Introduction to translation:
What if our universe originated from another universe that preceded it? For nearly three quarters of a century, this hypothesis has been the focus of scientists, and it seems that we are close to finding evidence of its validity, but to understand the matter from his door, we turned to Bernard Carr, an emeritus professor of mathematics and astronomy at Queen Mary University of London, one of those who worked at the center of this hypothesis for more than half a century, to explain its story and its consequences, in a rich but easy material as possible, it will open a door for you to dive into the interior of fun and exciting cosmology.
Translation text:
Almost 50 years ago, when I was a PhD student, I wrote an article in New Scientist about the abundance of evidence for the existence of black holes, regions of space where gravity is too strong for light to evade its grip. Since then, my endeavor has yielded so much that we have no doubt about their existence today, that they are formed from the remnants of collapsed stars, that giant black holes are located at the center of galaxies, and that we have even taken pictures of two of them. But in that article I wrote at the time, I also mentioned some speculative possibilities suggesting that tiny black holes may have formed early in the universe, shortly after the Big Bang.
I started working on this idea under the direction of astrophysicist Stephen Hawking, who also began ruminating on this possibility just a few years ago. Working together has set the course of my career, much of which I have devoted to studying what we now call "primordial black holes." However, our understanding still clouds the fog around these holes, but there are good reasons to believe that they were formed in the early universe, and some of them may still exist today, and most interestingly, their existence could lead to answers to a whole host of undecipherable cosmic mysteries.
More bizarre probability
Rather than everything coming into existence in a single moment, the Great Reversion theory suggests that our universe arose as a result of a previous universe that collapsed and began to expand again. (Shutterstock)
Recently, crashing thoughts burst into my head, and I became concerned with the even stranger possibility that some black holes could be older than the age of the universe itself. It sounds like a wild idea, but it's not out of the question, a breakthrough that could radically change our understanding of cosmology. Most cosmologists believe that matter and energy that penetrate into all parts of space emerged into existence at a single moment 13.8 billion years ago, the moment they called the "Big Bang." The Big Bang was followed by a phase in which the universe felt its way into a new form and grew rapidly known as "cosmic inflation," a phase that the universe adopted before it took a more moderate approach to expansion. (Translator: The expansion of the universe is like drawing a set of dots on a balloon before blowing it, and then the balloon blows and you notice that the dots move away from each other, not because they run fast but because the balloon itself is inflating, the universe is so, galaxies are moving away from each other, not because they run away but because the universe is expanding.)
The problem with this picture is that we don't know for sure what exactly happened at the moment of the Big Bang (or what drove the universe to start swelling). Scientists often point out that our universe emerged from a point of origin called Singularity, a point characterized by its infinite density and gravity, at which all the laws of physics collapse, even Albert Einstein's general theory of relativity - which is our best description of gravity - collapses at this point, making describing it as the usual equations that explain reality difficult to achieve, and understanding them closer to impossible.
Instead of everything coming into existence in one moment, the Big Reversion theory suggests that our universe arose as a result of a previous universe that collapsed and began to expand again (meaning that the universe that preceded our universe went into a state of contraction until a certain point in the Big Bang for us, after which it began to expand again, leading to a new universe, our universe). The Big Bang theory is more like a Big Bang but without singularity, since the universe always has a finite density (as opposed to the singularity, which is characterized by its infinite density).
The idea that black holes may have formed in the early universe dates back to the early seventies. (Shutterstock)
The "Great Reversion" scenario corresponds to certain attempts to unify the laws of physics, among them models of quantum cosmology, cyclic quantum gravity (a theory that attempts to integrate quantum mechanics with general relativity), as well as some alternative theories of gravity. If our universe arises from apostasy, it makes sense that it will be beset by the same fate that preceded it and end up collapsing. This type of repetitive regression, in which the universe goes through periods of expansion and pressure, is called a "cyclical" or "cascading" universe. But this theory can only make sense if the universe is to collapse, which in turn depends on the dark nature of energy, the mysterious force that causes the universe to break apart faster and faster. However, if we find evidence of these regressive and cyclical patterns, it would have enormous implications for both how the universe began and what it ultimately would become.
But finding such evidence is not easy, because everything that existed in the previous universe may have been destroyed when the universe was hit by a great collapse, and this may lead us to the most important question: Was everything really destroyed at that moment, or was the outcome of this process not like this? I believe there is a chance of survival for some of the black holes that have continued their quest from a previous universe groping their way to our present universe and still exist today.
The idea that black holes may have formed in the early universe dates back to the early seventies. Stephen and I knocked thinking about whether black holes could have formed from the fluctuations of density that occurred near the Big Bang. Although our calculations showed that this was indeed possible, we ran into a hindrance, because a few years ago, Russian researchers Yakov Zeldovich and Igor Novikov explained that any black holes formed in the early universe would presumably adopt a rapid pace in their growth to reach a huge mass today. The researchers eventually concluded that primordial black holes never formed.
