Post by Andrei Tchentchik on Aug 25, 2020 15:38:26 GMT 2
(.#499).- Antimatter in Cosmology.
Antimatter in Cosmology.
Richard Taillet
Teacher, Physical Researcher.
Published 07/01/2005 - Modified 10/28/2015.
Archives
This chapter is a little more advanced than the previous ones and is intended for the reader wishing to go a little further ...
We said above that the reactions that create antimatter create matter at the same time. How then can we understand that we live in a Universe apparently made up almost exclusively of matter? To provide some answers to this question, you have to take a look around the Big Bang. Small detour so ...
A - The Big-Bang model in two words
A large number of astronomical observations indicate that we live in an expanding Universe. This means that in the past it was denser than today. It also means it was warmer. Extrapolating this remark in the past, one arrives at the idea that the matter must have been, in the past, in an extremely dense and hot state.
It is the Big-Bang model. To tell the story of the Universe in chronological order, we take as a starting point a state so hot and dense that the quarks are not linked together in nuclei, but form a sea quarks-gluons. The Universe expands, cools, and several things happen ... First, the quarks condense to form the first nucleons, protons and neutrons. Bad luck, the neutron is unstable, and disintegrates into a proton after a few minutes, or even less in very dense environments ... Fortunately, the story is running fast at that time, and before all the neutrons have not disintegrated, the temperature becomes low enough for them to recombine with protons to form more complex nuclei, deuterium, helium, lithium, beryllium, boron, it is the primordial nucleosynthesis. Finally, these nuclei bind to the electrons everywhere present to form atoms. At this precise moment, the Universe becomes transparent to the radiation it contains, it can propagate freely and we are still observing it now in the form of cosmological background radiation.
B- The matter-antimatter asymmetry
One of the problems raised by this scenario is that of matter-antimatter asymmetry. The interactions described by the standard model of particle physics retain the baryonic number, that is to say that as many baryons are created as there are antibiotics (the baryons are the "heavy" particles, like neutrons or protons) ... If at the beginning there are as many baryons as there are antibiotics, this situation will continue all the time. The most natural situation would therefore be as follows: an equal quantity of baryons and antibaryons is present, most of these particles annihilate quite early, except for a few particles here and there which have escaped the battle. We would live in a place where chance has meant that it was baryons that survived, there would be other places where antibaryons would have survived. However, quantitatively, this scenario poses a problem, because on the one hand we can show that the quantity of surviving baryons would be much lower than that which we observe, and on the other hand if regions of the Universe which contain mainly matter adjoins regions in which antimatter dominates, we should observe significant annihilations at their borders. We can still try to see the antibaryons that would have survived if this hypothesis is correct, and we mentioned above the possibility of detecting helium antinuclei from hypothetical anti-stars.
Andrei Sakharov (1921-1989)
If now the Universe actually contains more matter than antimatter, two things one: either from the start the Universe contains more baryons than antibiotics, or not, but there has been net creation of baryons at some point in the history of the Universe. The first possibility does not satisfy the cosmologists, who want to understand the Universe by making the minimum of a priori hypotheses on the "initial state". We will discuss a little more before the second possibility. The matter-antimatter asymmetry would be generated during the Big-Bang, during the creation of the baryons themselves (from the first to the second box in the following drawing, in which the black balls represent the precursors of baryons, whatever they are , the blue balls represent antimatter and the orange balls the matter). After mutual annihilations, there is almost only excess material (from the second to the third box).
Sakharov showed in 1967 that three conditions had to be met at some point in the history of the Universe to explain the appearance of a matter / antimatter asymmetry during the first stage.
- Reactions violating the conservation of the baryonic number, that is to say favoring the creation of baryons over that of antibaryons: if not of course starting from a symmetrical situation one arrives at an equally symmetrical situation.
Reactions retaining the baryonic number cannot not create more matter than antimatter.
Reactions that violate the conservation of the baryonic number can.
- Violation of symmetries C and CP. This is the technical way of saying that among the previous reactions which do not keep the baryonic number, there should not be as many which create baryons as there are reactions which create antibaryons. In other words, if the CP symmetry is respected, the reactions which create baryons (and therefore violent B) would be compensated by similar reactions which create antibaryons (and therefore also violent B) and overall we would not be more advanced! By repeating the drawings introduced above, the following two reactions must not have the same properties.
