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 of interest to scientists, and it seems that we are close to reaching evidence of its validity, but to understand the matter from its door we turned to Bernard Carr, an emeritus professor of mathematics and astronomy at Queen Mary University in London, and he is one of those who worked in The center of this hypothesis for more than half a century, to explain to us its story and its consequences, in a rich material, but as easy as possible, that will open a door for you to dive into the innermost interesting and exciting science of cosmology.
Translation text:
Almost 50 years ago, when I was a PhD student, I wrote an article for this magazine New Scientist about the abundance of evidence for the existence of black holes, regions of space where gravity is so strong that light cannot escape its grip. My endeavours, since then, have yielded so much that today we have no doubt about their existence, that they are formed from the remnants of collapsing stars, and that giant black holes lie at the centers of galaxies, and we have even taken a picture of two of them. But in that article I wrote at the time, I also mentioned some speculative possibilities that micro-sized black holes might have formed in the early universe, shortly after the Big Bang.
I began working on this idea under the supervision of astrophysicist Stephen Hawking, who had also begun ruminating on this possibility just a few years earlier. Our work together defined the course of my career, much of which I have devoted to the study of what we now call "primordial black holes". However, our understanding of these holes is still clouded by a fog, but there are good reasons to believe that they formed in the early universe, and some may still exist today, and the most interesting thing is that their existence may lead to answers to a whole host of cosmic mysteries which cannot be decoded.
An even stranger possibility
Recently, jittery thoughts have exploded in my head, and I have become interested in an even stranger possibility, which is that some black holes may be older than the age of the universe itself. It sounds like a wild idea, but it's not far-fetched, and it's a breakthrough that will fundamentally change our understanding of cosmology. Most cosmologists believe that matter and energy permeating all of space sprang into existence in a single moment 13.8 billion years ago, a moment they call the Big Bang. The Big Bang was followed by a stage in which the universe touched its way to a new form and grew at a tremendous speed known as "cosmic inflation", a stage that the universe adopted before it followed a more moderate approach in its expansion. (Translator: The expansion of the universe is like drawing a group of dots on a balloon before it is inflated, then after that the balloon is inflated and you notice that the dots are moving away from each other, not because they are running fast but because the balloon itself is inflating, the universe is likewise, the galaxies are moving away from each other, not because they are running away It's 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 caused the universe to begin to inflate). Scientists often point out that our universe emerged from a point of origin called the singularity, a point characterized by its infinite density and gravity, at which all the laws of physics collapse, even Albert Einstein's theory of general relativity – which is the best description we have of gravity – collapses at this point , which makes describing it with the usual equations that explain reality difficult to obtain, and understanding it closer to impossible.
This led some cosmologists to speculate that the universe began with a Big Bounce, so instead of everything coming into existence in one instant, the Big Bounce theory suggests that our universe arose as a result of a previous universe that collapsed and began expanding again (meaning that the universe that preceded ours It 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, which is ours). The Big Bounce theory is similar to the Big Bang but without a singularity, given that the universe has always had a finite density (as opposed to a singularity, which is characterized by its infinite density).
The Big Bounce scenario is compatible with certain attempts to unify the laws of physics, including models of quantum cosmology, loop quantum gravity (a theory that attempts to combine quantum mechanics with general relativity), as well as some alternative theories of gravity. If our universe arose from a bounce, then logically it would suffer the same fate as the one before it and end up collapsing. This type of recursive bounce, in which the universe goes through periods of expansion and compression, is called a "cyclic" or "cascade" universe. However, this theory can only make sense if the universe is destined to collapse, and this in turn depends on the nature of dark energy, that mysterious force that is causing the universe to fall apart faster and faster. But if we find evidence for the existence of these regressive and cyclical patterns, it has huge implications for both how the universe began and how it ends up.
But finding evidence like this is not an easy matter, because everything that existed in the previous universe may have been destroyed when the universe was rocked by a great collapse, and this may lead us to a more important question: Was everything actually destroyed at that moment, or was what resulted from this process? with this picture? It is my belief that there is a chance of survival for some of the black holes that continued their quest from a previous universe groping their way to ours and still exist today.
