter makes up 85% of the total mass, whereas dark energy and dark matter constitute 95% of the total mass-energy content.

Because dark matter has not yet been seen directly, so it should barely interact with ordinary baryonic  and radiation. Primary candidates for dark matter are some of the new types of primary particles, which have not yet been discovered, especially, weak-interactive massive particles (WIMPs), or gravitational-intersting vast particles (GIMPs). Many experiments are being done actively for direct detection and study of dark matter particles, but no one has yet succeeded. Dark matter is classified as cool, hot or hot according to its velocity (more accurate, its free streaming length). Current models favor a cold dark matter scenario, in which structures emerge by gradual accumulation of particles.

Although the existence of dark matter is generally accepted by the scientific community, some astrophysicists are conspired by some observations, which do not fit the Dark Matter theory, argue for various modifications of standard laws of general relativity. , Such as modified Newton dynamics, tensor vector-scaler gravity, or entropic gravity. These models try to account for all comments without applying a supplemental non-baryonic  case.

Classification of dark matter

Dark matter can be divided into cold, hot and hot categories. These categories refer to velocity rather than the actual temperature, it shows that due to the expansion of the universe, at a slow pace before the random motion of the universe, how far things went away - this is an important distance, which is free streaming length (FSL) ).  Primordial density is fluctuated compared to this length, which is washed away by particles spread in very low-lying areas, while large fluctuations are unaffected; Therefore, this length determines a minimum scale for the formation of later structures. The categories are determined in relation to the size of a protogalaxy (an object which is later developed into a dwarf galaxy): Dark metal particles are classified as cold, warm or hot according to their FSL; Very small (cool), similar to (hot), or very large (warm) than a protogalaxy.

The above mixture is also possible: a theory of mixed dark matter was popular in the mid-1990s, but after the discovery of Dark Energy it was rejected.

Cold dark matter leads to the bottom-up formation of the structure with galaxies in the latter stage, while the hot dark matter resulted in a top-down formation scenario, with large amounts of aggregation early, later individually There is fragmentation in galaxies; The latter is excluded by high-redshift galaxy observations.

Cold dark matter

The cold dark matter provides the simplest explanation for the most cosmological observations. It is dark matter which is composed of component with an FSL which is very small than a protogal. This is the focus for dark matter research, because hot dark matter galaxies or galaxies do not seem capable of supporting cluster formation, and most particles slow the candidates quickly.

Components of cold dark matter are unknown. Possibilities come from large objects such as new particles like WIMPs and axes such as MACHOs (like black holes and pre-star stars) or RAMBOs (such as the cluster of brown dwarfs).

The study of Big Bang nucleosynthesis and gravitational lensing convinced most cosmologists that the macro can not make more than a small fraction of the dark matter. a. According to Peter: "... only the candidates with really adornable black matter are new particles." In particular, Jamie Ferns proposes a particle with negative mass.

The 1997 DAMA / NaI experiment and its successor DAMA / LIBRA in 2013 claimed to directly detect dark matter particles passing through the Earth, but many researchers are in doubt, because negative results from similar experiments are incompatible with DAMA results Seem to be

Many supersymmetric models offer dark matter candidates as WIMPy lite supersymmetric particle (LSS). Individually, heavy sterile neutrinos are present in the non-supersymmetric extension in the standard model, which explain the small natrinos mass through the secretion system.

Warm dark matter

Hot dark matter contains particles with an FSL that are equal to the size of a protogalaxy. Predictions based on hot dark matter are very similar to cold dark matter, but with small density on small scale. This reduces the estimated abundance of dwarf galaxies and can reduce the density of black matter in parts between large galaxies. Some researchers consider it a better fit for comments. A challenge for this model is the lack of essential mass particle candidates ~ 300 eV to 3000 eV.

None of the known particles can be classified as hot deep matter. A posted candidate is sterile neutrinos: a heavy, slow form of neutrinos which, unlike other neutrino, does not interact through weak force. Some modified gravitational theories, such as scalar-tensor-vector gravity, require "hot" dark matter to work on their equations.

Hot dark matter

Hot dark matter contains particles whose FSL is much larger than the size of a protogalex. Neutrinos is worthy of such particle. They were independently searched, long before the prey of dark matter: They were posted in 1930, and came to know in 1956. The neutrinos mass is less than 10-6 compared to the electron. Neutrinos only interact with normal matter through gravity and weak force, which makes it difficult to detect (weak force only works at a short distance, thus a neutrino only forces a weak force event only when It hits a nucleus on the head). This makes them 'light up the weak particles' (WILPs), unlike the WIMP.

Three known tissues of neutrinos are electron, muon and tau. Their mass is slightly different. As soon as they move, neutrinos oscillate between flavors. It is difficult to set a precise upper limit on the mass average of three neutrino (or for any three personally). For example, if the average neutrinos mass was greater than 50 eV / c2 (less than 10 if5 of the mass of the electron), then the universe would collapse. CMB data and other methods indicate that their average mass is probably not more than 0.3 EV / C2. Thus, celebrated neutrinos can not interpret dark matter.

Because the fluctuation in the size of the galaxy's size is washed by free-streaming, the hot dark matter means that the first things that can form huge supercluster-shaped pancakes, which then break into galaxies. Observations of Deep-field indicate that the galaxies exist first, followed by clashes of clusters and superclusters together with galaxies.