Some cosmologists believe that there was a dark and empty universe, very similar to the one in the distant future, which could have been at the origin of our own big bang
“The last star will cool little by little and in the end it will fade. With his death the universe will once again be an empty place devoid of light, life, or meaning.”
Alastair Wilson , University of Birmingham
This was the warning of the physicist Brian Cox in an episode of the series Universe , from the BBC. The death of the last star will be only the beginning of an infinitely long and dark age. All matter will end up being devoured by monstrous black holes, which will later evaporate until they are reduced to faint flashes of light. Space would infinitely expand outward until even those faint flashes of light would be too spread out to interact in any way. There will be no activity of any kind.
Or maybe yes? Although it seems quite strange, some cosmologists believe that there was a dark and empty universe, very similar to the one that will exist in the distant future, which could have been at the origin of our own big bang .
The raw material
But before we get to that, let’s see how that “material” (that is, that physical matter) arose for the first time. If we are trying to explain the origin of stable matter made up of atoms or molecules, there really was none of that during the big bang (nor for the hundreds of thousands of years that followed). The truth is that we have a fairly detailed understanding of how the first atoms formed from simpler particles once conditions cooled enough for complex matter to be stable, and also how these atoms later fused together. with heavier elements within stars. But that knowledge does not answer the question of how something could come out of nothing.
So let’s go back a little further. The first long-lived particles of matter of any kind were protons and neutrons, which when joined form the nucleus of the atom. These came into existence approximately one ten-thousandth of a second after the big bang occurred. Before that, there was really no material at all in any of the usual senses of the term. But physics allows us to go even further back in time, to the physical processes that preceded the existence of stable matter.
This leads us to the so-called “ great unified epoch “, which in turn leads us to fully enter the field of speculative physics, since in our experiments we cannot generate the energy enough to reproduce the type of processes that were taking place at that time. But a plausible hypothesis is that at that time the physical world was made up of a hodgepodge of elementary particles of limited existence, among which were the quarks , that is, the fundamental units that make up the protons and neutrons . There was both matter and antimatter, and in practically equivalent amounts . Each type of matter particle, such as a quark , had an antimatter counterpart, a “mirror image” that was virtually identical to it, differing only in one aspect. However, matter and antimatter annihilate each other in a burst of energy when they meet, meaning these particles were constantly being created and destroyed.
But how did these particles come into existence? Quantum field theory tells us that even a vacuum that could supposedly correspond to zero space-time values is full of physical activity, and that this activity manifests itself in the form of energy fluctuations. These fluctuations can cause particles to appear, which however disappear soon after. All of this might sound more like mathematical wackiness than actual physics, but such particles have been detected in countless experiments.
The space-time vacuum state is altered by particles that are constantly created and destroyed, and that apparently “come out of nowhere”. But perhaps what all this really tells us is that the quantum vacuum, despite its name, is something instead of nothing . The philosopher David Albert is the author of a famous critique of approaches to the big bang which, based on this theory, promise to explain how something could have come out of nothing.
Let’s imagine that we wonder where space-time came from. In that case we could go back even further, to the really archaic “ Planck Epoch ”, a period so early in the history of the universe that it defies our best physical theories. This epoch spanned just one ten-millionth of a trillionth of a trillionth of a trillionth of a second after the big bang. At this point both time and space became the subject of quantum fluctuations themselves. Physicists usually work outside of quantum mechanics, which governs the microworld of particles, and also of general relativity, which applies to large cosmic scales. But to really understand the Planck Epoch we would need a unified theory of quantum gravity that fuses the two together.
We still do not have a perfect quantum gravity theory, but there are proposals such as string theory or loop quantum gravity . In these proposals, ordinary time and space are generally conceived as emergent elements, like waves on the surface of a deep ocean. And it is that what we experience as space and time is the product of quantum processes that operate at deeper, microscopic levels; processes that do not make much sense to us, who are creatures settled in the macroscopic world.