Astrophysicist Stephen Hawking. (Reuters)
After several days of extensive calculations, I enthusiastically rushed to his office to inform him of the good news that primordial black holes would never have grown so fast due to the expansion of the universe, which the researchers did not think about. I discovered that Stephen also came to the same conclusion by doing the calculations in his head, and it provoked sighs of sorrow deep down, because we both came to the same conclusion (which we thought was wrong and the spectrum of doubt loomed in our minds because of the two Russian worlds). After all, it seems that primitive black holes did exist from the start.
Fifty years later, we still haven't seen any of these black holes with certainty, although some believe there are signs of their presence, ripples in spacetime called gravitational waves. We can't say for sure it exists, but we're at least sure that thinking about it led Stephen Hawking to discover the radiation released by black holes, known as "Hawking radiation," as well as his discovery of the "black hole information paradox" (a problem that still exists in cosmology, which says that when black holes evaporate, they lose information about what has fallen to them).
Of course, it will be exciting and eloquent moments if scientists prove that primordial black holes actually formed in the early universe, and interest in this idea has already increased in recent years. We know that any black hole weighing less than a trillion kilograms – roughly the mass of a mountain – and the size of a proton should have already evaporated to the present day due to Hawking radiation, while any larger black hole would survive and resume its march to this day. About 10 years ago, Alan Cooley of Dalhousie University in Halifax, Canada, shared my interest in whether we live in a cyclical universe, fluctuating between deflation and inflation.
A great explosion and a strong crush
One can expect black holes to survive and resume their path to reach our current universe if at the time of the rebound they are larger than their usual size, because in this case they will not go through a phase of pressure and fusion together. (Shutterstock)
We started to think about whether black holes formed in a previous cosmic cycle, and we realized there were two possibilities. The first possibility indicates that it was formed as a result of the high density of the previous universe in the last moments of its collapse, in what is known as the "Big Crush" phase, which is exactly similar to the stage in which the universe was very dense after the Big Bang, but the difference here is that it occurs at a time when the universe is on the brink of the abyss, while the Big Bang phase occurs at a time when our universe is going through difficult labor and emerging into existence. So if black holes could have formed at the time of the Big Bang, they could also form in the "extreme crushing" phase of the universe.
In this case, black holes would have a minimum mass determined by the density of the universe at the rebound, and if that density is low enough, black holes could reach a size large enough to explain dark matter, or those mysterious things that prevent galaxies from disintegrating and flying away, or perhaps help identify the origin of supermassive black holes. To dig deeper into the topic, Jerome Quentin and Robert Brandenberger of McGill University in Montreal, Canada, collaborated to calculate the quantum and thermal fluctuations of a collapsed universe, and the two scientists discovered that black holes can indeed form, but only in one case: "matter" dominating the universe and not "radiation."
The second possibility suggests that black holes formed early in the previous universe, just like black holes formed from the collapse of stars or galactic cores in our universe (a region stacked at the center of highly bright galaxies).
Either way, the question arose: whether the black holes before the Big Bang would survive the rebound and cross into our current universe. We concluded that this depends on the space occupied by black holes in the universe at the time of recoil, and one can expect black holes to survive and resume their path to reach our current universe if at the time of the recoil were larger than their usual size, because in this case they would not go through a phase of pressure and fusion together. We ultimately concluded that this scenario should be possible in many cases.
It is usually assumed that none of the black holes that exist today formed after the inflation phase, unlike some rebound patterns that do not have an inflation phase. (Shutterstock)
In 2015, I teamed up with my colleague Timothy Clifton of Queen Mary University of London and Alan Cooley of Dalhousie University in Canada, in an attempt to address this question in a mathematically precise and rigorous manner. We derived some precise solutions to Einstein's equations of general relativity to be able to describe a regular set of black holes found in a universe in a rebound phase, and our results concluded that there are indeed possibilities that different black holes will survive these endless cycles of destruction and regeneration, which cosmologists call "cosmic rebound."
Later, we also investigated some cosmic consequences of this proposal claiming that black holes before the Big Bang could explain dark matter, provide the first building blocks for galaxies, and may also be the main factor in the genesis of the rebound itself. Perhaps what we don't realize is that the primordial black holes born before the cosmic inflation phase appear in the usual Big Bang scenario very weak and fragile, so it is usually assumed that none of the black holes that exist today formed after the inflation phase, unlike some rebound models that do not have an inflation phase.
These ideas gave way to new research that other researchers later elaborated on. In 2018, Carlo Ruffelli of the University of Aix-Marseille in France and theoretical physicist Francesca Fidoto of Western University in Ontario, Canada, decided to study the possibility that dark matter was made up of remnants of black holes that existed before the Big Bang, arguing that only a fraction of the size of the universe would be outside these black holes at the time of the rebound, although observers of these regions would see a homogeneous universe at later times. The most bizarre possibility is that the recoil is pressing on the universe so hard that all the black holes merge together. Even the supermassive black holes we know exist today could lead to the same situation if our universe eventually collapses on itself.
