Can we cross space-time with the multiverse?

What if the universe was one of many? The assumption is disturbing, it questions our understanding of the cosmos, it invites us to rethink space-time. This completes the lesson of humility instilled in us by astrophysics since the Copernican revolution 500 years ago. No, our Earth is not the center of everything, it is one of hundreds of billions of planets in the Milky Way, just one galaxy among more than 2,000 billion others in the visible universe. And now our universe may be a tiny part of the multiverse itself! Pure science fiction? Not at all: in fact, most current cosmological models predict the existence of plenty of these famous parallel universes beloved of the big screen…

Read more: Could there be a mirror universe on the other side of the Big Bang?

Starting, even, with the theory that has been a reference for more than a century: Einstein’s general relativity! This theory of gravity asserts that space-time is not static: it distorts and bends in the presence of matter or energy.

“According to relativity, space can expand to infinity, or retract itself into three dimensions, Benjamin Wandelt, professor of astrophysics and member of the Planck collaboration, explains. However, if the universe is infinite, then it is mandatory: somewhere in space-time there is an Earth-like planet populated by our look-alike, a real parallel world! This is because of infinity, everything that has a non-probability of zero happening… necessarily happens, and the infinity of time: so there will be worlds at infinity! It is even possible to estimate the step that separates us from our nearest: statistically, 1010 29 m. An extraordinary distance that unfortunately puts it forever out of reach… in question? Two physical limits are inherent to our space-time: the finite speed of light, which prevents us from seeing beyond 1028 meters or 46.5 billion light-years; and the expansion of the universe that separates regions of the universe from each other. In this multiverse of general relativity, parallel worlds thus follow each other in the same space-time that manifests ad infinitum…

Exponential expansion

If not cosmic inflation! This second model of the multiverse is an extension of general relativity. He hypothesized that, immediately after the Big Bang, our universe would have gone through a phase of exponential expansion during which space-time would have expanded mercilessly: distances would have expanded at least 1026 times a billionth of a billionth of a second!

Developed in 1979 by Alan Guth, a researcher at the Massachusetts Institute of Technology (MIT) in the United States, this model is now accepted by most cosmologists. However, according to its author, this inflation was in fact eternal: it actually stopped in our universe but continued in others, blowing bubbles of cascading universes from the dawn of time! The cosmic web of our universe will therefore not expand indefinitely, it will share a boundary with a neighboring bubble universe. “Each bubble will have basic physical properties different”, Described by Ranga Ram Chari, a cosmologist at the California Institute of Technology, USA. There the speed of light can vary from 300,000 km/s; There electrons can be lighter and protons can be bigger; Gravity is more or less intense… “Bad: If we study inflation within the framework of string theory, we can calculate that the vacuum energy of this bubble can take on 10500 different possible configurations, The researcher added. And the number of dimensions of space itself can be more than three!

Read more: Are there other universes besides ours?

A 10-dimensional hyperspace

Properly, string theory also has a model of the multiverse… It considers that the basic elements of matter will be small uniform strings, whose various modes of vibration will mimic the elementary particles we know – electrons, quarks, etc. Revolutionary consequences: our visible universe will be a membrane – or “brain” – with 3 dimensions of space, immersed in a hyperspace, itself, … up to 10 dimensions, “bulk”. “We will be stuck in one brain, but there will be many more, parallel universes to ours, Michaël Sarrazin explains, from the University of Bourgogne-Franche-Comté.

The physics of a hidden brain may be so close to us that we can even find stars and planets there. And if, by chance, two branes along an extradimension other than three in space come within 1015 meters, particles can even travel across the multiverse by jumping from one universe to another.

Michael Monnot

The idea of ​​a brain world is most successful in terms of theoretical formalization and potential collisions with experience— Michael Buckhoot, Researcher at the University of Bourgogne-Franche-Comme

Certainly, to date, no experimental evidence has supported string theory and its hidden dimensions. But Michael Sarrazin is sure of this: “Among the various concepts of multiple universes, the brain world concept is the most scientifically accomplished in terms of theoretical formalism and possible collisions with experience. »

Finally, a fourth theory predicts the existence of parallel worlds: the quantum multiverse. Quantum physics has indeed shown that a particle can be in two states at the same time until it is measured, causing it to move into only one of the two states – a curiosity illustrated by Schrödinger’s famous example of the cat. Besides, this change creates a problem for scientists. Why and how does the real choose one option instead of another? To solve this dilemma, astrophysicist Hugh Everett discovered in 1950, “The Many-Worlds Explanation”, that Schrödinger’s cat was somehow dead in one universe and alive in another. During a transition, two destinies will come true in parallel, in two separate realities that may never intersect again.

Additional Neutron Hunt

The stereo collaboration is trying to detect neutrons from a source, a nuclear reactor, located on the other side of a thick wall that they won’t be able to pass through. In green, the reactor is not operating: the detected neutrons are just part of the environment. In red, the reactor is on and a surplus of neutrons has been detected: so they can skip the wall and jump into a parallel universe before returning to our reality! Alas, the error bars in black are still too wide to conclude in the multiverse.

Read more: Does time stop at 3.5 billion years?

Overlay One World

Space-time would then hold all possibilities simultaneously. And we, poor people, will be sensitive to only one of them, like a radio fixed on a certain frequency, which does not receive the waves emitted by the various stations around it. “It’s a revolutionary idea that most people still don’t understand, Comment by Thibault Damore, expert at the Institute for Higher Scientific Studies. We must abandon the classical notion of a single space-time or multiple ‘parallel’ space-times: there is a single quantum world, the nature of which is wave superposition.

So, of course, every branch of physics has its own multiverse, the definition of which is variable geometry. “Most people think of our universe as a bubble of our space-time in which vacuum energy is constant,” Said Ranga Ram Chari. “There are multiverses Mathematical terms are collections of disconnected space-times”, Comment for his part Philip Brax, for whom anything outside the volume of space-time made up of all the points connected to us by light would be part of another universe.. Michaël Sarrazin takes an even different perspective: for him, the multiverse is the universe itself. “In string theory, our visible world must lie on one brane, and a parallel hidden world may lie on another brane, but our universe is the totality of these branes: it’s this hyperspace. More than 4 dimensions are what we call bulk.”

Yet, all these multiverse models are compatible with each other… The reality of the physical world may be crazier than science fiction imagines, a layered layer of interlocking multiverses, space-time bubbles with hidden dimensions and full of them. All possible states of each of their particles. A meta-multiverse, which one day the much-sought-after “theory of everything” will likely be relegated to just… “our universe”.

How to prove the existence of multiverse?

“Theoretically, whenever a neutron collides with an atomic nucleus, it can enter this hidden world and disappear from our brain.”, reveals Michael Sarrazin. To test this prediction, the researcher is looking for an excess of these wall-passing neutrons with data from the Neutrino Detector Stereo at the Lau-Langevin Institute in Grenoble. (See infographic) … without success so far. “Our tests still give us an upper limit, He revealed. There is a 1 in 30 billion chance that the neutron will pass into the hidden brane during the collision, otherwise it would have been detected. ” Multi-travel is not so easy.

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