Primitive black holes: have we found proof of their existence?

Since ancient times, humans have been curious to explain the most unpredictable and disturbing phenomena in the universe. Although the study of astronomy has been a constant in all civilizations, different astronomical events of a more “unpredictable” nature, such as comets or eclipses, were considered as “omen of calamities” and “actions of the gods”.

It is worth remembering the fall of the Saxon king Harold II in the year 1066 before the Norman invasion of William the Conqueror and attributed to the bad omen of the passage of a comet (later baptized as “Halley”). Or, when during the battle of Simancas (Valladolid) in 939 between the troops of León Ramiro II and the Caliph Ad al-Rahman, a total solar eclipse caused panic among the troops on both sides, delaying the battle for several days.

How would our ancestors have reacted to the existence in the universe of objects capable of swallowing everything that fell into them, including light? Fortunately, these objects would not pose any fundamental problem for ancient civilizations as they are so far from us that only with the modern instrumentation they have been able to be detected and even photographed.

In 2019, the collaboration of eight radio telescopes located in different parts of the world was able to take the first photo of a gigantic black hole (6.5 billion times more massive than our Sun). It is located about 55 million light years from us (it should be remembered that a light-year corresponds to a distance of about 9.5 trillion kilometers) at the center of the Messier 87 galaxy (M87).

The italics of the word photo is not accidental: how can a photograph be obtained of an object that catches light and, therefore, would not allow it to be seen? The answer is simple: we are not observing the object itself, but the remains of a star that is literally being swallowed by the black hole.

This stellar matter rotates at enormous speeds around the black hole and its brightness can be detected when it reaches temperatures of the order of a million degrees Celsius. This disk of matter that surrounds the black hole is called the “accretion disk” and the “edge” of the black hole (once through which nothing can escape) corresponds to the “event horizon”.

In the image above we can see the accretion disk and the event horizon of the black hole located in M87. Also compare its gigantic size in relation to our solar system.

Primitive or primordial black holes
A considerable part of the black holes in the universe are formed by the gravitational collapse of a star when, in its final phase, they exhaust all their fuel: they are called “stellar black holes”. Not all stars will generate black holes at the end of their life: the limit is at least three solar masses.

There is another particular type of black hole, the so-called “primitive or primordial”. As their name suggests, these were formed in the first moments of the Big Bang and, in theory, they can have any mass. Its size can range from that of a subatomic particle to several hundred kilometers. It is precisely the smallest ones that emit the most amount of radiation.

But, how is this phenomenon possible if they are objects that “do not emit radiation” and trap everything, even light?

The answer was provided by physicist Stephen Hawking in the mid-1970s. He postulated that quantum effects near the event horizon of a black hole would produce the emission of particles that could escape from it. That is, black holes that do not gain mass by other means will progressively lose their mass until finally evaporating and disappearing.

Capture of an atomic-sized primordial black hole
Would we be able to detect holes the size of a hydrogen atom, before evaporating completely?

In a recent post, an astrophysical scenario is suggested in which one of these atomic-sized black holes is captured by a supermassive black hole.

As the first approaches the event horizon of the second, the fraction of Hawking radiation that we can detect from Earth gradually decreases, until it reaches the size of a ray of light.