What is dark matter?
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Updated June 01, 2013
In astrophysics, dark matter means the area apparently undetectable. Various hypotheses have been issued and crawled on the composition of the hypothetical dark matter: molecular gas, dead stars, brown dwarfs in large numbers, black holes, etc..
In several types of astronomical objects (stars, galaxies), observed the movements are different from those which are expected in theory. When we try to deduce from the gravitational mass observed: it is as if an invisible mass density was present. Nobody dares, for the moment, challenging the laws of gravity, then what this theoretical matters is made?
Is that dark matter exists, really?
Dark matter is not black but invisible. The dark matter would be more abundant than baryonic matter.
Cosmology tells us that the composition of the Universe is composed of: 73% of dark energy and 27% of material distributed as follows (23% of non-baryonic matter and 4% baryonic matter). The observations (or rather the lack of direct observations) imply rather a non-baryonic A baryon is in particle physics, a class of particles, including the most famous are the proton and the neutron. The term "baryon" Bary comes from the Greek word meaning "heavy" and it refers to the fact that the baryons are generally heavier than other types of particles. , and therefore still unknown. The dark matter would be more abundant than baryonic matter. Cosmology tells us that the composition of the universe is composed of: 73% of dark energy and 27% of material distributed as follows (23% of non-baryonic matter and 4% baryonic matter).
Image: Sloan Digital Sky Survey Team, NASA, NSF, DOE
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Does dark matter exists?
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One of the major problems of modern astrophysics is that we do not know the nature of the substance of the matter in the universe. The luminous matter, the only one we see directly, appeared to represent less than one-tenth the mass of the universe. The dark matter and dark matter or missing mass means that the matter does not emit light in a broad sense, this is radio waves to gamma rays. This light is our only source of information. However, astronomers agree that between 90% and 99% of the matter in the universe does not emit light. Yet the solar system shows that the bulk of the estate is in the Sun (99%), not bright planets represent only 1% of the estate. On the other hand, the study of light from the stars indicates that there is, dust which absorbs some of the visible light in the infrared re-emitting, and the neutral hydrogen that shines in radio waves or of hydrogen ion in which emits ultraviolet and X-ray. But this field is well because the light is, more distribution is not different from that found in stars like the Sun. So why, in a number of astronomical objects, observed the movements are different from those which are expected in theory?
When trying to calculate in terms of the effect of gravitational masses there that everything happens as if an invisible mass density was present.
It is clear that the universe contains more matter than we see. The dark matter is it or is there another explanation for this oddity?
In the early 19th century the last planet Uranus is known, the movement calculated for Uranus does not correspond to that observed. In 1845, astronomers Adams and Le Verrier have the same idea to solve this puzzle. They imagine that this is due to a new planet beyond Uranus and calculate the position that it should occupy. A telescope is pointed toward the position in 1846 and predicted the new planet is discovered, it is Neptune. The same is true concerning the observation of galactic clusters that show they also contain a large amount of mass in another form, called dark matter.
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Amount of dark matter...
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Clusters of galaxies are objects of choice to study the problem of dark matter, because we can study their mass distribution by several independent methods (The movement of galaxies, the properties of hot gas they contain, the phenomena of gravitational lens that can observe, disruption of the cosmic background radiation that they generate, modeling their training by gravitational collapse). Various hypotheses have been advanced on the composition of the hypothetical dark matter: molecular gas, dead stars, brown dwarfs in large numbers, black holes, etc...
However, the lack of direct observation rather entail non-baryonic and therefore still unknown. Cosmology tells us that the composition of the universe is composed of: 73% of dark energy and 27% of material distributed as follows (23% of non-baryonic matter and 4% baryonic matter). The study of galaxy clusters indicates that 90% of their mass is in an invisible and study galaxies indicates that the vast majority of their mass is invisible.
The observation of clusters of galaxies can show that dark matter is distributed in a less concentrated, more extensive than ordinary matter. The numerical simulations on the properties of the early universe, to find the distribution of dark matter around clusters of galaxies. Indeed, these simulations indicate that on small scales, dark matter would tend to form clumps, with individual values ranging from that of Earth to that of a galaxy. The dark matter would be a pancake dough encompassing clusters of galaxies containing a multitude of small lumps. Ben Moore has developed farms of computers specifically dedicated to this type of problem (the image here against a need 6 months of calculation).
