Le silence des trous noirs de l’univers

Black holes, we often talk about it as if it were an evidence, as if their origins were perfectly clear. You have probably already heard the classic story about their formation. A very massive star arrives at the end of its life, it no longer manages to resist its own gravity, it explodes into a supernova and what remains collapses on itself to form a black hole. It's a nice picture, it works well to explain the concept, and above all, it's true in a lot of cases, but if we stop there, we're wrong. Because behind this simple version, there is a much deeper question, a question that bothers a little. Does it really happen all the time like that? Or more simply, is it really the only way to make a black hole? And there, things get complicated. When we look at the distant universe, so young, we come across weird observations. For example, we discover supermassive black holes in the center of the galaxy, already very early, after the Big Bang. Monsters that weigh millions, sometimes billions of times the mass of our sun, and that are there while the universe was only a few hundred million years old. But wait, it doesn't make sense. Because to get to such a mass, it takes time, a lot of time. A star must form, live, explode, become a basic black hole, then this black hole grows by swallowing matter around it. But in such a young universe, this time does not exist. So, how do we explain their presence? It is here that we suspect that there may be something else, that black holes are not only giant star corpses. Maybe there are other ways to create them, other ways, other mechanisms that we don't see directly. And maybe even that, in some cases, a black hole can appear without having visible precursors. As if nature could bend space-time directly, without going through the stage of stars. It seems crazy, but why not? Because finally, what do you really need for a black hole to exist? Do you need ordinary matter, like those that make up planets and stars? Do you need a pre-aligned structure? A cloud of gas, ready to collapse? Or is it enough just to have the right physical conditions? A certain density, a certain pressure, and hop, a black hole forms on its own. And here, inevitably, we think of the beginning of the universe. A young, unstable, still shapeless, bubbling universe. No galaxies, no stars, just a burning, dense soup, which was dilating at full speed. And in this chaos, there must have been areas a little heavier than others, local fluctuations. Some of these areas could have collapsed directly, without going through the classic process. This is what we call primordial black holes, objects born almost from nothing, just because the laws of gravity allowed it, at a time when the universe was still a child. And there, everything changes, because if these primordial black holes exist, it totally changes the vision we have of the universe. And that means that the black holes are not only the remains of dead stars. And that means that they could have been present from the very beginning, even before the birth of the first galaxies. And above all, it opens the door to dizzying explanations. Maybe these hidden black holes could be related to black matter, this invisible thing we've been looking for for decades. Maybe they are replicas of a universe that we will never fully understand. But be careful, it's not that simple. We can't just say they exist, and move on to something else. Because each hypothesis raises other questions. If these primordial black holes are really there, why don't we have direct evidence yet? And if on the contrary they don't exist, how to explain the presence of these supermassive monsters so early in the history of the cosmos? And that's where it becomes almost philosophical. We no longer speak only of the physics of stars, but of a question about the very nature of space-time. So yes, we can say that the simple version, with the stars that die and collapse, is insufficient. But if we look closer, if we dare to question this reassuring image, we discover a much stranger universe. A universe where a black hole could appear as an evidence, without warning. Just because the fundamental conditions are there. And that's what's fascinating. Because deep down, no one really knows. We have models, theories, scenarios, but no certainty. The black holes can be the corpses of the stars. Or the fossils of a primordial universe. Or something else that we haven't even imagined yet. And as long as we don't know exactly how they are born, we will remain with this impression that they escape us. As if they were carrying a secret that exceeds what we are able to understand at the moment. And it is this doubt, this gray area, that makes the question so exciting. Because wondering where the black holes come from is not just a cosmic detail. It is asking the question of what it takes for space-time to bend and create a horizon. And that is probably one of the most dizzying questions we can have about the universe. Hello everyone, welcome to this brand new video. I hope you will like it. Just before we start, as usual, we will read a little comment. But hey, it wouldn't hurt me to watch it a second time. In any case, it exceeded my expectations. This channel is validated for me. Thank you. It's so nice. Thank you very much. I'm glad you like it. And so it brings me to a little question. How long do you like videos to last? Is it too long when it lasts an hour? Or on the contrary, is it good? I have the impression