Black Matter

In Astrophysical, the black matter (or matter sinks ), translation of the English dark matter , indicates the apparently undetectable matter, called upon to give an account of unexpected effects, in particular about the Galaxie S. Différentes assumptions were emitted and explored on the composition of this hypothetical black matter: molecular gas, died stars, dwarf brown in great number, black holes, etc However, the observations (or rather lack of direct observations) would imply a baryon , and thus still unknown nature rather not . The black matter would however represent an abundance five times more important than the baryon matter, to constitute from 22% to 27% of the total density of the observable Univers, according to the models of formation and evolution of the Galaxie S, as well as the cosmological model .

Indirect detection of the black matter

First indices

In 1933 the Swiss astronomer Fritz Zwicky decides to study an small group of seven Galaxie S in the cluster of Coma. Its objective was to calculate the total Masse of this cluster by studying the speed (or rather the dispersion speeds) of these seven galaxies. It could thus - using the laws of Newton - deduce the mass from it known as “masses dynamic”, then to compare it with the mass known as “masses luminous”, which is the mass deduced from the quantity of Lumière emitted by the cluster (by making the assumption of a reasonable distribution of the star populations in the galaxies).

The dispersion speeds (or in other words, how speeds of these 7 galaxies differ from/to each other) is directly related to the mass present in the cluster by a formula similar to the third law of Kepler. In fact, a star cluster can be compared with a Gaz, whose particles would be stars. If the gas is hot (and thus light), the dispersion speeds of the particles is high. In the extreme case, the particles having a sufficient speed leave the gas (evaporation). If the gas is cold (and thus heavy), the dispersion speeds is weak

Zwicky was surprised to note that the speeds observed in the coma cluster were very high. The dynamic mass was 400 times larger than the luminous mass! At the time, the methods and the precision of measurements were not enough good not to exclude from the errors of measurement. Moreover, massive objects such as the brown dwarf , the white dwarf , the neutron stars and the black holes, all of the objects far from radiant, were known little about, just like their distribution. In the same way for the interstellar dust and the molecular gas.

Zwicky announced its observation to its fellow-members, but those did not seem to be interested in it. Zwicky very did not have good reputation because of sound strong character and its measurements were criticizable because of large the Incertitude S of measurement.

This same phenomenon was observed again in 1936 by Sinclair Smith during the calculation of the total dynamic mass of the Virgo Cluster. This one was 200 times more important than the estimate given by Edwin Hubble, but it could, according to Smith, to be explained by the presence of matter between the galaxies of the cluster. Moreover, the galaxy clusters were still considered by a great number of astronomers as of the temporary structures whose galaxies could escape, rather than of the stable structures. This explanation was enough to justify excessive speeds.

The question of the difference between the dynamic mass and the luminous mass does not interest and dark in the lapse of memory for several decades. At the time, the astronomers had other questions considered to be more important, like that of the Expansion of the Universe.

Curves of rotation punt of the spiral galaxies

It is only one forty years later, in the Années 1970, that the question of the existence of this missing matter - that one will name “black matter” ( Dark Matter in English) - remade surface. Starting from the analysis of the spectrum S of the Galaxy S, the astronomer American Vera Rubin studied the rotation of the spiral galaxies. The problem is the same one as the comparison between the dynamic mass and the luminous mass of the galaxy clusters. It is a question of knowing if “luminous mass”, i.e. the mass which is deduced from the presence of stars, is quite equal (except for some corrections) to the dynamic mass.

It should be noted that the dynamic mass is normally the only true mass, since it is about a measurement of the mass deduced from its gravitational influence. Any mass being subjected to the force of Gravitation, it has no reason there to think that the dynamic mass observed is false. It is not also simple for the luminous mass. To measure the latter, the assumption is made that all the mass of the galaxy (or the galaxy cluster) consists of stars. These stars radiate, and if one knows (but it is very difficult) their distribution (mass, number, age, etc), the close infra-red is thus a good “tracer” of mass (it is not very sensitive at the height radiation of massive stars and makes it possible to detect the emission of less massive stars which prick in optics and the infra-red).

