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[EN VIDÉO] Interview: what particles make up dark matter? According to calculations and observations, there would exist in space a large quantity of invisible matter. This mysterious mass, baptized dark matter, is still today an enigma with which many researchers are rubbing shoulders. As part of its Questions d’experts video series on physics and astrophysics, publisher De Boeck interviewed Richard Taillet, researcher at LAPTH, to tell us more about this dark matter.
All the measurements concerning the observable Universe tell us that it is very mainly composed of matter and not ofantimatter. It’s a riddle of cosmology because the theory of big Bang based on the physical known implies that as much matter as antimatter should have been produced during this very hot and very dense period of the known cosmos, when not even nuclei ofatoms did not yet exist.
One could imagine that antimatter and matter were distributed like water and oil in a emulsion but we would then potentially have small-scale contact zones between these regions which would lead to annihilations between particles of matter and antimatter, generating a flow of gamma rays noticeable. Nothing like this has been observed in the known universe, which therefore does not seem to contain galaxy or star made of antimatter.
However, many anti-electrons and anti-protons were observed in the flow of cosmic rays outside the Earth, as shown in particular by the detector AMS (Alpha Magnetic Spectrometer) aboard theISS. It is also known that in connection with the explosion of supernovae, particle acceleration processes occur in the Milky Way causing matter particles to collide, resulting in antimatter-producing reactions. We also know that the pulsarthose neutron stars very dense and in rotation, can produce particles of antimatter, especially positrons, the antiparticles from electrons. The ejecta of supernovae also contain radioactive elements which decay into positrons.
A short presentation of pulsars. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © NASA Goddard
Pulsar or dark matter positrons?
In the end, therefore, there must be a tiny bit of antimatter, produced by sources astrophysicswhich circulates through the magnetic fields turbulent parts of our Galaxy like the particles of pollen performing a movement Brownian stochastic. But the amount of positron discovered by AMS and other orbiting detectors is higher than expected.
Some see it as indirect proof of the existence of dark matter particles postulated by the standard cosmological model.. Indeed, some theories proposed concerning these particles not only allow them to annihilate by colliding, which produces antiparticles of normal matter, but imply that these annihilations are more frequent in the heart of galaxies, where black matter would focus on these theories.
But a lot ofastrophysicists think that anomalous cosmic ray flux measurements concerning antimatter can also very well be explained by the relatively close presence of Solar systemat the scale of the Milky Wayof a population of as yet undetected pulsars.
The discovery announced today by a group of researchers and concerning the PSR pulsar J2030 + 4415 (also called J2030) which is located at around 1600 light years from Sun towards the Constellation du Cygne is therefore not insignificant. It is exposed in an article published in The Astrophysical Journal a version of which is freely available on arXiv.
A presentation of the discovery with the J2030 pulsar. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © Chandra X-ray Observatory
Astrophysicists have taken a closer look at the emissions in the field of X-rays of the pulsar and discovered that the beam of matter and antimatter particles, already highlighted in 2020, was much larger and longer than expected according to observations made with the famous satellite Chandra from NASA. Data collected in February 2021 shows that the particle jet is three times longer than previously thought and that it exceeds 60 trillion kilometers, that is to say almost 6 light years! It’s a record and it’s impressive when you realize that the diameter of a neutron star is a few tens of kilometers at most.
The jet of particles must contain a significant amount of antimatter in the form of positrons and the fact that it extends so far from its source implies that pulsars can inject far more positrons into the interstellar medium than previously thought. . This is therefore one more reason to think that the flow of anti-electrons detected with AMS is in fact not anomalous and that it does not testify to the existence of dark matter particles.
Cascading antimatter production
Let us give some explanations on the origin of these jets of particles emitted, one thinks it, with the magnetic poles of the pulsars.
We know, in fact, as predicted by the theory, that thecollapse gravitational pull of a rotating star resulting in the formation of a rapidly rotating neutron star, which sometimes signals itself as a pulsar by its pulses radio collimated, considerably amplifies the initial magnetic field of the star.
On the surface of the rapidly rotating neutron star in its own magnetic field, the theory of relativity implies that this field also gives a electric field powerful for an observer at rest on the surface solid of the star. This field strips electrons from the atoms of iron of his crust and accelerates them to the point that they emit photons gamma. These photons are energetic enough to produce by various processes ofquantum electrodynamics electron-positron pairs which, in turn, are accelerated and radiate gamma photons which can again produce other pairs of particles and antiparticles.
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