that you watch videos that are very long, like 1h, 1h30. Rather than videos that are shorter, like 30 minutes. Like this one, I think it's going to be about 30 minutes. So here's a little question. Maybe you like both. I think the longer videos are more suitable to watch in the evening. Because they are very long. And when you go to sleep, I don't know, at least it continues. While the shorter videos are more suitable to watch during the day. When you have things to do. When you eat or do another activity. Because maybe you have an activity that lasts 30 minutes. And a video that lasts 30 minutes. It fits well. I don't know. In any case, these are the first feedbacks I've had. But it interests me a little bit to know which length you prefer. In any case, I hope you still like this channel as much. It's always a pleasure to make these videos for you. You saw, at the moment, we are very active. We make a lot of videos at the moment. We have up to 4 videos per week. It's huge. And for that, you can really thank the team that also works on these videos. I'm not alone. And fortunately, because otherwise I couldn't. Anyway, don't hesitate to subscribe. To join the channel if it's not already done. And besides, there are 60% of people who watch my videos who are not yet subscribed. It's free. In addition, you just have to press the little red button. And it supports me greatly. So thank you to all those who will do it. Let's talk a little bit in the comments. With that, I leave you with the video. Let's go. What if a gigantic black hole formed even before the birth of the first stars? This is the incredible question that raises today a new discovery of the James Webb Space Telescope. Astronomers have spotted a very distant luminous point in the universe. Located more than 13 billion light-years from Earth. This point is Abel 2744 QSO1. A supermassive black hole, an object so dense that even light cannot escape it. And what we see is the past. As the light of this object took 13 billion years to reach us, we see it as it was only 700 million years after the Big Bang. In other words, at a time when the universe was still a baby. But it's not just its age that intrigues scientists. What really surprises is the environment in which it is. Normally, supermassive black holes are hidden in the heart of large galaxies. Surrounded by stars, dust, gas. But here, around QSO1, there is almost nothing. No visible large galaxies. No traces of star formation. Very little matter. In short, an almost empty space. And that's not all. Analyzes show that the gas around this black hole is almost free of metals. What are called metals are all the heavier elements than hydrogen and helium. So like oxygen, carbon, which are created by stars. So if there is almost no metal, it means that no generation of stars has ever lived here. It's as if the corner of the universe had never been touched by the light of the stars. And yet, in the middle of this void, there is a gigantic black hole. 50 million times more massive than our sun. And the most amazing thing is that it is more massive than the tiny galaxy around it. In fact, it is the opposite of what we usually observe. In today's universe, black holes are much smaller than their galaxies. Here, it is as if the black hole had arrived first and that the galaxy had not yet had time to form around it. So how could such a monster appear so early? And especially in such a deserted place. This black hole does not correspond to any classic model of formation. It should not exist. And yet it is there. We think that a black hole is born when a very massive star dies. Then it grows little by little by absorbing gas and merging with other objects. But this process is slow. It takes time and a lot of matter. But here, there is not enough gas, no star, almost no metal. And yet, the black hole is already huge. So, scientists consider two scenarios. Maybe this black hole formed all at once from the direct collapse of a huge cloud of gas without going through the stage of the stars. But for this to be possible, it would take a perfect environment. But these conditions are extremely rare in the universe, even at its beginning. This is why scientists wonder if this black hole would not be even older than we thought. It was born just after the Big Bang. Even before the formation of the first stars in the galaxy. And we are talking about a primordial black hole. But whatever its origin, Abel 2744 QSO1 raises the question of what we thought we knew about the beginnings of the universe. It forces scientists to rethink the order of things. And if the black holes were not only the results of galactic formation, but the first engines of the construction of the universe. A practically naked black hole, surrounded by almost nothing, but yet already immense. We agree, in the collective unconscious, a black hole is scary, nothing less. We imagine a gulf that swallows everything. A kind of cosmic vacuum cleaner that devours the stars and planets. And it's true that said like that, it sounds terrifying. But in reality, a black hole is not a magical entity or supernatural. It is above all a logical consequence of gravity pushed to the extreme. In fact, the definition is quite simple. A black hole is not a star. Because there is no physical delimitation. But it is a region of space where the density is so enormous, that even light cannot escape it. And as light is literally the fastest thing in the universe, if it is trapped, well, everything is trapped. And what's crazy