Cosmic enigma: did primordial black holes give rise to supermassive black holes?
In the black hole family, there are several quite different members. First of all, there are stellar black holes, historically the first discovered (theoretically) by physicists, then (through observation) astronomers. They form following the death of a star whose initial mass exceeds 20 or 40 times that of the Sun. These stellar black holes are between three and tens of solar masses, because the star that formed them has lost mass in the meantime.
Supermassive black holes live at the centers of galaxies. These are Gargantua ranging from millions to several billion solar masses. The mechanism leading to their formation remains a mystery, but we observe that they are there, very numerous and very massive, even in the early days of the Universe. Except we don't really know why. This is one of the great astrophysical enigmas, the subject of a study published on arXiv.
Intermediate mass black holes are the missing link between the two mentioned above. They are between 1000 and 50,000 solar masses — a range yet to be specified — and could exist in the molecular clouds giants or globular clustersFor example. Numerous observational clues show that they do indeed exist.
There would nevertheless be a final category: primordial black holes. Still elusive, they have been theoretically predicted for a long time like the other three. During the first second of the Universe, conditions were very different from those of today, then excessively hot and dense. The Universe was entirely bathed in radiation, although absolutely opaque, and if we found small density fluctuations here and there, then matter could have collapsed into low-mass black holes.
The smallest give birth to the biggest monsters!
The study in question here proposes that it is precisely these primordial black holes which would be the germs, or seeds, of supermassive black holes. It would have been enough for them to quickly merge with each other. Certainly, this is not the most common formation hypothesis, because we are instead looking for primitive, very massive stars (called Population III), or immense clouds of gas capable of quickly giving gigantic masses to the holes. supermassive blacks.
Nevertheless, this hypothesis is very interesting and relatively elegant, because it would help to explain why we no longer see these primordial black holes today. Yes, they would no longer be tiny at all, having potentially given birth to the biggest monsters in the Universe!
There is a way to test this hypothesis of primordial black holes as the seed of supermassives
This is all well and good, but a scientific hypothesis without a way to test it is not worth much. In fact, the authors are offering a soon-to-be-accessible way to test their model! If these primordial black holes merged in such large numbers, then they produced gravitational waves. And if they were around thirty solar masses at the beginning of the Universe – a hypothesis considered plausible – then a laser interferometer such as the Einstein telescope, a Ligo-type project, but with arms 10 km long instead of 4 km, would be able to detect these residual gravitational waves.
It would be a fitting return to history if the enigma of the formation of supermassive black holes were solved thanks to a telescope called Einstein, whose equations revealed the existence of black holes and gravitational waves!
