Post by Andrei Tchentchik on Aug 25, 2020 15:39:09 GMT 2
(.#500).- Dark matter, antiprotons and positrons.
Dark matter, antiprotons and positrons.
Richard Taillet,
Teacher Physics Researcher
Published on 07/01/2005 - Modified on 28/10/2015
Archives
We will now address some much more speculative points, which concern important aspects of modern cosmology, and in which antimatter also has its role to play.
A - Dark matter
Numerous indications seem to indicate that the Universe contains a large quantity of matter which one does not detect in a direct way. We infer its presence by the gravitational effect it has on its surroundings, but we have not yet been able to find a visible counterpart. Current models strongly favor the hypothesis that this dark matter is mainly formed by new particles (we say that they are non-baryonic), described in extensions of the standard model of particle physics, such as for example supersymmetry or theories of great unification that we mentioned.
Representation of the distribution of dark matter (in blue) in a cluster of galaxies (the points)
If this assumption is correct, it is possible that these particles are stable when they are isolated, but can annihilate each other when they meet. It could result from these annihilation the creation of radiation, but also of protons, antiprotons, electrons, positrons, in short a bit of everything!
Distortion effect of the appearance of galaxies by the effect of gravitational lens due to dark matter.
B - Antiprotons and positrons coming from dark matter?
There are many different theoretical models describing dark matter (precisely because we do not know what it is, theorists can give free rein to their vivid imagination ...), and each of these models predicts different values the amount of antiprotons and positrons that should result from its annihilations.
However, we observe that our local environment contains few antiprotons, and that those that we observe have properties (essentially their energy distribution) compatible with a classical origin (nuclear reactions, see above). This therefore makes it possible to eliminate the models which predict an excessive formation of antiprotons. The ideal would be to see an excess of antiprotons compared to those of classical origin, and that this excess is well explained by a single dark matter model ... We are very far from this situation.
On the other hand, this hypothesis could explain the excess of positrons observed by HEAT. If this assumption is correct, dark matter should have rather specific properties, so as not to give too many antiprotons when it annihilates, but many positrons.
C - A dark matter signal in the center of the Galaxy?
As we saw above, the INTEGRAL satellite detected a powerful electron-positron annihilation signal (1043 annihilations per second, or 10 billion tonnes of matter). We do not understand where the positrons come from which annihilate in this way with the electrons, and we could imagine that they come from the dark matter, which would be present with a greater concentration in this place. Attention, this is still very speculative ... This hypothesis poses a problem: the positrons resulting from the disintegration of a massive particle are created with an important energy, but they annihilate at rest (in the positronium, see above) . They must therefore be slowed down, which should be accompanied by the emission of a significant amount of braking radiation, which is not observed. It was necessary to "invent" a particular type of dark matter which circumvents this problem, by allowing the positrons to be created directly at low energy. Physicists do not like having to review the fundamental properties of models as soon as an observation becomes troublesome!
D - Antiprotons coming from the evaporation of mini black holes?
Several cosmological scenarios predict that during the complex history of the Universe, black mini-holes, with masses of a fraction of a gram, could form. For these small black holes, Hawking evaporation is a phenomenon which plays an important role in their evolution, and we think that it could be important enough to lead to the emission of very energetic radiation, whether under the form of photons or particles. If this is the case and if these mini black holes are actually present in our Universe, we could possibly deduce their presence from an observation of an excess of antimatter. This is really very speculative!
The moment to conclude
Richard Taillet,
Teacher Physics Researcher
Published on 07/01/2005 - Modified on 28/10/2015
Archives
There is nothing mysterious about antimatter, we observe it, we create it, we store it, we use it ... It provides astrophysicists with another way of observing the Universe around us. It also makes it possible to highlight very specific processes, which we do not (yet?) See with other means.
This original vision of the world around us raises questions that arise from the very origin of our Universe.
The theoretical invention then the experimental discovery of antimatter, followed finally by its practical use as a probe of the Universe constitutes an almost ideal model of the science in progress, which we hope will be reproduced for the other windows which begin to open up to our Universe: gravitational waves and neutrinos.