about black holes, is that they are, in a way, the simplest objects in the universe. It may seem paradoxical, because they are always associated with mysteries. To incomprehensible phenomena. But once they exist, they obey a rule called the theorem of galaxy. Don't ask me, I didn't invent the names. But to make it simple, this theorem says that a black hole has no hair. And I swear to you that it exists and that it is a true definition. More seriously, it means that a black hole, no matter how it was formed, no matter what it swallowed before, it can be summarized in only three parameters. Three values are enough to describe it completely. Its mass, its electric charge, and its rotational speed. That's all. So of course, depending on these three values, we have different types of black holes. If we take a black hole that has no charge and does not rotate, we call it a Schwarzschild black hole. This is the simplest case, a bit like the basic model. But if we add a rotation, so that it rotates on itself, we get a Kerr black hole. And if, in addition to rotating, it is charged electrically, then we enter the domain of the black holes of Kerr-Newman. So you see, it's classified a bit like variants. But in the end, it's still in these three parameters. No need for more. This is where the word calvicity makes sense. The black hole loses all the characteristics of what it created. It has no more hair, no more details. It only keeps its mass, its charge, and its rotation. But to get there, we have to explain how we make a black hole. And there, we go back to the basics. Life and death of stars. A star is a huge ball of gas, mostly composed of hydrogen, and which holds thanks to a fragile balance. Inside, we have nuclear reactions that transform hydrogen into helium, and later, into heavier elements, such as carbon, nitrogen, etc. These reactions produce energy, light, and above all, they exert a pressure towards the outside. This radiative pressure is what prevents the star from crushing under its own weight. As long as it burns its fuel, everything is fine. The balance holds. But at some point, the fuel ends up missing. And there, everything depends on the mass of the star. If it is small, like our sun, it quietly ends its life in white wool. If it is a little more massive, it can become a neutron star, a kind of ball of ultra-dense matter, where protons and electrons have crushed to form neutrons. And if the star is really huge, we reach a limit. The famous limit of Tolman-Oppenheimer-Golkoff. Basically, beyond a certain mass, even the pressure of neutrons is no longer enough to hold against gravity. To make you realize, the pressure of neutron degeneration simply means that neutrons can no longer superimpose, be in the same place in space. And so, if gravity takes it away, there is only one outcome left. Total collapse. The star collapses on itself, without resistance, and a black hole is born. But it's not the only way. We can also make a black hole by collision. For example, when two neutron stars fuse, the energy released is colossal. And sometimes, the result is a black hole. We can also create it by accumulation of matter. A dense object that swallows more and more gas around it, ends up exceeding the critical limit and collapsing. And depending on their size, we distinguish three main categories. Stellar black holes are those born directly from the death of a star. They weigh between a few solar masses and a few dozen solar masses. Then there are supermassive black holes, those found in the center of galaxies. They are monsters. They weigh millions, even billions of times the mass of the Sun. And then there is an intermediate category. Intermediate black holes. Between a hundred and a few thousand solar masses. These are more difficult to observe, but they probably exist in star clusters. So be careful, this classification, depending on the mass, has nothing to do with the parametric classification we were talking about earlier. These are two different ways to look at black holes. On the one hand, according to their mass, on the other, according to their fundamental parameters. The standard model, the one that creates black holes from stars or collisions, works pretty well. But it has its limits. We said it earlier, in the very young universe, we found supermassive black holes when there was not yet time to form big enough stars. Then to make them explode, then to let them grow. And this is where an even stranger hypothesis appears. That of primordial black holes. The idea is simple, but dizzying. Maybe some black holes never had stars to form. Maybe they appeared directly in the first moments of the universe. Just after the Big Bang, when the universe was an incredibly dense and warm soup, there were fluctuations, small variations in density, like bumps in a tight fabric. And some of these bumps could have been so dense that they collapsed immediately to give black holes. Not star corpses, but fossils of the primordial universe. We even think it could be related to the inflaton. This hypothetical field that would have caused cosmic inflation. This ultra-fast expansion just after the Big Bang. If this field was not perfectly homogeneous, if there were no small irregularities, fluctuations, this difference could have created heavier areas that would collapse on themselves. It would give primordial black holes, scattered all over the universe. When we say they form from nothing, we must understand what it means in cosmology. Nothing is not the complete