By analyzing the spectrum of the spiral galaxies seen by the section, like the Galaxy of Andromède, it is possible to deduce the curve from it from rotation. The curve of rotation describes the number of revolutions of the galaxy according to the distance to the center. This curve of rotation is a direct measurement of the total distribution of matter in the galaxy. The maximum speed of rotation of a spiral galaxy is with a few kilo Parsec S of the center, then it is supposed to decrease, while following a Keplerian decrease . Indeed, the stars with the Périphérie of the galaxy are in orbit around the center, in the same manner that the Planet S are in Orbite around the Sun. The stars in periphery of the galaxy thus turn less quickly than those more close to the center. The curve of rotation, after a maximum, starts to go down again.

However, Vera Rubin observed that the stars located at the periphery of the Galaxie of Andromède - as for others spiral galaxies - seem to turn too quickly (speeds remained practically constant as one moved away from the center). The curve of rotation of the spiral galaxies, or in any case some of them, was punt. Speed did not decrease whereas one moved away from the center. Many other similar observations are carried out in the Années 1980, coming to reinforce those of Vera Rubin. This observation raises deep questions, because the curve of rotation measures the dynamic mass well. No assumption about the age, of the distribution of mass of stars is necessary. The only assumption is that the stars which are the source of the light which forms the analyzed spectrum are many tracers of the mass of the galaxy. How to imagine whereas the stars, main components of matter in the spiral galaxies, turn in a not-Keplerian way, i.e. quite simply any more the laws of the gravitation do not follow?

A possible explanation is to imagine the existence of a gigantic nonvisible matter halation surrounding the galaxies; a halation which would represent until 90% total mass of the galaxy, even more in some dwarf galaxies . Thus all the stars are almost in the center of the true extension of the “galaxy” (this time made up of the visible galaxy and the matter halation sinks), and thus turn normally. In other words, the stars, even those with the visible periphery of the galaxy, are not “enough far” from the center to be in the going down again part of the curve of rotation. It remains to directly observe this famous matter to confirm that it is the good explanation. Nobody still reached that point until today.

The presence of black matter is the a possible explanations, and today most convincing. It with the immense advantage of being simple and of going in the good sense. Indeed, the astronomers suspected well that the galaxies contain stars far from luminous (like the brown dwarf, dwarf white, black holes, neutron stars) which can constitute an important part of the total mass of the galaxy, but which is not visible with the usual optical instruments. With the measurement of the curve of rotation punt further possible from the center, the observation of the spiral galaxies in others wavelengths (in order to better characterize the presence of not very luminous objects in the visible field) was one of the major efforts of astronomy to study the problem.

Recent observations

According to results published in August 2006, black matter would have been observed distinctly of the ordinary matter thanks to the observation of the Amas of the ball made up in fact of two close clusters which entered in collision approximately 150 million years ago. The Astronome S analyzed the effect of gravitational Lentille in order to determine the total distribution of mass in the pair of cluster and compared this distribution with that of the ordinary matter such as data by the direct observation of the emissions of x-rays coming from extremely hot gas of the clusters, which one thinks that it constitutes the majority of the ordinary matter of the clusters (the galaxies contributing does very little of it). The very high temperature of gas is due precisely to the collision during which the ordinary matter interacts between the two clusters and is slowed down in its movement. The black matter as for it would not have interacted, or very little, which explains its different position in the clusters after the collision.

The best proof of the existence of the black matter would come however from a truly direct observation, i.e. of the interaction between black matter particles with terrestrial detectors, such CDMS, XENON or WARP, or creation of such particles in an accelerator (like LHC for example). This type of description would have the advantage of precisely determining the mass of such particles and of analyzing in-depth the form of their interactions.