In the Planck Epoch our ordinary knowledge about space and time is blown up, so that we cannot continue to apply the ordinary logic of cause-effect relationships. Despite this, all possible theories of the field of quantum gravity hold that there was some kind of physical substance during the Planck Epoch; some sort of quantum precursor to ordinary space and time. But where did that come from?
Even if we take into account that in Planck’s epoch causality did not work in any of the usual ways, it would still have been possible to explain one of the components of the universe in terms of its correspondence with another. Unfortunately, today even our best physicists utterly fail to provide us with answers in this regard; Until we make further progress towards a “theory of everything” we will be unable to offer a definitive answer. The most we can say with certainty at this point is that physics has so far detected no confirmed examples of something coming out of nothing.
Cycles that arise almost out of nowhere
In order to really answer the question of how something can come out of nothing, we would need to be able to explain the quantum state of the universe as a whole during the beginning of the Planck Epoch. All attempts to carry out this task remain highly speculative, and there are even some that appeal to the existence of supernatural forces as an architect of the universe . But there are other theories that remain within the field of physics, such as the multiverse (according to which it contains an infinite number of parallel universes) or the cyclical models of the universe (which would be born and reborn again and again ).
Roger Penrose , winner of the 2020 Nobel Prize in Physics, has proposed a suggestive, but also controversial model of the cyclic universe , called “conformal cyclic cosmology”. Penrose was inspired by an interesting mathematical connection between a very hot, dense, small state of the universe (which is what it was like at the big bang) and an extremely cold, empty, expanded state of the universe (which is what it was like at the big bang). will be in the distant future). His radical theory to explain this correspondence is based on the fact that these states became mathematically identical when they reached their respective limits. As paradoxical as this may seem, a complete absence of matter could have caused the rise of all the matter that we see around us today in the universe.
From this point of view, the big bang would have come almost out of nowhere; it is what would have remained after all the matter in the universe had been engulfed by black holes that would later have evaporated, generating photons that would wander through the vacuum. In this way, the entire universe would have emerged from something that, seen from another physical perspective, would be the closest we could get to absolute nothingness. But that nothing would still be something; we would still be talking about a physical universe, even if it were empty.
But how is it possible that the same state of the universe is cold and empty from one perspective and hot and dense from another? The answer lies in a complex mathematical procedure called “conformal rescaling,” a geometric transformation that alters the size of an object, but not its shape.
Penrose showed how the cold and dense state, on the one hand, and the warm and dense state, on the other, could be related through these rescalings in such a way that they could correspond through the shapes of their respective space-times, although not their sizes. The truth is that it is difficult to understand how two objects can be identical according to this theory when their sizes are different, but Penrose argues that size as a concept ceases to make sense in such extreme physical environments.
In conformal cyclic cosmology, the direction of explanations goes from the old and cold to the young and hot: the dense and warm state exists because so does the cold and empty. But this “because” does not have the usual meaning (that of a cause followed in time by its effect). It’s not just that size is no longer relevant in these extreme states; is that time also ceases to be. In fact, the cold and dense state and the warm and dense state are located on different timelines. The cold, empty state would continue indefinitely in its own time geometry from an observer’s perspective, but would empower the hot, dense state to occupy a new timeline.
To try to understand that the dense and warm state is the product of cold and emptiness, it can be helpful to approach the question from some kind of non-causal perspective. Perhaps we could say that the warm dense state arises from , or is rooted in , or is discovered by , the cold and empty state. These are typically metaphysical ideas that have been developed in depth by philosophers of science , especially in the field of quantum gravity , where the classical cause-effect logic breaks down. And it is that, when we reach the limits of knowledge, it is difficult to separate physics from philosophy.
Conformal cyclic cosmology offers detailed, albeit speculative, answers to the question of where our big bang came from. But even if Penrose’s theories were validated by future developments in cosmology, one might think that we are still unable to answer a deeper philosophical question; the question of where physical reality itself comes from. That is, the question of how the whole system of cycles works.
In this way, we end up facing the stark question of why there is something instead of nothing (which, on the other hand, is one of the great metaphysical questions).
