Primordial Black Holes. Could they have existed? Do they still exist?


About a year ago I read the article in New Scientist considering the question of black hole seeds. It was saying that some of the detected supermassive black holes have the redshift corresponding to the time when the Universe was less than a billion years old. But what’s more important in 2000 Nasa’s Chandra X-ray observatory spied a very distant and enormously powerful quasar called SDSSJ1030+0524.  Calculations showed that the black hole’s mass which powered this quasar is more than a billion solar masses while the Universe was about 900 million years after the Big Bang when the signal from this quasar started to travel towards the Earth.

Before this discovery it was thought that supermassive black holes are the relicts of the very first stars in the Universe each of those had a mass of more than 100 solar masses, underwent a supernova explosion and formed a black hole of about 100 Solar masses. Then those black holes should have engulfed a lot of material, and eventually sank towards the center of a galaxy becoming a supermassive black hole.

Modern theories tell us that the more gas a black hole gulps the more radiation shines out sweeping away the nearby material and cutting off the food supply. Eventually a black hole is starved and can’t accumulate more mass for a long period of time.

Calculations provide the information according to which the black hole in SDSSJ1030+0524 could theoretically accumulate such a mass in roughly 700 million years but this requires a perfect food supply that is barely possible in a real Universe since the black hole surroundings are likely to be somewhat messy. If our SDSSJ1030+0524 were the only such black hole we could assume it is just a freakish example of perfectly tuned events, but astronomers have detected several more since then, so there has to be a natural way of producing these monsters.

According to this there should have been bigger seeds for such supermassive black holes. When this article was released there were 4 probable candidates for this.

The first one is a collapse of entire cluster of stars in the early Universe into one black hole up to several thousand solar masses. Some regions in the early Universe were spectacularly active, clusters of stars tended to form in those regions and some of those stars were extremely heavy. They tended to sink towards the center of cluster and almost inevitably ran into one another. After that they formed a one giant “star” that collapsed on itself to form a black hole pretty soon. Such a black hole would have a mass of about few thousand times that of the Sun.

The second possibility (hold up, you might start to laugh very hard in a moment) is a collapse of entire galaxy core into a black hole of up to million solar masses. It may sound extremely improbable but the idea was initially proposed by Martin Rees that makes it not as ridiculous as it might seem. One major problem here is spin. Even early protogalaxies rotated a bit and accumulation of much material should have forced the incoming material to move faster and faster and eventually gravitational attraction should have been balanced out. This might be overcome, however, if the core of a galaxy is overdense and slow-spinning. In this case it could become unstable and the rotation velocity could have transferred outwards. The remaining core would collapse into a much denser thing in this case but not into a black hole yet. A small black hole would be sitting at the heart of such a knot but surrounding cocoon of material would provide an enormous supply of gas and stuff into the black hole. The result would be a supermassive black hole of about a million solar masses in just several million years.

The next possible explanation is also very speculative. It considers those black hole seeds as formed within the so-called dark stars. Such stars could have been powered by dark matter. That is, the dark matter particles could lie at the center of stars colliding and annihilating with each other. This would produce a “gentle” heat with no harmful radiation that would repel the incoming gas, thus a star will accumulate mass almost indefinitely. Such stars may become unimaginably heavy and eventually collapse to form a black hole of about 100 thousand solar masses.

The last suggestion mentioned in the article is simultaneously the most radical. It is saying that the first black holes could have formed directly in the Big Bang or, to be more precise, a fraction of a second after it, at the moments known in physics as state transitions. Those hypothetical holes are called primordial black holes and I want to consider them a bit more. There is not a lot of information about them in the article but luckily, I’ve just finished the book written in 1970s by Steven Weinberg where this possible formation of black holes was considered.

Our Universe is known to be highly isotropic and homogeneous on large scales. However, right after the Big Bang quantum fluctuations could have been strong enough to generate kind of perturbations in that homogeneity. Amplitude of those perturbations is given by the local deviation of density of a space metric from a homogeneous space metric. Simplifying this, we can say that it is a difference between the amount of material given by exactly homogeneous space-time fabric and the amount of material which actually located within some volume of space. Hence, some regions of space could have been denser than the others.

What is relevant for us here is that no physical law forbids those perturbations to be extremely large at least in a small volume. If such a perturbation leads to extremely high density in a given volume, it could stop the expansion of space in that volume. This volume with a given material would collapse into a black hole in this case. Obviously, all the material and radiation would be trapped within the black hole. Such an object is called a primordial black hole and has certainly a different formation from either a stellar mass black hole or a supermassive black hole.

The calculations give us a presumable size of such a perturbation when it could have lead to collapse of the material into a black hole. This presumable size equals the product of the speed of light and the time passed after the Big Bang. The black hole could have formed if the local mass was several times higher than the average. So you can see that the earlier those perturbations take place – the lesser mass is required for the formation of a primordial black hole. The further in past we follow the inverse sequence of our Universe – the lesser mass a black hole could have had. Literally, the mass of a primordial black hole could be any low, for example – 10 to the power of 20 g for a black hole formed at 10 to the power of minus 18 s; a million g (1 ton) for a black hole formed at 10 to the power of minus 32 s; 1 g for a black hole formed at 10 to the power of minus 38 s from the Big Bang. This relation of the mass and time exists because the density of material in the Universe is getting higher and higher when we approach the moment of Big Bang.

The last thing I want to emphasize here is that no primordial black hole has ever been observed yet. It doesn’t mean that they don’t exist or that they have never existed. But it is saying that their amount isn’t large, otherwise we would detect them for sure. Or… they can lie at the centers of galaxies in a form of supermassive black holes? Scientists still aren’t sure about it but this is a good thing about science, we always have to explore new possibilities, otherwise science won’t move forward.

Thank you.


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