The existence of parallel universes may seem like a fantastic question that only science fiction writers can ask themselves and which has nothing to do with modern theoretical physics. But the idea that we live in a multiple universe of parallel universes has long been considered scientifically sound - albeit highly controversial. Yet the search for ways to test this theory, including scanning the sky for collisions with other universes, is well under way.
It is important to keep in mind that multiple universe theory is not really a theory, but rather a consequence of our current understanding of theoretical physics. This is an important difference. We can't give up and say, "Okay, let's have a multiverse." That our universe could be one of many others follows from current theories like quantum mechanics and string theory.
Many-worlds interpretation
You may have heard of the thought experiment with Schrödinger's cat, a creepy animal that lives in a closed box. The act of opening the box allows us to find out one of the possible stories of our future cat, including the one in which he is both alive and dead. The reason this seems impossible is because our human intuition is simply not familiar with this outcome.
However, according to the strange rules of quantum mechanics, such a future is quite possible. The reason this can happen is because of the vast space of possibilities in quantum mechanics. Mathematically, a quantum mechanical state is the sum (superposition) of all possible states. In the case of Schrödinger's cat, the cat is in a superposition of the "alive" and "dead" states.
But how can we bring all this in line with our common sense? It can be assumed that of all these states, only one is "objectively true": which we observe. But it can be assumed that all possibilities are true and that they exist in different universes of a multiple universe.
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String landscape
String theory is one of our most (if not the most) promising areas of research that can combine quantum mechanics and gravity. This is extremely difficult, since gravitational force is difficult to describe at small distances, where atoms and subatomic particles function - in the realm of quantum mechanics.
But string theory, which claims that all fundamental particles are made of one-dimensional strings, can describe all known forces of nature simultaneously: gravity, electromagnetism, and nuclear interactions.
However, for string theory to work mathematically, it requires a minimum of ten physical dimensions. Since we can only observe four dimensions: height, width, depth (spatial) and time (temporal), the extra dimensions of string theory must be hidden somehow.
To use this theory to explain the physical phenomena we know, these extra dimensions must be “compactified,” curled up so that they cannot be seen. Perhaps at every point in our four major dimensions, there are six additional indistinguishable dimensions.
The problem, or, as some would say, feature, of string theory is that there are many ways to do this compactification - 10 ^ 500 possibilities. Each of these compactifications leads to a universe with different physical laws - with different masses of electrons and gravitational constants. However, there are also energetic objections to the compactification methodology, so the issue cannot be considered solved.
From all this, the question arises: in which of the possible string landscapes do we live? String theory itself does not provide a mechanism for this prediction, making it useless due to its untestable nature. Fortunately, the idea behind our exploration of the cosmology of the early Universe turned this bug into a feature.
Early universe
At the time of the earliest universe, even before the Big Bang, the universe underwent a period of accelerated expansion - inflation. Inflation was originally intended to explain why the temperature of the present observable universe is nearly uniform.
However, this theory also predicted a spectrum of temperature fluctuations around this equilibrium, which was later confirmed by the Cosmic Backgroung Explorer, Wilkinson Microwave Anisotropy Probe, and PLANCK spacecraft.
Although the exact details of this theory are still hotly debated, inflation is well received by physicists. However, the implication of this theory is that there must be other parts of the universe that are still accelerating.
However, due to quantum fluctuations in spacetime, some parts of the universe will never reach the final inflation state. This means that the universe, at least according to our current understanding, will be in a state of perpetual inflation. Some of its parts may eventually become other universes, those, in turn, different. Such a mechanism produces an infinite number of universes.
If you combine this scenario with string theory, there is a possibility that each of these universes has a different compactification of extra dimensions, and therefore different physical laws.
Testing the theory
Such universes, predicted by string and inflation theory, which live in the same physical space (unlike many quantum mechanical universes that live in mathematical space), can overlap or collide. They will inevitably collide, leaving possible signatures in the cosmic sky that we can try to look for.
The exact details of these signatures depend on specific models - from cold to hot spots in the cosmic microwave background to anomalous voids in the distribution of galaxies. However, since collisions with other universes must take place in a specific direction, any signatures are expected to break the uniformity of our observable universe.
Scientists are actively looking for these signatures. Some are peering into the imprints of the cosmic microwave background, the afterglow of the Big Bang. However, no such signatures have yet been found. Others are looking for indirect confirmation in the form of gravitational waves, ripples in the fabric of space-time that appear when massive objects pass through it. Such waves can directly confirm the existence of inflation, which will further strengthen the theory of multiple universes.
Whether we can prove their existence or not is still unknown. But given the enormous implications of such evidence, the search is certainly worth continuing.
ILYA KHEL