But here we want to focus on explanations that are limited to the realm of physics. There are three big options on the fundamental question of how the cycles began. There might not be any kind of physical explanation. Or it could be infinitely repeated cycles, each of which would make up a universe by itself, in which the initial quantum state of each universe would be the consequence of some characteristic of the previous universe. Or there could be a single cycle with a single universe repeating itself, so that the beginning of the cycle somehow explained its own end. The last two options do not require a specific causality, which gives them a special appeal. And it is that, in this way, nothing would be left out of a purely physical explanation.
Penrose conceived an infinite sequence of new cycles driven by a series of reasons partly linked to the interpretation of quantum theory that he believed to be the most accurate. In quantum mechanics a physical system exists in a superposition of several different states at once and only “chooses” one at random when we measure it. For Penrose, each cycle implies random quantum events that occur in a different way, which means that each cycle will be different from both the previous one and the next one. This really is good news for experimental physicists, as it would allow us to glimpse the old universe that gave rise to ours through fuzzy traces, or anomalies, in the leftover radiation generated by the big bang that can observe the Planck satellite.
Penrose and his collaborators believe that they could have already detected these traces in the information provided by the Planck satellite on the radiation emitted by supermassive black holes in a previous universe. However, the validity of these observations has been questioned by other physicists , so we remain without absolute certainty.
The indefinite succession of new cycles is fundamental in Penrose’s theory. But in conformal cyclic cosmology one can move naturally from a multi-cycle model to a single-cycle model. In the latter case, physical reality would consist of a single cycle that would range from the big bang to a state of maximum vacuum in the distant future… And then that same big bang would occur again. , which would result in an identical universe over and over again.
This last possibility is compatible with another interpretation of quantum mechanics, the so-called “multiple universes interpretation”. This holds that whenever we measure a system that is in a superposition, this measurement does not randomly select a state. Instead, the result of the measurement that we observe is just one possibility (the one that takes place in our own universe). The other results of the measurements take place in other universes of the multiverse, which are in effect completely independent of ours. Hence, it does not matter how small the chance of something happening is, because if it is not zero, it will have happened in some other of the quantum parallel worlds. There are people just like you who have won the lottery, been swept up into the clouds by a terrifying typhoon, spontaneously combusted, or all three.
Some people believe that these parallel universes could also be observable in terms of cosmological data, as traces caused by another universe colliding with ours.
The quantum theory of multiple universes can bring a new approach to conformal cyclic cosmology, although not one with which Penrose agrees. Our big bang could have been the second birth of a single quantum multiverse containing an infinite number of different universes existing simultaneously. Everything possible ends up happening (and then it would happen again, and again, and again).
An ancient myth
For a philosopher of science, Penrose’s proposal is fascinating. It opens up new possibilities for explaining the big bang because it takes our reasoning beyond the usual cause-effect logic. We speak, therefore, of a great starting point to explore the different ways in which physics can explain our world and that therefore deserves more attention from philosophers.
For a lover of myths, in addition, Penrose’s proposal is beautiful. In its preferred quantum possibility, that of continuous cycles, lies the promise of an infinite series of new worlds to be born from the ashes of its predecessors. And in the possibility of the single cycle, it is an impressive reworking of the old conception of the ouroboros or serpent world. In Norse mythology, the serpent Jörmungandr is the daughter of Loki, a cunning swindler, and the giant Angrboda. Jörmungandr devours his own tail, and the circle he creates in doing so holds the balance of the world. But the ouroboros myth has been enacted by cultures around the world, including some as archaic as ancient Egypt.
The ouroboros that would be a unique cyclical universe is majestic in itself. In its gut it would contain both our own universe and the rest of the disturbing and wonderful alternative possible universes that quantum physics contemplates. And the point where the head meets the tail would be an absolute vacuum, but at the same time a space teeming with energies at temperatures of hundreds of billions of billions of billions of billions of degrees Celsius. Even Loki the shapeshifter would be impressed.
Alastair Wilson , Professor of Philosophy, University of Birmingham
This article was originally published in The Conversation . Read the original.