The idea that black holes may have formed in the early universe dates back to the early 1970s. At this point, Stephen and I ruminated on whether black holes could form from fluctuations in density that occurred near the Big Bang. Although our calculations showed that this was indeed possible, we ran into a snag, because a few years earlier, Russian researchers Yakov Zeldovich and Igor Novikov had shown that any black holes that formed in the early universe would have accelerated their growth to the massive mass of today. And since accurate calculations and reliable observations ruled out this hypothesis, given that we would have seen traces of such huge black holes if they really existed, the researchers concluded in the end that primordial black holes never formed.
On the other hand, my first paper with Stephen Hawking proved this conclusion wrong by the two Russian researchers. After several days of intensive calculations, I rushed excitedly to his office to inform him of the good news I had reached, which is that primordial black holes could not have grown at such a rapid pace. Never due to the expansion of the universe, which the researchers did not think about. I discovered that Stephen had also come to the same conclusion by doing the calculations in his head, and this made me sigh of sorrow, because we both came to the same conclusion (which we thought was wrong and we were suspicious because of the Russian scientists). After all, primordial black holes seem to have already existed from the beginning.
Fifty years later, we still haven't seen any of these black holes with certainty, although some believe that there are signs that suggest their presence, which are ripples in space-time called gravitational waves. It is true that we cannot be sure of its existence, but we are at least certain that thinking about it prompted Stephen Hawking to discover the radiation that black holes emit, known as “Hawking radiation”, as well as his discovery of the “black hole information paradox” (a problem that still exists in cosmology, It says that when black holes evaporate, they will lose information about what has fallen into them.
Of course, it will be an exciting and significant moment if scientists prove that primordial black holes did indeed form in the early universe, and interest in this idea has really increased in recent years. We know that any black hole weighing less than a trillion kilograms – about the mass of a mountain – and the size of a proton would have already evaporated into our time due to Hawking radiation, while any larger black hole would survive and resume its trajectory into our day. this. Still, there is an even more intriguing possibility. 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 contraction and inflation.
Big bang and strong crush
We started to think about whether black holes formed in a previous cosmic cycle, and we realized that 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 crunch” stage, which is exactly like 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, while the Big Bang phase occurs when our universe is in its throes and springs into being. So if black holes could have formed at the time of the Big Bang, they could also have formed in the "big crunch" phase of the universe.
In this case, black holes would have a minimum mass determined by the density of the universe at the bounce, and if this density is low enough, black holes could reach a size large enough to account for dark matter, or that mysterious stuff that keeps galaxies from disintegrating and flying away. Or maybe they help learn about the origin of supermassive black holes. To delve deeper into this topic, Jerome Quentin and Robert Brandenberger of McGill University in Montreal, Canada, collaborated to calculate the quantum and thermal fluctuations of a collapsing universe, and the two scientists discovered that black holes can actually form, but only in one case, which is when "matter" is dominant in the universe. Not "radiation".
As for the second possibility, it indicates that black holes formed at an early stage in the life of the previous universe, just like the black holes that were formed from the collapse of stars or galactic nuclei in our universe (which is a congested region in the center of very bright galaxies).
Either way, the next question occurred to us: whether pre-Big Bang black holes would survive the bounce and cross into our present universe. We concluded that this depends on the space occupied by black holes in the universe when bouncing, and one can expect black holes to survive and resume their path to reach our current universe if their size at the time of bouncing is greater than their usual size, because in this case they will not go through a stage of compression and merging together. We concluded in the end that this scenario should be possible in many cases.
In 2015, I teamed up with my colleagues Timothy Clifton at Queen Mary University of London and Alan Cooley at Dalhousie University in Canada to try to approach this question in a mathematically rigorous way. We worked out some precise solutions to Einstein's equations of general relativity to be able to describe a regular group of black holes present in a universe undergoing a bounce, and our results concluded that there are indeed possibilities for different black holes to survive these endless cycles of destruction and renewal, which is what cosmologists call a "bounce" cosmic".
Later, we also investigated some cosmological consequences of this proposal claiming that pre-Big Bang black holes could explain dark matter, provide the first building blocks for galaxy formation, and may also be the main factor in the genesis of the bounce itself. What we probably don't realize is that primordial black holes born before the stage of cosmic inflation appear in the usual scenario of the big bang to be very weak and fragile, so it is usually assumed that any of the black holes present in our time formed after the stage of inflation, unlike some models of regression that do not There is a stage of hypertrophy.