Image: Results of simulations made by Ben Moore. Overview of the distribution of dark matter into a universe a billion light-years away and a region of 10 000 light-years behind. Both zooms represent the region being respectively 100 light-years and 1 light-year.
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Nature of dark matter
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Two main theories compete on the nature of dark matter. The dark matter hot and cold dark matter. These theories are based on the mass of particles making up the dark matter and speed. In the case of dark matter called "hot" particles have speeds close to that of light, while composing a black substance called "cold" would be more massive and more slowly. The speed of these particles involved in the formation of large structures of the universe. If the universe was dominated by hot dark matter, the high speed of the particles would constitute a first step in forming a structure smaller than the super cluster of galaxies, which then becomes fragmented clusters of galaxies, then galaxies, and so on. This is the scenario called "top down", since the largest structures form first, then divide. The best candidate to be the hot dark matter is the neutrino The neutrino particle is imagined for the first time in 1930 by Wolfgang Pauli, even before the discovery of the neutron was detected in 1956 by Frederick Reines and Clyde Cowan. This particle, insensitive to electromagnetic forces and the strong nuclear force is emitted during a beta decay, accompanied by an electron. The neutrino interacts very little with other particles, making it a good candidate for dark matter. The mass of the neutrino was estimated very low or even zero. The neutrino is the most abundant particle in the universe, after the photon..
If the cold dark matter dominated Universe, the particles will travel a distance smaller and thus erase the density fluctuations on tracts smaller than in the case of hot dark matter.
Ordinary matter will then come together to form the first galaxies (from clouds of gas), who themselves will gather in clusters and super clusters. This is the scenario called "bottom up". Both theories were defended by Yakov Borisovitch Zeldovitch for hot dark matter, and James Peebles for the cold dark matter. Currently, the model of cold dark matter that seems to prevail. Indeed, the galaxies are in dynamic equilibrium, which suggests they were created before the masses who need more time to reach that balance. However, theories introduce today a little hot dark matter. It is necessary to explain the formation of clusters; cold matter could not only enable the in so little time.
Image: Lyman alpha forest obtained by a numerical simulation, in an area of 30 million light-years behind. It is possible to detect large clouds of primordial hydrogen, through their absorption properties. There is a red shift of a factor which depends on the distance. This allows you to see through the absorption lines of clouds, how matter is distributed in the universe.
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The dark matter remains secret
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The Big Bang theory to calculate the number of baryons in the Universe (4 helium atoms and hydrogen), formed in the primordial nuclei synthesis. Astrophysicists have calculated the rate of baryonic matter which would be about 4% of the critical density. Or, to explain the flat geometry of the universe, the total area of the universe must represent 30% of the critical density (the remaining 70% of dark energy). It is therefore 26% of the critical density in the form of non-baryonic matter and therefore constituted by other particles that baryons. Many other indices converge to indicate that the universe contains a large quantity of material in a form non-luminous. In addition, the model of the Big Bang is in remarkable agreement with the observations, provided that the universe contains about 30% of dark matter and about 70% of dark energy. Yet more and more astrophysicists believe that dark matter does not exist and rather than try to explain anomalies in a field unobserved even unobservable, they said it would be more appropriate to review the physical laws that constitute the standard model and who are in any way called into question by other more fundamental problems. It would then be possible to solve several problems at once without making new assumptions. Some physicists are turning to such string theory.
String theory adds six new dimensions to the four usual (the three dimensions of space and time) and place the dark matter in these new dimensions that we are inaccessible. The electromagnetic forces and strong and weak nuclear would be confined to four dimensions and could not leave. However, gravity could disperse into other dimensions, and thus decrease in intensity compared to other forces. It is clear that it is difficult to give a complete and coherent vision of dark matter, because the subject is still in turmoil. As always in astrophysics, there is finally more questions than answers. If you can easily make an idea of this dark matter, it does not lift the mystery. This new component of our Universe, will certainly detect by other means in the near future. If it fails to explain many astrophysical observations because this is a test current theories incomplete. Perhaps we are on the wrong path since the beginning and the scientific world awaits a fresh and revolutionary.
Image: The dimensions that were case, the particle physicists.
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