These gradual mergers will play a role in the birth of larger black holes with a sequence of increased mass until the whole universe eventually turns into a single black hole. No one knows what might happen in this case, but according to a new study by cosmologists Daniela Perez and Gustavo Romero of the Argentine Institute of Radio Astronomy, and a team led by theoretical physicist Maxence Corman of the Perimeter Institute in Canada, the two groups, although the details of their calculations differ, agreed after studying the behavior of one black hole during rebound that the black hole can survive these endless cycles of destruction during rebound, and that its size may shrink for some time. On the other hand, this contraction may also prompt us to reflect on the possibility that these black holes will not eventually merge completely into a single black hole.
Empirical evidence is also required
The existence of primordial black holes formed in this universe is just an assumption, meaning that the idea of black holes from a previous universe is still more speculative and speculative. (Shutterstock)
It is true that all these efforts are remarkable and cannot be considered to lead to a vacuum, but is it not natural to wonder about the evidence? Fortunately, a recent study led by Yi Fu Kai of the University of Science and Technology in China, who is interested in the idea that primordial black holes may be responsible for the birth of giant black holes crouching at the centers of current galaxies, offers us some hope that we may one day be able to identify black holes before the Big Bang, meaning we will be able to distinguish them from other black holes that have formed in our universe.
The mass of these massive black holes ranges from one million to 10 billion times the mass of our Sun. Through our observations of the distant universe, we realized that these holes of magnitude did exist too early in the universe to form the usual astrophysical processes. So it's still vague and incomprehensible, because how can it grow so big and so fast in such a short period? To answer this question, there is a possibility, though not a popular view, that massive black holes originally produced primordial black holes.
In the same vein, this possibility begs us to the most important question: Is there a way to know whether these primordial black holes came from a Big Bang or a cosmic backfire? So Kay and his colleagues decided to model density fluctuations in both the inflation and rebound scenario to compare the two models, and the team predicted that the number of supermassive black holes would decrease sharply as the mass increased in the rebound phase. At the moment, we don't have enough data to distinguish between the two scenarios, but future observations from the James Webb Telescope could provide us with that.
The existence of primordial black holes formed in this universe is just an assumption, meaning that the idea of black holes from an earlier universe is still more speculative. However, it remains important to explore this possibility, not to mention the sense of joy and excitement that will come to mind once this idea is confirmed. Just as thinking about primordial black holes led to important insights into quantum gravity, thinking about the existence of black holes before the Big Bang could lead to more tangible physical insights, even if it turns out that the universe is not periodic (or continuing in a series of endless expansions and contractions).
Hawking radiation is one of the most important discoveries in twentieth-century physics, for its ability to unite three different physical fields together: quantum theory, general relativity, and thermodynamics. (Reuters)
I recently retired, and I think it's strangely fitting that my career, which began with studying the formation of black holes at the beginning of this universe, ends with their final form at the end of the universe. My article concluded 50 years ago that "black holes are theoretically widespread, though elusive during their observations," but I am now more optimistic about finding primitive black holes whether they formed in a previous universe or not.
Perhaps the most important point of the idea of primordial black holes was that it led Stephen Hawking to think about the quantum effects of black holes, especially when he discovered that black holes emit Hawking radiation. It is important to realize that only primordial black holes can be so small that this radiation causes them to disappear, and thinking about this point also led Hawking to conclude the phenomenon of the "black hole information paradox" (according to the laws of quantum mechanics, we can infer the state of any system at any moment by the wave function, but Hawking argues that after the black hole has vanished, we will not have a clear wave function to infer information associated with particles that were inside the black hole).
This means that Hawking's study constituted a claim that contradicted the laws of quantum mechanics that assert the impossibility of information being lost or destroyed in a black hole, while Hawking argued that annihilation would lurk once the black hole evaporates. But he later changed his mind, and we are still thinking about a solution to this paradox to this day. Hawking radiation is one of the most important discoveries in twentieth-century physics, for its ability to unite three different physical fields together: quantum theory, general relativity, and thermodynamics. So it's undeniable that Hawking's findings are such a remarkable result in the end that the physicist John Wheeler, who coined the term black hole, once told me that talking about Hawking's radiation and black holes is like the pleasant feeling that flows into our souls when "rolling candy on the tongue."
As someone who has previously collaborated with Stephen Hawking, I find myself fortunate enough to be so close to Hawking and to follow these developments that he has been working on. So even if primordial black holes never formed, the discovery of Hawking's radiation during these searches shows how important it was to think about them.
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This article is translated from New Scientist and does not necessarily reflect Meydan.
Translation: Somaya Zaher.