Distribution of the black matter in the Universe

Within the galaxies

Starting from the number of revolutions of stars and galaxies (on the level of the clusters), it was possible to measure the mass of this black matter, and to also deduce its distribution from it. A great quantity of this matter should be within the galaxies, not in the galactic disc but in the form of a halation including the galaxy. This configuration allows a stability of the galactic disc. Moreover, certain galaxies have rings perpendicular to the disc and composed of gas, dust and stars. There still, the matter halation would explain the formation and stability that of such rings require. On the other hand, it is impossible that the black matter is in the galactic disc, because one should then observe an oscillation perpendicular to the disc in the movement of the stars which we do not see.

Following the example luminous matter, it also decreases as one moves away from the center of the galaxy, but in a way much less marked. Thus, the proportion of luminous matter would vary the dominant one in the middle of the galaxies to negligible to the periphery. The study of satellite galaxies (small galaxies turning around other galaxies) obliges to imagine very wide halations: approximately 200 or 300 kpc. By comparison, the Sun is located at approximately 8,6 kpc center of our galaxy. The Galaxie of Andromède - galaxy nearest to us - is located at 725 kpc, that is to say a little more of the double of the ray of the black matter halation of our galaxy. Blow, these halations should be common between close galaxies (like pips in the same apple).

Between the galaxies, on a cluster scale

The movements of galaxies within the clusters revealed the same problem as the motion study of stars in the galaxies and thus suggest the presence of black matter between the galaxies; although nothing proves yet that these two problems are dependant. On a galaxy scale, the black matter rate would be up to ten times that of the luminous matter, but on the level of the clusters, it would be much more important: up to thirty times “visible” mass of these clusters.

In 1996, the astrophysicist Yannick Mellier undertook with its team to measure the quantity of black matter in all the Universe and to draw up a chart of its distribution between the galaxy clusters using the gravitational Cisaillement. The idea is to make a statistical study with large scales of the deformation of the images of the galaxies due to the gravitational interaction of the black matter present between the Earth and these structures, deviating the luminous rays sent by those (their image thus arrives to us deformed). A statistical study with very large scales (the area of the sky studied was apparent size of the moon and on a depth of 5 billion light-years) makes it possible to neglect the local deformations due to the other galaxy clusters.

This study led in March 2000 to a first cartography (still in the form of outline). The black matter should take the shape of long filaments which intersect, the quantity of matter of the universe should represent a third of that making it possible to reach the Densité criticizes, the remainder making up of black energy.

A new similar study is in hand, always by the team of Yannick Mellier, with this time a larger camera CCC, making it possible to study a surface twenty times larger than at the time of the first study. This one will make it possible to obtain a more detailed chart of the black matter with large scales.

Formation of the great structures of the Universe

The black matter poses many problems, but can solve some of them others. One can utilize it to explain the formation of the great structures of the universe (Galaxie S, galaxy cluster, supercluster, etc).

The problem is the following. One supposes that little time after the Big Bang, the Univers, composed of Proton S, of Neutron S, electron S, Photon S and others particle S is about homogeneous, i.e. uniform in any point, because its temperature is too high to allow the particles which form the Atome S to gather. Today, when one observes the distribution of the objects in the Universe, one notices that they are not distributed in a uniform way; it is thus supposed that was needed that matter concentrates a little more in certain places, formant of the fluctuations which one calls “paramount fluctuations”.

And to locate these fluctuations of density on the cosmological diffuse Fund, it is enough to locate the differences in temperatures coming from this fossil radiation. The recorded average temperature is of approximately 2,7 K. zones slightly hotter would indicate a density of matter a little stronger. It was enough that these fluctuations are about the thousandths of degree to explain the formation of the galaxies starting from these matter regroupings.

Unfortunately for this theory, the satellite COBE launched in 1992 revealed only temperature variations of about the hundred thousandths of degree, which is well too weak so that the great structures of the Universe can be formed starting from these paramount fluctuations in only 13,7 billion years.

It is there that one utilizes the black matter to save the theory. The protons, neutrons and electrons could not gather to form the atoms because of the pressure of the photons. On the other hand, the black matter does not interact with the photons and would thus not have undergone this pressure, which would have enabled him to create fluctuations of density well (invisible) before the ordinary matter. These fluctuations could thus have attracted, by gravitation, the ordinary matter during decoupling matter-radiation of the paramount Nucléosynthèse (decoupling which released the photons and made the Universe transparent).