void. It is rather the absence of structures as we know them today. At the time, there were no galaxies, no stars. Just a dense, hot state, mostly filled with hydrogen and helium. This is where the primordial nucleosynthesis took place. That is, the formation of the first atomic nuclei, a few minutes after the Big Bang. And that, we still have the proof today. The proportions of helium and hydrogen observed in the universe correspond exactly to what the Big Bang model predicts. So, we know that this phase has existed. And in this phase, there could have been conditions to make black holes directly, without going through the stars. If these primordial black holes exist, then they change everything. Because there could be phenomena that we do not understand yet, like the origin of black matter. But that's another story. Today, when we look far into the universe with our telescopes, we come across a problem that makes a lot of astrophysicists We have discovered huge black holes, which make millions or billions of times the mass of our sun, which already existed very early in cosmic history. And when I say early, it's not a few billion years after the Big Bang. As we said, it's a few hundred million years, while the universe was still a baby. This means that we find them at times where, in theory, they did not even have enough time for them to grow so much. And yet they are there, sitting in the center of the first galaxies, as if nothing had happened. Normally, with the models we have, the black holes would have to leave small, from massive stars, then they grow little by little, by swallowing gas, or by merging with other black holes. But this process takes a crazy time, especially in a universe that did not yet have a lot of organized matter to offer. So how to explain that some black holes already reached billions of solar masses, while the universe did not even have a billion years of age? These recent observations, especially thanks to the James Webb Telescope, are quite clear. There were no very massive structures, very early. And that, our equations have a hard time justifying it. Suddenly, astrophysicists find themselves exploring more exotic paths. And that's where the famous primordial black holes come from. This scenario has a seductive side, because it responds exactly to the problem of time. No need to wait hundreds of millions of years. If some black holes were present from the very first seconds, then, inevitably, they had a huge advance on the others, to grow, merge, and become the giants that we observe in the center of early galaxies. But the problem is that we do not yet have direct evidence of their existence. An elegant hypothesis, but for now, it remains a hypothesis. And that, it brings us to a huge limit in our current knowledge, our instruments. But with James Webb, which is today the most powerful telescope in the infrared, we can not go beyond a certain point. The further we look, the more we go back in the past. But the light becomes weak, stretched by the expansion. And our detectors reach their limits. So we juggle with the signals we have. The light of the quasars, very far away, the emissions of the nascent galaxies, and sometimes, the clues left by the gravity of these black holes on their environment. But to confirm that these are primordial black holes, it's too light. What we need are totally new means of detection. For example, gravitational waves. It's been exactly 10 years that with LIGO and VIRGO, we have already heard the signal of collision of black holes and neutron stars. But these instruments remain sensitive to masses, and therefore, to precise frequencies. To really test the idea of ​​primordial black holes, we will have to wait for the next detectors. Like LISA, which will be launched into space, or Einstein Telescope, and Cosmic Explorer, which should be able to listen to signals much older and more subtle. These machines could reveal if a tiny black hole, left by the Big Bang, continues to exist and merge today. And if one day we prove their existence, the consequences would be enormous. It would mean that our model of the formation of structures must be reviewed. It would mean that galaxies, instead of first appearing and then welcoming black holes in the center, may have been built around black holes already present. Basically, black holes would not be secondary products of cosmic evolution, but main actors from the start. And it would completely change the way the history of the universe is told. And if we really detected primordial black holes, it would also be an open window to a new physics. Because they are directly linked to the conditions of the ultra-young universe, long before light is released. It's like gravitational fossils, remains that nothing else can tell us. It could give us information on inflation, on the density of the universe at its beginning, or even on particles that we have never seen. In the meantime, we remain in this area of ​​uncertainty. We know that there is a problem with early black holes. We have a hypothesis that could solve it, but we have no proof. So each new observation, each technological advance, becomes a small step towards this answer. Maybe James Webb will find clues again. Maybe Lisa will listen to the distant sound of a fusion of primordial black holes. Or maybe we won't find anything, and the mystery will get even deeper. What is certain is that we have an important time for science. For the first time, we have the tools to ask these questions seriously. And even