These ideas gave way to new research, which was later taken up by other researchers at greater length. In 2018, Carlo Rovelli of Aix-Marseille University in France, and theoretical physicist Francesca Fedotto of Western University in Ontario, Canada, decided to study the possibility that dark matter is composed of remnants of black holes that existed before the Big Bang, and they argued that only a small part Of the size of the universe will be outside these black holes at the time of the recoil, although observers of these regions will see a homogeneous universe at later times. An even more exotic possibility is that the recoil compresses the universe so hard that all the black holes merge together. Even the giant black holes we know exist today could lead to the same situation if our universe eventually collapsed in on itself.
These gradual mergers will play a role in giving birth to larger black holes with a chain of increasing mass until eventually the entire universe 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 Korman of the Perimeter Institute in Canada, the two groups agreed – despite the differences in the details of their calculations – After studying the behavior of a single black hole during the bounce, the black hole can survive these endless cycles of destruction during the bounce, and that its size may shrink for some time. On the other hand, this shrinkage might also motivate us to consider the possibility that these black holes will not eventually merge completely into a single black hole.
Experimental evidence is also required
It is true that all these efforts are admirable and cannot be considered to lead to emptiness, but is it not natural to ask 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 lurking in the centers of current galaxies, has appeared to give us some hope that we may one day be able to identify Black holes before the Big Bang, which means we will be able to distinguish them from other black holes that formed in our universe.
The mass of these supermassive black holes ranges from 1 million to 10 billion times that of our Sun. Through our observations of the distant universe, we have come to realize that these supermassive black holes already existed too early in the life of the universe to be formed by usual astrophysical processes. Therefore, it is still vague and beyond our comprehension, as how can it grow to such a size and at such a high speed in that short period? In order to reach an answer to this question, there is a possibility, although not within the prevailing views, that the massive black holes were originally generated by the primordial black holes.
In the same way, this possibility brings us to the most important question: is there a way to know whether these primordial black holes came from a big bang or from a large cosmic bounce? So Kay and colleagues decided to model density fluctuations in both the inflation and rebound scenarios to compare the two models, and the team predicted that the number of supermassive black holes would decrease sharply as mass increased in the rebound phase. However, we do not currently have sufficient data to distinguish between the two scenarios, but future observations that the James Webb Telescope will lead to us may provide us with this.
The existence of primordial black holes that formed in this universe is just an assumption, which means that the idea of the existence of black holes from a previous universe is still a more speculative idea. Nevertheless, it remains important to explore this possibility, not to mention the sense of exhilaration and excitement that will come to us once this idea is confirmed. And just as thinking about primordial black holes has 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 cyclical (or continues in a series of endless expansions and contractions).
I have recently retired, and I find it oddly fitting that my career that began studying the formation of black holes at the beginning of this universe should end with their final form at the end of the universe. My article concluded 50 years ago that “black holes are common in theory though elusive in observations”, but now I am more optimistic about finding primordial black holes, whether they formed in a previous universe or not.
Perhaps the most important point of the idea of primordial black holes is that it prompted Stephen Hawking to think about the quantum effects of black holes, particularly when he discovered that black holes emit Hawking radiation. It is important to be well aware that only primordial black holes can be so small that this radiation is enough to cause them to disappear, and thinking about this point also prompted 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 means of the wave function, but Hawking believes that after the black hole has vanished, we will not have a clear wave function to infer the information associated with the particles that were inside the black hole).
This means that Hawking's study constituted a claim that contradicts the laws of quantum mechanics, which assert that information cannot be lost or destroyed in a black hole, while Hawking argued that annihilation will lurk once the black hole evaporates. But he later changed his mind, and we are still pondering a solution to this paradox to this day. Hawking radiation is considered one of the most important discoveries in twentieth-century physics for its ability to unite together three different areas of physics: quantum theory, general relativity, and thermodynamics. So there is no denying that what Hawking came up with is a wonderful result in the end, so much so that the physicist John Wheeler – who coined the term black hole – once told me that talking about Hawking radiation and black holes is like the pleasant feeling that flows into our souls when we "roll candy on the tongue ".
As someone who previously collaborated with Stephen Hawking, I consider myself fortunate enough to be so close to Hawking and follow these developments that he was working on. So even if primordial black holes never formed, the discovery of Hawking radiation during these searches shows just 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's website.
Translated by: Somaya Zaher .