On this assumption, these are thus the fluctuations of density of the black matter which would be at the origin of the formation of the galaxies and the galaxy clusters, distributed in a nonuniform way in the Universe. Unfortunately remain to explain why the matter sinks would have adopted a nonhomogeneous distribution, contrary to the ordinary matter…

Nature of this matter sinks

Hot black matter and cold black matter

Two great theories clash as for the nature of this black matter: hot black matter and cold black matter. Those rest on the mass of the particles composing the black matter and consequently, at their speed. In the case of black matter known as “hot”, the particles have speeds close to that of the light, while those composing black a matter known as “cold” would be more massive and thus slower.

The rate of travel of these particles intervenes in the order of formation of the great structures of the Universe. If the Universe were dominated by hot black matter, very the high speed of the particles constituting it would initially prevent the formation of a structure smaller than the supercluster of galaxies which then splits up in galaxy cluster, then in galaxies, etc It is the scenario known as “top to the bottom”, since the largest structures are formed initially, for then dividing. The best candidate to constitute the hot black matter is the Neutrino. On the other hand, if the cold black matter dominated the Universe, the particles will traverse a smaller distance and thus will erase the fluctuations of density on extents smaller than in the case of hot black matter. The ordinary matter then will gather to initially form galaxies (starting from gas clouds), which themselves will gather in cluster, then supercluster. It is the scenario known as “of bottom upwards”. The candidates with the cold black matter are WIMP and MACHO.

These two theories were defended by Yakov Borisovitch Zeldovitch for the hot black matter, and James Peebles for the cold black matter. Currently, it is the cold black matter model which seems to carry it. Indeed, the galaxies are in dynamic balance, which lets think that they were created before the clusters - of which all do not seem yet stable - with which it takes more time to reach this balance. However, the theories introduce a little hot black matter today. This one is necessary to explain the formation of the clusters; cold matter only not being able to allow it in if little time.

Research on the side of the ordinary matter

The scientists turned initially to the matter ordinary (or baryon) to carry out their research and reviewed all the types of objects which could contribute to this gravitational field, the such clouds of gas, the stars dead or the black holes.

Gas clouds?

In the Years 1990, precise cartographies of the sources of emission of X-rays in the universe - obtained thanks to the satellite Rose - highlighted the presence of gigantic clouds of Gaz ionized within the galaxy clusters; clouds of several million degrees not emitting a visible light. Moreover, these clouds seemed to contain ten times more matter (at least, luminous) that galaxies of these clusters, perhaps was this finally the required missing matter? Unfortunately not. On the contrary even, these clouds are the proof of the presence of black matter around the galaxies. Indeed, to reach of such Temperature S, the particle S constituting the cloud must be accelerated at very high speeds (approximately 300 km/s), and this acceleration comes from the force of Gravitation. However the quantity of gas is insufficient to generate such a field of gravity. In the same way, the star S cannot alone prevent the gas cloud from escaping. The gravitational influence of the dark matter is here also necessary to explain the containment of these clouds near the galaxies. Moreover, the shape of these clouds can help the astronomers to study the distribution of the black matter in the neighborhoods.

Conclusions of programs MACHO, EROS and REUNION

It is estimated that the three quarters of the baryon matter of the Universe consist of Hydrogène. The atomic hydrogen clouds in which the stars are present are insufficient to explain this strong gravitational interaction which makes turn stars in periphery of galaxy more quickly than envisaged, and multiplies only at best by two the mass of the galaxy; it misses at least five more times the mass of the galaxy. The astronomers were then interested in the more compact objects and not emitting a light (or to be detected too little), the such brown dwarf (stars which do not reach the stage of star because not massive enough) or the dwarf white (dead stars made up of heavy elements). These objects are called “MACHO”, for Massive Compact Halation Objects (massive compact objects of the halation).

