The Universe is full of black holes. Dark objects of immense gravitational power that swallow everything that comes within their reach without anything, neither matter nor energy, being able to go outside again. So far, scientists have discovered them in various types and sizes. Those of “stellar mass”, for example, arose after the gravitational collapse of very massive stars, are the smallest, and usually have masses equivalent to tens of times that of the Sun.Of course, there are much older ones. Another category, intermediate-mass black holes, by the way quite rare and difficult to find, have, according to some, between a thousand and 100,000 solar masses, although others think they can reach one million.

But it is in the centers of galaxies that the largest are found, the so-called “supermassive black holes”, with masses equivalent to several million, even billions of suns. Without going any further, in the heart of the Milky Way, our galactic home, sleeps Sagittarius A *, a black hole of four million solar masses that, despite its size, turns out to be one of the smallest of its kind.There they are, to prove it, beasts like M87 * (the first black hole that we have managed to photograph), with its 6.5 billion solar masses. Or the one at the center of the elliptical galaxy NGC 4889, with 21,000 million solar masses.

Incredibly, there are even bigger black holes. They are part of another category, that of the ultra-massive, which includes all those with masses greater than 10,000 million soles. There we find, among others, the gigantic black hole of 40,000 million solar masses in the center of the Holmberg 15A galaxy. Or TON 618, the largest of all black holes found so far, a monster among monsters, almost 70 billion times more massive than the Sun and almost at the limit of what researchers think possible. The calculations, in effect, indicate that black holes with more than 100 billion solar masses cannot exist within a galaxy. The question, at this point, is this: Could even bigger black holes exist somehow? The surprising answer is yes, although none have been found so far

“Incredibly large” black holes. In a study just published in the Monthly Notices of the Royal Astronomical Society, Bernard Carr of Queen Mary University of London explores a new category, that of “incredibly large” black holes, which would be those with masses greater than 100,000 million suns and are known as SLABs, an acronym for “Stupendously Large Black Holes.While there is currently no evidence that SLABs exist, Carr says, “it is conceivable that they could exist, and could be outside galaxies, in intergalactic space. Yet surprisingly, the idea of SLABs has been largely neglected until now.

Until recently it was thought that supermassive black holes formed within a host galaxy, where they grew by swallowing stars and gas from their surroundings or by merging with other black holes during collisions between galaxies, in which case, as shown. He said, there is an upper limit of mass that is around 100,000 million solar masses. Beyond that, the black hole’s accretion disk would be so massive that it would fragment under its gravity.

But in their new study, Carr and his team propose another possibility for supermassive black holes to form without having to be bound by that growth limit. Thus, the researchers suggest that SLABs could be primordial black holes, formed shortly after the Big Bang and long before the first galaxies. And since primordial black holes don’t form from the collapse of a star, they could have a wide range of masses, from the very small to the incredibly large.

The very small ones, if they ever existed, may have already evaporated due to the effects of Hawking radiation. But, at the other extreme, the most gigantic could have survived. Based on these assumptions, the researchers calculated how far these black holes could grow. And the conclusion is extraordinary: between 100,000 million and a quintillion (a 1 followed by 18 zeros) of solar masses. Carr and his team could not resist the temptation to calculate the size of such an object: with that mass (a quintillion suns) the black hole would be more than 620,000 light-years across.

To get so incredibly large, these primordial black holes would have to have absorbed a lot of mass early in their existence and in a relatively short time. And researchers believe that original “food” could have been dark matter.If the early universe was rich in dark matter, particularly a form known as weakly interacting massive particles (WIMP), then an initially small primordial black hole could consume it to grow rapidly.

Since dark matter does not interact with light, what was eaten by the black hole would not emit radiation or heat to dampen the growth rate. As a result, these black holes could be huge even before the Universe cooled down and the first galaxies formed.Later, when the Universe finally cooled down and continued to expand, these gigantic extragalactic black holes would no longer have more matter around them to devour, so they would not generate around them those bright rings of the matter about to be devoured that is common. supermassive black holes at the centers of galaxies.

In other words, they would be practically invisible to us.But Carr and his colleagues think they could just as well be detected by studying the way the space around them behaves. As is known, gravity is capable of bending space-time, which makes a beam of light coming from a distant object, and passing through these distorted regions follows a curved path.