F I N .
Dark matter, antiprotons and positrons.
Richard Taillet,
Teacher Physics Researcher
Published on 07/01/2005 - Modified on 28/10/2015
Archives
We will now address some much more speculative points, which concern important aspects of modern cosmology, and in which antimatter also has its role to play.
A - Dark matter
Numerous indications seem to indicate that the Universe contains a large quantity of matter which one does not detect in a direct way. We infer its presence by the gravitational effect it has on its surroundings, but we have not yet been able to find a visible counterpart. Current models strongly favor the hypothesis that this dark matter is mainly formed by new particles (we say that they are non-baryonic), described in extensions of the standard model of particle physics, such as for example supersymmetry or theories of great unification that we mentioned.
Representation of the distribution of dark matter (in blue) in a cluster of galaxies (the points)
If this assumption is correct, it is possible that these particles are stable when they are isolated, but can annihilate each other when they meet. It could result from these annihilation the creation of radiation, but also of protons, antiprotons, electrons, positrons, in short a bit of everything!
Distortion effect of the appearance of galaxies by the effect of gravitational lens due to dark matter.
B - Antiprotons and positrons coming from dark matter?
There are many different theoretical models describing dark matter (precisely because we do not know what it is, theorists can give free rein to their vivid imagination ...), and each of these models predicts different values the amount of antiprotons and positrons that should result from its annihilations.
However, we observe that our local environment contains few antiprotons, and that those that we observe have properties (essentially their energy distribution) compatible with a classical origin (nuclear reactions, see above). This therefore makes it possible to eliminate the models which predict an excessive formation of antiprotons. The ideal would be to see an excess of antiprotons compared to those of classical origin, and that this excess is well explained by a single dark matter model ... We are very far from this situation.
On the other hand, this hypothesis could explain the excess of positrons observed by HEAT. If this assumption is correct, dark matter should have rather specific properties, so as not to give too many antiprotons when it annihilates, but many positrons.
C - A dark matter signal in the center of the Galaxy?
As we saw above, the INTEGRAL satellite detected a powerful electron-positron annihilation signal (1043 annihilations per second, or 10 billion tonnes of matter). We do not understand where the positrons come from which annihilate in this way with the electrons, and we could imagine that they come from the dark matter, which would be present with a greater concentration in this place. Attention, this is still very speculative ... This hypothesis poses a problem: the positrons resulting from the disintegration of a massive particle are created with an important energy, but they annihilate at rest (in the positronium, see above) . They must therefore be slowed down, which should be accompanied by the emission of a significant amount of braking radiation, which is not observed. It was necessary to "invent" a particular type of dark matter which circumvents this problem, by allowing the positrons to be created directly at low energy. Physicists do not like having to review the fundamental properties of models as soon as an observation becomes troublesome!
D - Antiprotons coming from the evaporation of mini black holes?
Several cosmological scenarios predict that during the complex history of the Universe, black mini-holes, with masses of a fraction of a gram, could form. For these small black holes, Hawking evaporation is a phenomenon which plays an important role in their evolution, and we think that it could be important enough to lead to the emission of very energetic radiation, whether under the form of photons or particles. If this is the case and if these mini black holes are actually present in our Universe, we could possibly deduce their presence from an observation of an excess of antimatter. This is really very speculative!
The moment to conclude
Richard Taillet,
Teacher Physics Researcher
Published on 07/01/2005 - Modified on 28/10/2015
Archives
There is nothing mysterious about antimatter, we observe it, we create it, we store it, we use it ... It provides astrophysicists with another way of observing the Universe around us. It also makes it possible to highlight very specific processes, which we do not (yet?) See with other means.
This original vision of the world around us raises questions that arise from the very origin of our Universe.
The theoretical invention then the experimental discovery of antimatter, followed finally by its practical use as a probe of the Universe constitutes an almost ideal model of the science in progress, which we hope will be reproduced for the other windows which begin to open up to our Universe: gravitational waves and neutrinos.
F I N .