The theory of dwarf white was consolidated by work of Oppenheimer (2001), but was disputed thereafter (in particular Bergeron, 2001,2003,2005). This assumption remains outstanding for lack of trigonometrical measurement of Parallaxe and thus of distance on the dwarf white ones of their study. According to work of Oppenheimer, the limit lower of the contribution of the mass of dwarf white of the halation than the missing mass of the galaxy is of 3%, to compare with the higher limit provided by EROS which is of 35%. There exist nevertheless problems with this assumption: the missing mass of the galaxies is rather important all the same and ten times would thus be needed more dead stars than of alive stars. However while observing in remote space, one should see populated galaxies of these still alive stars (their light coming us from one time much older), therefore galaxies much more luminous; but it is not the case. Moreover, the proportion of Supernova E should also be more important in these remote galaxies. The supernovas releasing of the heavy elements, the proportion of these elements should as be ten times more important as that currently detected.

For the dwarf brown ones, the problem was to detect them. In 1986, the astronomer Bohdan Paczyński explains how to detect these massive objects but not emitting a light, using the effect of gravitational Lentille. A massive object passing in front of a star would deviate the luminous rays emitted by this star. Concretely, the effect of lens will create one second image of this star and will superimpose it on that of star; the luminosity becomes at this time (when the object passes right in front of star) more important. The problem was however the scarcity of the phenomenon: the number of chances to observe at one moment an effect of gravitational lens due to dwarf brown (by supposing that the black matter in is primarily made up) is of one on a million.

Profiting from cameras CCC to large field (recovered military programs), the astronomers could at the beginning of the Années 1990 study a great number of stars at the same time, increasing the chances to observe effects of gravitational lens. Two programs of observation were born: EROS (Experiment for the Search for Dark Objects) in 1990 and MACHO in 1992; the first concentrating on research of less massive and smaller objects. These programs stopped in 2003 and 2001, with a not very convincing assessment. Few effects of gravitational lens were observed and the scientists had to conclude that less than 10% of the halation of our galaxy could be formed of dwarf brown, which is insufficient once again.

The purpose of the Programme REUNION ( Andromeda Galaxy Amplified Pixel Experiment ) began towards 1994 and was to detect effects of gravitational lens by observing this time either the Grand Cloud of Magellan like MACHO and EROS, but the galaxy of Andromède. The distance being larger, the probability that the light is deviated by a compact object is too. Here also, few effects of lens are observed.

Black holes?

Much more massive than the MACHO or stars, the black holes could have been good candidates. Some of them could reach a mass of 10.000 solar masses (in particular the black holes supermassifs, in the center of the galaxies). However, it would be necessary, in a galaxy, nearly a million black holes of such a mass to fill this lack of matter; a too significant number within sight of the effects on stars near a black hole. Indeed, the black holes cross per moment the galactic disc and disturb the movement of stars. With such a number of black holes, the movements of these stars would be strongly amplified, which would make the disc galactic good thicker than what is currently observed.

Remain the stellar black holes (about some solar masses), not easily detectable, and the black holes of a few tens or solar hundreds of masses, whose nature of their formation remains still mysterious. In all the cases, the track of the black holes as being the famous black matter was forsaken, and the astronomers are leaning on another matter shape, nonbaryon.

Nonbaryon matter?

The theory of the Big Bang makes it possible to calculate the number of baryons of all the Universe, i.e. the number of atoms of Hélium 4 and hydrogen, formed at the time of the paramount Nucléosynthèse. The astronomers arrived from there ata baryon matter rate from approximately 4% of the critical density. However, to explain the geometry punt of the Universe, the total matter of the Universe must account for 30% of the critical density (the 70% remainders being black energy). It thus misses 26% of the critical density in the form of nonbaryon matter; i.e. constituted by other particles that the Baryon S.