if we don't have the answers yet, the simple fact that the universe could be populated with objects born a few fractions of a second after the Big Bang, hidden forever, is already quite dizzying, to make you want to keep looking. Abel 2744-QSO1 is not just a strange object. It is a real cosmic enigma. And above all, it is a unique opportunity to test our theories on the birth of the very first black holes of the universe. Today, scientists are studying two great clues to try to understand where this black hole comes from, so massive, so isolated, and so young. The first hypothesis is that of the direct collapse of a gas cloud. In this scenario, no star has had time to form. Here, we are talking about a huge very pure gas cloud, present shortly after the Big Bang, which suddenly collapses under its own weight, and directly forms a black hole. But for it to work, several conditions must be met. First, the gas must be perfectly stable, neither too hot nor too agitated. There must be no starlight nearby. Otherwise, the gas would risk being heated, and therefore it would prevent the collapse. And finally, this gas must also be quite dense, so that gravity takes over all other forces. In theory, it is possible, but in practice, these conditions are very rare. Simulations show that this kind of environment can exist, but that it is often unstable and does not last very long. That is why scientists are looking at a second hypothesis, that of the primordial black hole. It is even more radical, but also more fascinating. What if Abel 2744 QSO1 was a primordial black hole? That is to say, formed just after the Big Bang, long before the first stars or galaxies. At that time, the universe was just a chaos of particles, very hot, very dense, and rapidly expanding. And some areas could have been a little denser than others, forming bumps in the matter. And if these bumps were quite compact, they could have collapsed directly into a black hole, without needing stars. Abel 2744 QSO1 would therefore be one of these black holes, born at the beginning of the universe. It would not be the result of a galaxy, but rather one of the first bricks around which a galaxy would have begun to form. But this construction would not be finished yet, even 700 million years after the Big Bang. The problem is that we can not yet cut between these two hypotheses. When observing such a distant object, it is like trying to look through a fog, with very low light that has traveled for 13 billion years. And yet, Abel 2744 QSO1 is there, a huge black hole, isolated, surrounded by almost nothing, and that should not exist according to our current models. An example that contradicts all our rules. So, is Abel 2744 QSO1 the first of a new class of cosmic objects? Or is it, on the contrary, a unique case, a rare cosmic accident, lost in the immensity of the cosmos? Well, today, we do not know yet. But with each new observation of the James Webb telescope, a new piece of the puzzle is added. And maybe, one day, the complete image will finally reveal to us how the first monsters of the universe were really born. So, can we really form a black hole, from nothing? At first glance, the question seems completely crazy. How could something so extreme, so dense, come out in a universe that, at the beginning, had no stars, no galaxies, nothing structured? And yet, recent observations force us to take this hypothesis seriously. When James Webb points his instruments to the cosmic past, and that he detects massive black holes at a time when, normally, there should not have been any, it is all our understanding that wavers. Because if one of these monsters appeared before everything else, then it means that nothing from the very beginning was not really empty. He could already hide the necessary conditions to create a black hole. And this is not just a hypothetical thing. Several clues are beginning to go in this direction. The idea of ​​primordial black holes formed a few fractions of a second just after Big Bang, which is gaining more and more weight. If it is correct, it completely changes the way we tell the story of the universe. Black holes would not only be the final product of the evolution of stars. They would become the main actors from the start. Maybe even seeds around which galaxies were built. In other words, black holes would not be consequences, but causes. It's dizzying, but it's still a possibility among others. So for now, you got it, we don't have direct evidence. So science is moving forward, as always, step by step, observation after observation. It will be necessary to secure other black holes even older, more massive, like Abel 2744-QSOR, and wait to see if new instruments detect unexpected signals. The future missions, that they observe the light of the first quasars, or that they listen to the gravitational sounds of the fusion of these objects will be decisive. Because basically, what we call nothing, in the cosmological context, is not an absolute void. It's a dense, hot, unstable state, filled with particles and energy. Maybe in this initial chaos, the seeds of what would become the most extreme structures of the universe were already hidden. And if it's true, then the universe tells us something deep. Even nothing can contain the seeds of the end. Thank you very much for following this video. I put two other videos in link with black holes If you want to know more about it. I hope you liked it. See you soon. Don't forget to subscribe and like the video if you liked it. See you soon.