Do we live in a multiuniverse? – Magazine ?

The strange idea of ​​a multiple universe is not so strange in certain branches of physics.

Right now someone like you may also be starting to read an article in their favorite magazine. That person would have the same family and friends as you, would share your same hobbies and live in the same house in the same city. The only difference is that this double of yours would live in another universe, an exact replica of ours, and it would be so far away that we could never communicate with it in any way. Oddly enough, this idea has dazzled quite a few theoretical physicists and cosmologists over the past decade: the possibility that our universe is not the only one that exists, but one of a multitude. There would therefore not be a single universe, but rather a multiple universe or “multiverse”. Some universes would have the same laws of physics that we know and would be, in essence, very similar to ours. In others, the values ​​of the fundamental constants could change, giving rise to very different universes. There could even be universes with unknown physical laws and structures that would test the human imagination.

The meaning of the universe

The word “universe” comes from Latin universe and is composed of unus (one and versus (that turns, that goes around); that is, one and everything that surrounds him. Traditionally, the universe is considered synonymous with “everything that exists” or “everything there is.” In recent decades, however, this meaning has taken on a different nuance, thanks to our greater knowledge of the laws that govern the cosmos. Today we have to admit that even with the most sophisticated technology, we are only able to see a small portion of the universe: the one whose light has reached us. In this sense, the universe would rather be equivalent to “everything we can see.” Our observable universe It has a limit called the cosmic horizon. The objects located on that horizon are the furthest that we can see. Its light left us shortly after the birth of the universe, about 13.8 billion years ago. From our position, the cosmic horizon delimits an enormous sphere that encloses the observable universe. This is what is known as the Hubble bubble.

How big is our Hubble bubble? In a static universe it would be very simple to calculate the distance to the horizon. Since light has had 13.8 billion years to travel, the most distant objects visible would be 13.8 billion light years away. But our universe is not static, it is expanding. Space itself expands, separating galaxies from each other as if they were on the surface of an inflating balloon. This implies that, while the light travels this enormous distance, the object that emitted it continues to move away. When we receive its light, the object is not 13.8 billion light years away, but much further away. According to calculations, the most distant objects we can currently see are about 42 billion light years away. Light from objects at a greater distance has not had time to reach us. Those objects are beyond our cosmic horizon.

To the horizon and beyond

The Hubble bubble therefore marks the border of our observable universe, but that does not mean that everything ends there. Beyond there must be regions whose light will never reach us, like islands separated by a vast ocean of space and time (possibly an infinite ocean). Each of these domains, so remote that it is impossible to have contact with them, would be like another universe. This is the simplest version of the multiverse idea, known as a “level I multiverse.” In this hypothetical scenario, each of the parallel universes would have started with a different distribution of matter and could have evolved differently, but they would all have the same laws of physics as our Hubble bubble. The WMAP satellites and Planck have studied for the last decade the microwave background radiation, a residue of the Big Bang that permeates all of space. The data support the idea that space continues beyond the limits of our observable universe. This same data indicates that space is uniformly filled with matter on large scales, which means that other universes have to broadly resemble our own. All of this fits with a level I multiverse. If we accept that we live in this multiverse, then we can come to a surprising conclusion: at least one of its countless universes may be identical to ours. Consider our Hubble bubble, with all the particles of matter. According to estimates, our observable universe has room for 10,118 particles. How many different arrangements of the particles are possible? According to the calculations of physicist Max Tegmark, this number is of the order of 2 raised to the power of 10118. Although this quantity is enormous, it is finite. This implies that the particle configuration must be repeated somewhere, as long as the multiverse is large enough. Tegmark has estimated that there will be a twin universe to ours at a distance of 10 to the power of 10118 meters from us (a 1 followed by 10118 zeros). In that remote place, a replica of you is looking up from a magazine with the same shocked face.

Bubble universes

Now let’s go back in time 13.8 billion years to the Big Bang. Exactly 10-36 seconds after the birth of the universe something extraordinary happened: for just an instant, the cosmos expanded exponentially, increasing in size 1030 times. This implies that a bean-sized region of space would stretch to a size larger than our observable universe. And all in a thousandth of a billionth of a trillionth of a second. This event is what is known as cosmic inflation. It was proposed by the American physicist Alan Guth in 1981 to explain some characteristics of our universe. Although no one still knows with certainty what mechanism could have caused it, today it is accepted by the vast majority of scientists (see As you see? No. 186). And regarding the topic at hand, the inflationary universe is the seed from which the so-called “level II multiverse” is born.

In many versions of inflationary theory, the rapid spatial expansion that drove our region of the universe would not be a one-time event, but could be repeated over and over again. Like a child blowing soap bubbles, cosmic inflation would randomly generate new bubble universes. Each one would be isolated from the others, without the possibility of connecting in any way, and their characteristics could be very different. Experts think that the properties of elementary particles and the values ​​of the fundamental constants of physics arose during inflation. Different inflationary processes could therefore give rise to very different universes. The vast majority would be empty and boring. If the electromagnetic force were a little greater, hydrogen would not fuse inside the stars and they would not form. If the gravitational force were somewhat less, the matter would have dispersed without forming galaxies. If the protons were a little heavier, they would decay into neutrons and there would be no atoms. In these other universes, the exact combination that makes the formation of complex structures, including life, would not exist, as it does in ours.

gigantic membranes

Another branch of physics where the notion of multiverse arises naturally is string theory. Since its emergence in the mid-1980s, string theory has been the most promising—if not the only—candidate for the theory of everything (see As you see? No. 108). According to string theory, everything in the universe is made up of tiny threads of energy. These strings vibrate in different ways, like those of a violin, to form elementary particles. This elegant theory has, however, a surprising requirement: it needs seven more spatial dimensions than we see.

String theory not only includes strings, but other larger objects, some types of membranes. These could have more than three dimensions, and would not be limited in size, like ropes. If they had enough energy, they could reach an enormous size, as big as our universe. Some theorists propose that our universe could be inside a three-dimensional membrane. If this idea is correct, then why don’t we see the membrane? The answer from string theory is that photons, the particles of light, would be trapped in the membrane; They could move freely through it, but they couldn’t get out. Thus, the membrane appears completely transparent and invisible. For the same reason, we would not be able to see any of the extra dimensions that the theory predicts, nor other membranes floating in a higher-dimensional space. All of these possible universes, which would also make up a level II multiverse, could be right there, next to us, much closer than we suspect.

Although some physicists have high hopes for string theory, the truth is that no one has yet seen any signs of extra dimensions, much less a membrane or a string.

Science or science fiction?

Despite everything, some cosmologists have suggested possible clues to the reality of the multiverse. If we lived in a level II multiverse, another bubble universe could have collided with ours and the trace of this catastrophic event should have been imprinted in the cosmic radiation that permeates the sky. Finding this signal would be a good argument in favor of the existence of other universes.

Another possibility would be to measure the shape, or geometry, of the space. The options being considered are: spherical, flat or hyperbolic – like a saddle. A sphere occupies a finite volume, which would not fit with a level I multiverse. Astronomical data collected to date indicates that the observable universe is flat, although this would not be definitive either. Just as the Earth appears flat in our daily lives, the universe could exhibit a different geometry beyond our cosmic horizon.

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