The neutrino

The Neutrino is a particle postulated for the first time in 1930 by Wolfgang Pauli, before even the discovery of the Neutron (one year later), and which was detected in 1956 by Frederick Reines and Clyde Cowan. This particle - insensitive with the electromagnetic forces and the strong nuclear force - is emitted at the time of a beta decay, accompanied by an electron. The neutrino thus interacts very little with the other particles, which makes a good candidate for the black matter of it.

The mass of the neutrino was considered very weak, even null. With the problem of the missing mass of the Universe, the physicists wondered perhaps whether the neutrino did not have a mass, low, but nonzero. More especially as the neutrino is the most abundant particle in the universe, after the Photon. However, the experiments Super-Kamiokande and SNO ( Sudbury Neutrino Observatory ) revealed a mass too much low so that this particle can constitute the essence of the black matter. The neutrinos can represent, at best, 18% of the total mass of the Universe.

WIMP

WIMP ( Weakly interactive massive particles ) form a class of heavy particles, interacting slightly with the matter, and constitute excellent candidates with the not-baryon dark matter. Among those one finds, the neutralino postulated by the extensions supersymmetric of the standard model of the physics of the particles. The idea of the Supersymétrie is to associate with each Boson a Fermion and vice versa. Each particle is thus seen allotting a super-partner, having identical properties (mass, load), but with a Spin different from 1/2. Thus, the number of particles is doubled. For example, the Photon is found accompanied by a Photino, the Graviton of a Gravitino, the electron of a Sélectron, etc Following impossibility of detecting a boson of 511 keV (partner of the electron), the physicists had to re-examine the idea of a exact Symétrie. Symmetry known as is broken and the superpartenaires are found with a very important mass. One of these superparticules called LSP ( Lightest Supersymmetric Particle ) is lightest of all. In the majority of the theories supersymmetric, known as without violation of the R-parity, the LSP is a stable particle because it cannot disintegrate in a more light element. It is of more neutral of color and charge electric and thus only sensitive to the weak Interaction; it for this reason constitutes an excellent candidate with the not-baryon dark matter.

This supersymmetric particle lightest is in general (according to the models), the Neutralino, a combination from these three superparticules: the photino (partner of the photon), of the zino (partner of the Z₀ boson) or of the higgsino (partner of boson of Higgs). Recent measurements with the CERN indicate that its mass is higher than 32 GeV/c ². The neutralino is, in theory, stable thus very abundant at the point to represent the essence of the matter of the Universe. It is for this reason the subject of many research. The detection of neutralinos can be direct, by interaction in the detector, or indirect, via the research of the products of annihilation.

The matter detection sinks supersymmetric is a field of extremely dynamic physics, in particular from the point of view of the techniques. The localization of the detectors is with the image of this diversity: in terrestrial orbit (AMS, PAMELA), under the ice of the south pole (AMANDA, IceCube), in seamen circle (ANTARES), or in the underground laboratories (EDELWEISS, MACHe3).

An useless assumption?

For more and more of astronomers, this black matter does not exist: rather than to seek to explain the anomalies by an even inobservable unobserved matter, it would be according to them more judicious to re-examine the physical laws who constitute the standard model, and who are in any event called in question by other problems even more fundamental. It would be then possible to solve several problems without putting forth at the same time new assumptions.

Theory of the cords and axions

Certain physicists turn for example to the Théorie of the cords. The theory of the cords adds six new dimensions to the four usual ones (three dimensions of space and time) and would place the black matter in these new dimensions which are inaccessible for us; this is why it would not be detected. The forces electromagnetic and nuclear strong and weak would be confined in our four dimensions and could not leave them. On the other hand, the gravitation could disperse in other dimensions, and thus drop in intensity compared to the other forces.

Another theoretical particle, the Axion, which would be extra-light (1 µeV), stable and which would also interact very little with the matter - a practically undetectable particle thus - would make another good candidate with the black matter. This particle would solve inter alia, the problems arising from the Antimatière (why the matter carried it on the antimatter). Various programs were launched since 1996 to try to detect axions, of which CAST ( CERN Solar Axion Telescopes ).

Theory MOND

And others still…

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