La carte des fluctuations du rayonnement fossile déduite des observations de Planck. © ESA

The oldest dark matter clusters observed in the Universe are not as expected!


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[EN VIDÉO] The secrets of the Planck mission
The Planck satellite is a formidable time machine, capable of revealing several secrets to us about the origin, structure and composition of the universe. Cosmologists and astrophysicists have used it to map on the celestial vault, with unequaled precision, the temperature and polarization fluctuations of the oldest light in the world, that of fossil radiation. This video produced by the HFI-Planck consortium, the communication agency Canopée and with the help of Jean Mouette, from the IAP (Institute of Astrophysics of Paris), explains what this mission consists of.

It took twenty years to demonstrate the existence of neutrinos and about forty for that of the Brout-Englert-Higgs boson. How much longer will it take to demonstrate the existence of particles of black matter or on the contrary to demonstrate their non-existence?

Recall that the neutrino had been postulated by Wolfgang Pauli to maintain the law of conservation of energy in certain nuclear decays which seemed to violate it. But to do that had to admit that the energy that seemed to disappear into nothingness was in fact carried away by a massless particle, without electrical charge and interacting so little with low-energy matter that it could cross 300 Earths without breaking. Stop. We now know that there are in fact three types of neutrinos and that they have masses, although very small.

Dark matter particles are just as ghostly, but paradoxically we need them to account for the existence of galaxies and structures that bring them together in the form of clusters containing from a few hundred to a few thousand galaxies very roughly. It is under the effect of their field of gravitation that ordinary matter collapsed faster than it should have on its own. We know for various reasons that these particles of dark matter do not resemble those which we know on Earth and which are in particular produced in the collisions of protons at the LHC, although they are being tracked there.

However, even though dark matter is one of the pillars of Standard cosmological modelit could not exist and the observations which it reports could perhaps also be explained by modifying the equations celestial mechanics Newton. One wonders currently if the first discoveries of galaxies which seem very primitive by the telescope James-Webb, because observed as they were more than 13 billion years ago, are not precisely a beginning of a refutation of the dark matter theory and a confirmation of the Mond theory.

For 13.8 billion years, the Universe has continued to evolve. Contrary to what our eyes tell us when we contemplate the sky, what composes it is far from being static. Physicists have observations at different ages of the Universe and carry out simulations in which they replay its formation and its evolution. It would seem that dark matter has played a big role since the beginning of the Universe until the formation of the large structures observed today. © CEA Research

A new surprise in this regard may be nearing the end of its nose in an article published in Physical examination letters by an international team of researchers led by members of Nagoya University in Japan. It deals with certain results obtained with the Japanese Subaru telescope, in Hawaii, within the framework of the research campaign of the Subaru Hyper Suprime-Cam Survey (HSC), in combination with other observations obtained by the Planck satellite of the’ESA in the form of his famous map of cosmic radiationthe oldest light of theUniverse observable, emitted about 380,000 after the big Bang in a few thousand years.

Dark matter that distorts images of galaxies

Let us also remember what the astrophysicist and cosmologist Laurence Perotto, member of the Planck collaboration, had explained to Futura in the file that she had allowed us to write where she explained to us the nature of the fossil radiation and the analyzes that she and her colleagues had planned to make of this radiation in search of fundamental keys for the cosmology and theoretical physics: The effect of gravitational lens makes it possible to reconstruct the integrated gravitational potential of the surface of the last diffusion till today. It is an interesting probe of the structures of the Universe. Thus, if we succeed in this reconstruction, Planck would become an autonomous experiment sensitive to the whole evolution of the Universe, from the primordial universe of the time of the last diffusion until us. “.

The surface of last scattering is that of a fictitious sphere surrounding any observer in the cosmos observable and showing him the regions from which the photons fossil radiation when the observable Universe became transparent because its density became so low that the photons of that time could then travel without colliding with charged particles which would scatter them over large distances.

The evoked gravitational lens effect, in this case the so-called weak one or even gravitational shear, is an effect of deflection of light rays by a gravitational field leading to deformation of the initial image of a galaxy by a large mass interposed between this galaxy and an observer. We can deduce from the deformation the mass of the body producing it, so that measuring gravitational lensing effects makes it possible to probe mass distributions in the observable cosmos, including masses of dark matter which itself does not radiate.

In a vacuum, light usually travels in a straight line. But in a space deformed by a massive celestial body, like a galaxy, this trajectory is deviated! Thus, a light source located behind a galaxy has an apparent position different from its real position: this is the phenomenon of gravitational mirage. This video originates from the web documentary “ The Odyssey of Light (…) and has been integrated into the web documentary ” Embark with Dark Matter ( © CEA-Anime

This effect has been used to estimate the presence and distributional changes of dark matter up to about 8 to 10 billion years ago. As Laurence Perotto also explained to us, the weak gravitational lensing effects produced by mistresses of the galaxies and the galaxies in the foreground of the last scattering surface contaminate the study of the fossil radiation and it is necessary to somehow subtract this noise from the signal to go back to the primitive state of the fossil radiation. This makes it possible in particular to track down the mythical B-mode primitives of the polarization of the fossil radiation. The highlighting of these modes would convincingly demonstrate the existence of a vertiginous inflationary phase of the expansion of space during the Big Bang.

Clusters of dark matter that have been structured since the Big Bang

But, as the cosmologist had mentioned in the excerpt from her file that we have given, the measurement of the weak gravitational lensing effect could in theory inform us about the presence and the variable characteristics over time and space of dark matter from the appearance of the first galaxies until today using fossil radiation.

The Japanese-led team managed to make precisely such observations beyond 8 billion years by measuring the effects of galaxies detected with the HSC on Planck’s measurements of the background radiation. We had not gone further before because the galaxies, whose images were distorted by gravitation, were too faint to make valid measurements.

But now researchers can go back about 12 billion years into the observable cosmos.

Remarkably, although still to be confirmed, the size characteristics of dark matter concentrations between 8 and 12 billion years ago do not seem to follow the predictions of the Standard cosmological modeldark matter density fluctuations during this period appear weaker than expected.

Yuichi Harikaneone of the authors of the discovery and Professor at theCosmic Ray Research Institute from the University of Tokyo, does not hesitate to explain: ” Our conclusion is still uncertain. But if true, that would suggest that the whole model is flawed as you go back further in time. It’s exciting because if the result holds after reducing the uncertainties, it could suggest an improvement in the model that could provide the nature of dark matter itself. »

With this goal in mind, cosmologists still need to increase the volume and accuracy of available data, which they will soon be able to do with the commissioning of the Vera C. Rubin Observatory, formerly called the OHSA.

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