The number of black holes in the observable Universe would be gigantic!

The equations of the theory of general relativity ofEinstein admit a fairly diversified zoo of cosmological solutions if we allow space to be non-isotropic and with non-trivial topologies. We can, in the first case, have a cosmos in rotation or in an oscillating form between a sphere and a cigar for example. In the second case, space can be crumpled, in the words of Jean-Pierre Luminet, and be for example the analog of a torus or Poincaré dodecahedron.

In the case of a torus, the geometry of space would be flat and we could believe it to be of infinite size, as a plane would be, and yet we would be wrong. All these considerations are there only to remind that even if the analyzes of the data of the Planck satellite concerning cosmic radiation from big Bang rather favor a Universe with a flat geometry and therefore perhaps of infinite size in space, we still have no certainty about this question and even less about the shape and topology of space (its finite character or not in the weather is also subject only for the moment to speculations, for example those of the Nobel Prize in Physics Roger Penrose on a Cyclic Universe).

Still, we can still talk about the concept of the observable Universe, that is to say all the regions of space from which light rays have been able to reach us since its birth there. about 13.8 billion years old. It should be noted in passing that the behavior of these rays, which can serve as a cosmological probe to test different models of the Universe, has recently been examined more closely by the members of the collaboration RayGal.

An observable universe 90 billion light-years in diameter

About the current observable Universe, whose characteristics are not really debated and do not really depend on the cosmological model considered, we can get an idea thanks to a famous video, The Known Universe, available on the channel Youtube of the’American Museum of Natural History.

Let’s go back to the comments that Futura had already made regarding this video years ago. The journey into the observable Universe begins here on the roof of the world, i.e. Tibet, then takes us through the Solar system to show us the position of orbits planets relative to the Milky Way. We then go beyond the wave sphere radio generated by Humanity from the 1910s and whose front is about 100 light years to furrow the fine disc of the spiral galaxy what is our Milky Way.

The journey continues through the map of galaxies and quasars obtained by large readings such as the Sloan Digital Sky Survey (SDSS) to eventually reach the last scattering sphere located more than 46 billion light-years from Earth today. It corresponds to the regions from which left 13.8 billion years ago the photons from cosmic radiation. We then return to Earth with a strange impression of metaphysical vertigo associated with the awareness of the enormity of the scales of space in which Humanity has been caught for millennia, somewhere between All and Nothing, carried by a wave having led Big Bang alive.

It has long been speculated about the exact content of this observable Universe, how many galaxies, stars, protons ? We have indirect ways of estimating these numbers and today a group ofastrophysicists run by members of the International School for Advanced Studies (in Italian, Scuola Internazionale Superiore di Studi Avanzati or SISSA), a famous Italian university located in Trieste, has just published an article evaluating the number of black holes of stellar origin in the observable Universe.

A precision, the radius of the observable cosmos is about 45 billion light-years and not 13.8 as one might naively believe. Indeed, since the Big Bang, which we do not know if it begins with a real birth of time, space is expanding and nothing limits a priori her speed. Any calculation made taking into account this expansion, the regions from which the photons of the fossil radiation have left for 13.8 billion years are found today indeed at about 45 billion light-years and they are probably currently occupied by galaxies much like those in the vicinity of the Milky Way.

During all this time, stars formed and some were massive enough to collapse giving rise to black holes in galaxies as well as in globular clusters orbiting these galaxies. There are also reasons to think that globular clusters would be the place of real chain reactions catalyzing the formation of binary black holes more and more massive, according to new numerical simulations describing the evolution of clusters, taking into account general relativity. Such black holes binaries also form in galaxies since many stars are actually born in pairs. In any case, mergers will intervene giving more massive black holes as the detections ofgravitational waves made with Ligo and virgo have demonstrated it.

All stellar black holes since the start of the Big Bang

We succeeded in modeling the evolution of the star formation rate in galaxies and then the evolution of these stars and the number of stellar black holes that they will form. These developments are also related to the evolution of the chemistry galaxies due to the nucleosynthesis elements in the massive stars, elements which they disperse by becoming supernovae. This changes the metallicity galaxies and stars, i.e. the abundances of elements heavier than thehydrogen, according to the jargon of astrophysicists. However, metallicity influences the formation and evolution of stars.

In short, as explained in a publication in The Astrophysical Journalwhich can also be consulted freely on arXiv, the researchers estimate that about 1% of the protons and neutrons produced by the Big Bang can be found today in the form of black holes of stellar origin, no larger than a few hundred solar masses, in the observable cosmos. They also obtained a distribution function in galaxies and globular clusters of the number of black holes of stellar origin of a given mass and it takes into account, by involving the catalyzed formation of black holes in globular clusters, particularly massive black holes of several tens of solar masses which had surprised researchers after their detection with Vigo and Ligo.

All of these stellar black holes are too massive to have started to evaporate by Hawking radiation. The temperature of such radiation for the masses involved being lower than that of fossil radiation, these stars are colder than it and they absorb it rather than radiate it. The balance sheet of stellar black holes in the observable Universe therefore counts all the black holes of stellar origin formed and existing since the birth of the first stars.

The researchers then arrive at the staggering number of 40 billion billion stellar black holes in the cosmos, i.e. 40*10¹⁸ stars.

We end by recalling that the supermassive black holes have not been taken into account although they contain from one million to several billion solar masses and are present in all major galaxies. This is hardly surprising because it is still not really known how these giants were born, perhaps from germs in the form of primordial black holes.