One of the most important motivations for assuming the existence of particles of black matter never yet demonstrated on Earth in collisions with particle accelerators or in detectors buried under mountains is to be able to quickly give birth to galaxies that we observe already formed at least a few hundred million years ago after the big Bang.
Indeed, the analysis of the temperature fluctuations of the cosmic radiation allows us to deduce in the first place the fluctuations of density in the normal matter, which interacts with the light, only about 380,000 years after the Big Bang. If we calculate the timecollapse of these fluctuations of matter according to the laws of gravitation known, galaxies would not have had time to form today. But if we imagine density fluctuations of a material that cannot emit light (or very little) more important than for the material formed of protons, neutrons and electronsthen galaxies can be born very early.
However, we know that in their heart there also exist very early supermassive black holes already containing at least one million masses solar and in some cases at least a billion. It is difficult to explain the early appearance of these black holes giants, even if they can be made to grow rapidly by accretion of material in large quantities, in this case with dark cold matter filaments.
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
Black holes born before or after fossil radiation?
Among the explanations explored are those of primordial black holes which would have formed from density fluctuations in the material before the emission of the fossil radiation. They would therefore be fossils of the Big Bang itself and would not have formed by the collapse of matter once it was over.
It is tempting to relate these primordial black holes to dark matter, not only by proposing that dark matter particles do not really exist but are a gas primordial black holes, some of the most massive of which would be black holes of a few tens of solar masses detected with gravitational waves issued during their merger and who have crossed the telescopes » Ligo and Virgo, but by causing these black holes to form primarily through the collapse of overdensities of dark matter.
In the previous article below, Futura exposed one of the theories of the formation of supermassive black holes by the collapse of dark matter, but a collapse produced well after the Big Bang and before the emission of the cosmic radiation.
In the article published today in Physical Review Letters by Hooman Davoudiasl, Peter Denton, and Julia Gehrlein based in Brookhaven National Laboratory and a version of which is freely available on arXivit was before the emission of fossil radiation that supermassive black holes would have formed from dark matter.
Pierre Brun is a particle physicist at Irfu and works at the frontier between particle physics and cosmology. He is interested in a theory that postulates the existence of a particle called “axion”, which would solve certain problems related to the violation of symmetry in the laws of physics of strong interaction. Neutral and light and interacting very weakly with matter, the axion has all the characteristics to be a particle of dark matter. © CEA Science
A testable theory with gravitational waves
The calculations of the three physicists theorists rely on a particular class of very light particles of the type known as axions and which were initially postulated to account for an enigma with the equations of the QCDthe theory that describes the world of hadrons composed of quarks linked by the strong nuclear force and its gluonsthe cousins of photon.
We still don’t know if the axions exist and in the case of the work published today, the considered axions are naturally related to superstring theory and they are extremely light since their mass would be a hundred billion billion billion times lighter than that of a proton.
Very light dark matter particles are more and more often considered because many models with massive particles of the kind called Wimps (for Weakly Interacting Massive Particles) are now disadvantaged.
The primordial gas of axions postulated by Hooman Davoudiasl, Peter Denton, and Julia Gehrlein would then have undergone what is called a phase transition of the first order (like those of water with the temperature or that leading to the appearance of a Bose-Einstein condensate), leading it to collapse directly into supermassive black holes of the order of a billion solar masses.
The phenomenon would have generated a bath ofgravitational waves now having a frequency band very precise but which cannot yet be demonstrated with detectors such as Virgo and Ligo. But it would soon be within reach of the detection method based on the study of pulsar populations within the framework of research conducted with NANOGrav (North American Nanohertz Observatory for Gravitational Waves).
Supermassive black holes were born from dark matter
Article of Laurent Sacco published on 05/04/2016
Black holes are everywhereuniverse and some think that they could constitute part of the dark matter. This could have caused the direct collapse of superdensities of matter at the beginning of the history of the cosmos, giving rise to the first supermassive black holes.
And if the stars compacts located in the heart of large galaxies and containing several million to several billion solar masses were not black holes described by the theory of general relativity ? The idea cannot be ruled out. However, there really is no alternative, except perhaps wormholes or, at the very least, objects whose actual description hardly differs from the classical theory, built between 1965 and 1975 from Einstein’s theory. The concept of event horizon could thus be relaxed, as proposed a few years ago stephen hawking.
In any case, it is necessary to explain the birth of these objects. Concerning the black holes, various hypotheses have been put forward but none have been accepted. It seems clear, however, that they evolved by merging and accreting matterlike the galaxies which shelter them. But where do their germs ?
It is difficult to bring into play the formation of very massive stars which would then have collapsed into black holes because stars containing more than a few hundred solar masses pose problems. We have also tried to involve primordial black holes; in theory, these are easy to train. However, the observations ended up disfavoring their existence. This means that the observable cosmos began its history with initial conditions that were not conducive to the formation of supermassive black hole seeds in this way.
Seeds of black holes of two million solar masses
The enigma of the origin of these black holes seems to appear in a new light if we believe two articles published in the journal MNRASaccessible on arXiv. They come from new numerical simulations conducted by a group of researchers from the universities of Osaka, Japan, and Kentucky, United States.
Several simulations of the formation of the large structures of the universe have already been carried out with supercomputers for decades. We can cite that of Millennium or DEUS. The first simulations carried out within the framework of cosmologies with black matter only took this matter into account exotic. Others then involved a mixture of normal matter and dark matter. It is within this last framework that the researchers worked, refining their calculation methods.
It turned out that, after a while, fluctuations in the density of dark matter led to matter rapidly falling into the heart of the overdensity zones by gravitational collapse. Regions containing nearly two million solar masses were thus able to collapse, later, more completely and directly, giving rise to supermassive black holes. These regions would have appeared in about 2 million years, during the formation phase of the first stars and galaxies.
The path leading these concentrations to actually become black holes in a second phase remains to be explored. According to astrophysiciststhe reality of the first phase could be established thanks to future observations of the space telescope James Webb.
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