- Break in thermodynamic equilibrium. Otherwise the reactions which create baryons are exactly compensated by the reactions which destroy them (reversibility of the reaction rates at thermodynamic equilibrium). The following two reactions should not be done at the same speed.
What about these three conditions in the standard model of particle physics?
- In the standard model, the violation of B is present, at an extremely low level at current energies or even those of accelerators, but at an appreciable level in the primordial Universe.
- The violation of C and CP mentioned here is observed in the reactions of particle physics, but at a level too low to play a significant role in this context.
- The third condition seems easily fulfilled, because the expansion of the Universe tends to upset the thermodynamic equilibrium. In fact it turns out that the expansion is too slow to break the thermodynamic equilibrium effectively enough to satisfy the third condition. On the other hand, the Universe undergoes during its history a phase transition during which the thermodynamic equilibrium is violently broken.
Detailed studies show that the standard model does not explain the matter / antimatter asymmetry in the Universe. Cosmologists and theoretical physicists are leaning towards extensions of this standard model, such as the Great Unification theories or supersymmetry.
It turns out that anyway, for other reasons, the particle physicists want to go further than the standard model. They would like to unify the fundamental interactions, that is to say describe these interactions as several facets of the same interaction. This current was started with the unification of electrical and magnetic phenomena to give electromagnetism. In this context, an electric field for one observer can behave like a magnetic field for another. Later, the weak interaction was unified with electromagnetism to give the electro-weak interaction. The next step is to add the strong interaction. This is not finished, but there are very serious tracks, to which we give the sweet name of GUT (for Grand Unification Theories or in French Théories de Grande Unification). These theories predict larger baryonic number B, C and CP violations than in the standard model. The problem for the moment is that one cannot compare these theories with observations in a very precise way because one does not really know the detail of the great unified theory which it is advisable to consider. A crucial test of these theories is that they predict the instability of the proton. Several experiments try to surprise a proton in the process of disintegrating, and if they succeed we can say more.
F I N .
Antimatter in Cosmology.
Richard Taillet
Teacher, Physical Researcher.
Published 07/01/2005 - Modified 10/28/2015.
Archives
This chapter is a little more advanced than the previous ones and is intended for the reader wishing to go a little further ...
We said above that the reactions that create antimatter create matter at the same time. How then can we understand that we live in a Universe apparently made up almost exclusively of matter? To provide some answers to this question, you have to take a look around the Big Bang. Small detour so ...
A - The Big-Bang model in two words
A large number of astronomical observations indicate that we live in an expanding Universe. This means that in the past it was denser than today. It also means it was warmer. Extrapolating this remark in the past, one arrives at the idea that the matter must have been, in the past, in an extremely dense and hot state.
It is the Big-Bang model. To tell the story of the Universe in chronological order, we take as a starting point a state so hot and dense that the quarks are not linked together in nuclei, but form a sea quarks-gluons. The Universe expands, cools, and several things happen ... First, the quarks condense to form the first nucleons, protons and neutrons. Bad luck, the neutron is unstable, and disintegrates into a proton after a few minutes, or even less in very dense environments ... Fortunately, the story is running fast at that time, and before all the neutrons have not disintegrated, the temperature becomes low enough for them to recombine with protons to form more complex nuclei, deuterium, helium, lithium, beryllium, boron, it is the primordial nucleosynthesis. Finally, these nuclei bind to the electrons everywhere present to form atoms. At this precise moment, the Universe becomes transparent to the radiation it contains, it can propagate freely and we are still observing it now in the form of cosmological background radiation.
