The domain of infinity has something to fascinate, both scientists and science fiction authors. If the opinion on the existence of multiple or parallel universes divides so much, it is because nothing has yet been demonstrated. “Their existence is however the best way to answer unresolved scientific questions “, say some researchers, at the origin of many theories. All that remains is to test these hypotheses to hope to demonstrate (one day) that our universe is not the only one.
There is no doubt that the Universe is much larger than what we can observe. Therefore, the idea of the existence of multiple universes – also called multiverses – is not so far-fetched, and is even seriously considered by many scientists. According to the possible hypotheses, either the other universes are simply beyond the space horizon, or they have physical constants completely different from ours.
The hypothesis is not so recent since the philosopher Giordano Bruno issued it in 1600, then the physicist and mathematician Hugh Everett three centuries later (in 1957). The theory of the latter, also called theory of many worlds, is in fact an interpretation of quantum mechanics which allows him to describe "all reality". Based on Schrödinger's cat experiment, Everett assumes that universes divide and what is not "true" in one universe is in another (a divergent universe).
In the book "At the Crossroads of Worlds “, one of the characters explains how the multiple universes would separate:“Because this world, like all the other universes, was born from the result of probabilities. Let's take the example of the game of heads or tails:the coin you toss can land on heads or tails, but we don't know in advance which side it will fall on. If it is heads, it means that the possibility that it lands on tails has failed. But just before we throw it, both possibilities have the same chance. If, in another world, the coin lands tails at this moment, the two worlds separate. I am using the heads or tails example to make it clearer. In reality, these probability failures occur at the level of elementary particles .
Why consider the possibility of parallel universes? Because it is the best way to answer certain unsolved mathematical problems and equations, according to the researchers. In any case, the hypothesis of parallel worlds is not in contradiction with the theory of relativity and quantum physics, and would even make it possible to unify them.
Among the possibilities considered, the string theory (or theory of everything) assumes that matter is composed of tiny vibrating strings and that each vibration corresponds to a different particle. In this case, there would be more dimensions in the Universe than the four we know (three of space and one of time), dimensions that are invisible because they are very small and rolled up on themselves. With this theory, there could well exist 10 power 500 parallel universes...
“Inflation — considerable increase in the ‘ size’ of the Universe in its first moments — would have created not one, but an infinity of bubble universes, structured according to different physical laws (dictated by the strings), possibly very different from those which govern our own bubble “, advanced in 2012 Aurélien Barrau, astrophysicist at the subatomic physics and cosmology laboratory of the CNRS in Grenoble. “Elsewhere, lightless worlds, matterless worlds, ten-dimensional worlds…Each bubble-universe would have its own Big Bang, perhaps its own dimensionality. Almost everything would eventually become possible ". Some researchers have even modeled the existence of wormholes that would link the universes together:a black hole for the entrance and a white hole for the exit. What Aurélien Barrau describes on the other hand as "pure science fiction".
The possibilities of the topology of the Universe are multiple and its shape depends on matter, energy and the influence of fundamental interactions. In these other universes there may be a few look-alikes of ourselves and the laws of physics could be the same as completely different, who knows?
Another theory predicts that there would be a "mirror" universe, twin and symmetrical with respect to ours, except that it would be made of antimatter. Indeed, if our universe contains only matter, one can wonder where the antimatter went, since each particle of matter would be associated with a particle of antimatter which resembles it (same mass, same lifespan, but opposite charge). According to this theory, the twin universe would be "CPT-symmetric" with respect to ours, i.e. matter would be replaced by antimatter (C symmetry), time would flow in the other direction (T symmetry ) and the geometry would be inverted (P symmetry).
Stephen Hawking and Thomas Hertog then tried to prove the existence of multiverses in the study "A Smooth Exit from Eternal Inflation", revised in 2018 - a week before Hawking's death. According to the two scientists, if other universes appeared at the same time as ours at the time of the Big Bang (13.7 billion years ago), they must have left a trace in the form of radiation in our universe. Hawking and Hertog then developed mathematical formulas that would allow space probes to detect this radiation, proof of the existence of other universes.
In 2020, a "parallel universe" detected by NASA in Antarctica had made the headlines of many newspapers, but it was only anomalies detected in some data. Moreover, the hunt for multiverses through neutrons that can pass from one universe to another continues in a very serious way at the Laue-Langevin Institute in Grenoble. Everything would take place as if neutrons could cross barriers, in the form of oscillations analogous to neutrinos (elementary particles generated by nuclear reactions). The nuclear reactor of the Institut Laue-Langevin is not in its first attempt to validate the "crazy" theory. The experiment, renewed in 2021, would now indicate new limits on the existence of this neutron flux. To be continued.
A January 2022 study even manages to explain the low mass of the Higgs boson using string theory. "There are mechanisms stated in the string theory or in that of the eternal inflation of the Universe which explain very well how the coexistence of multiple universes can be consistent with the calculations “, report the authors. Everything then happens as if each of the universes contained Higgs bosons with heterogeneous masses. Our "bubble universe" would just be different from others, and maybe even a universe could only survive in the presence of the low-mass Higgs boson.
As long as nothing is demonstrated, all theories are still possible on the existence or not of other universes, in various forms. There could very well be just our universe, just as there could be many. Proponents of the latter theory claim in any case that experimental predictions tend to answer fundamental scientific questions.