B- The matter-antimatter asymmetry
One of the problems raised by this scenario is that of matter-antimatter asymmetry. The interactions described by the standard model of particle physics retain the baryonic number, that is to say that as many baryons are created as there are antibiotics (the baryons are the "heavy" particles, like neutrons or protons) ... If at the beginning there are as many baryons as there are antibiotics, this situation will continue all the time. The most natural situation would therefore be as follows: an equal quantity of baryons and antibaryons is present, most of these particles annihilate quite early, except for a few particles here and there which have escaped the battle. We would live in a place where chance has meant that it was baryons that survived, there would be other places where antibaryons would have survived. However, quantitatively, this scenario poses a problem, because on the one hand we can show that the quantity of surviving baryons would be much lower than that which we observe, and on the other hand if regions of the Universe which contain mainly matter adjoins regions in which antimatter dominates, we should observe significant annihilations at their borders. We can still try to see the antibaryons that would have survived if this hypothesis is correct, and we mentioned above the possibility of detecting helium antinuclei from hypothetical anti-stars.
Andrei Sakharov (1921-1989)
If now the Universe actually contains more matter than antimatter, two things one: either from the start the Universe contains more baryons than antibiotics, or not, but there has been net creation of baryons at some point in the history of the Universe. The first possibility does not satisfy the cosmologists, who want to understand the Universe by making the minimum of a priori hypotheses on the "initial state". We will discuss a little more before the second possibility. The matter-antimatter asymmetry would be generated during the Big-Bang, during the creation of the baryons themselves (from the first to the second box in the following drawing, in which the black balls represent the precursors of baryons, whatever they are , the blue balls represent antimatter and the orange balls the matter). After mutual annihilations, there is almost only excess material (from the second to the third box).
Sakharov showed in 1967 that three conditions had to be met at some point in the history of the Universe to explain the appearance of a matter / antimatter asymmetry during the first stage.
- Reactions violating the conservation of the baryonic number, that is to say favoring the creation of baryons over that of antibaryons: if not of course starting from a symmetrical situation one arrives at an equally symmetrical situation.
Reactions retaining the baryonic number cannot not create more matter than antimatter.
Reactions that violate the conservation of the baryonic number can.
- Violation of symmetries C and CP. This is the technical way of saying that among the previous reactions which do not keep the baryonic number, there should not be as many which create baryons as there are reactions which create antibaryons. In other words, if the CP symmetry is respected, the reactions which create baryons (and therefore violent B) would be compensated by similar reactions which create antibaryons (and therefore also violent B) and overall we would not be more advanced! By repeating the drawings introduced above, the following two reactions must not have the same properties.
- Break in thermodynamic equilibrium. Otherwise the reactions which create baryons are exactly compensated by the reactions which destroy them (reversibility of the reaction rates at thermodynamic equilibrium). The following two reactions should not be done at the same speed.
What about these three conditions in the standard model of particle physics?
- In the standard model, the violation of B is present, at an extremely low level at current energies or even those of accelerators, but at an appreciable level in the primordial Universe.
- The violation of C and CP mentioned here is observed in the reactions of particle physics, but at a level too low to play a significant role in this context.
- The third condition seems easily fulfilled, because the expansion of the Universe tends to upset the thermodynamic equilibrium. In fact it turns out that the expansion is too slow to break the thermodynamic equilibrium effectively enough to satisfy the third condition. On the other hand, the Universe undergoes during its history a phase transition during which the thermodynamic equilibrium is violently broken.
Detailed studies show that the standard model does not explain the matter / antimatter asymmetry in the Universe. Cosmologists and theoretical physicists are leaning towards extensions of this standard model, such as the Great Unification theories or supersymmetry.
It turns out that anyway, for other reasons, the particle physicists want to go further than the standard model. They would like to unify the fundamental interactions, that is to say describe these interactions as several facets of the same interaction. This current was started with the unification of electrical and magnetic phenomena to give electromagnetism. In this context, an electric field for one observer can behave like a magnetic field for another. Later, the weak interaction was unified with electromagnetism to give the electro-weak interaction. The next step is to add the strong interaction. This is not finished, but there are very serious tracks, to which we give the sweet name of GUT (for Grand Unification Theories or in French Théories de Grande Unification). These theories predict larger baryonic number B, C and CP violations than in the standard model. The problem for the moment is that one cannot compare these theories with observations in a very precise way because one does not really know the detail of the great unified theory which it is advisable to consider. A crucial test of these theories is that they predict the instability of the proton. Several experiments try to surprise a proton in the process of disintegrating, and if they succeed we can say more.
F I N .
