The theory of multiple universes. Where science ends and fiction begins?

The universe where there have always been and always will be. At least so we were told, and so it follows from the word “universe”. But whatever the true nature of the Universe, our ability to gather information about it is fundamentally limited. Since the Big Explosion took place 13.8 billion years, and the speed with which information travels — the speed limit, the speed of light is limited. Therefore, although the entire universe may indeed be infinite, the observable universe — not.

According to the leading ideas in theoretical physics, our universe could be just one small region of the vast multiple universes, which may be infinitely many. Some of these ideas are really scientific, and some are purely speculative, wishful thinking. Let’s learn to share them. But first, a little background.

Are there multiple universes?

Modern universe offers us some interesting facts that are easy to observe and to check, anyway, with the help of research facilities world-class. We know that the universe is expanding: we can measure the properties of the galaxies, their distance and speed of recession from us. The farther they are, the faster removed. In the context of General relativity, this means that the universe is expanding.

And if the universe is expanding today, which means that in the past it was smaller and denser. If you delve far enough into the past, you might find that she was also more homogeneous (because of gravity took a while to collect all the piles) and hotter (because smaller wavelengths of light means higher energy and temperature). This brings us back to the Big Bang.

But the Big Bang was not the beginning of the Universe. We can look into the past only to a certain point in time for which predictions of the Big Bang no longer true. There are some observations of things in the Universe that the Big Bang does not explain, however, explains the theory of cosmic inflation.

In 1980-ies it was developed quite a lot of theoretical consequences of inflation, including:

  • how to look like the sowing of large-scale structures;
  • that the fluctuations of temperature and density must exist at scales exceeding the outer horizon;
  • that all regions of space, even with the fluctuations must have a constant entropy;
  • must be high temperature made the Big Bang.

In the 1990s, 2000s and 2010s, four of these predictions were confirmed by observation with high accuracy. Cosmic inflation wins.

Inflation tells us that before the Big Bang the universe was filled with particles, antiparticles and radiation. Instead she was filled with energy inherent to space itself and this energy led to the fact that space was expanding quickly, inexorably and exponentially. At some point, the inflation ended, and all (or almost all) of this energy was converted into matter and energy, initiating the hot Big Bang. The end of inflation marked the beginning of the Big Bang. That is, the Big Bang was, but not in the beginning.

If this were the full story, we were in the hands of one extremely big universe. Its properties would be the same everywhere, the laws are the same, and the parts that were beyond the visible horizon, would be similar to the place where we are, but to call them multiple universes would be impossible.

That is, it would be impossible until then, until you remember that everything that exists physically has to be quantum in nature. Even inflation with all the unknown surrounding her, must be a quantum field.

If you need to inflation had the properties of quantum fields:

  • its properties must be of the uncertainty inherent in them;
  • field should be described by a wave function;
  • field values are stretched over time;

then you’ll come to an unusual conclusion.

Inflation did not end everywhere simultaneously, but rather in a separate, selected, independent areas, while the space between them continued to swell. Must be some huge regions of space where inflation ends and begins with the Big Bang, but they will never meet because of the divided regions of inflationary space. After the start of inflation will continue indefinitely and is guaranteed, at least in some places.

When inflation ends, we get a Big Bang. That part of the Universe we see is only part of the region where inflation has ended, beyond which many unobservable Universe. And there are a huge number of regions, separated from each other, with exactly the same story.

This is the idea of multiple universes. As you can see, it is based on two independent, well established and widely accepted aspects of theoretical physics: quantum nature of everything and properties of cosmic inflation. There is no way to measure it, as there is no way to measure the non-observed part of the Universe. But these two theories that lie at its core, inflation, and quantum physics, have shown their worth. If they are correct, multiple universes will be the inevitable result, and we are going to live in them.

What? There are many theoretical consequences, which are inevitable, but that we can’t know for sure because I can’t to check them out. Multiple universes is one of those consequences. Not that it was useful, it’s just an interesting prediction that follows from the theories.

Why do so many theorists writing on the topic of multiple universes? On the topic of parallel universes and their connections with our own? Why do they say that multiple universes tied to the strings of the cosmological constant and the fact that our universe is tuned for life?

Yes, because the best ideas they have.

In the context of string theory there is a huge list of options that can, in principle, take almost any value. This theory makes no predictions for them, so we are forced to estimate their values in the context of string vacuums. If you have heard about the incredibly large numbers, like the famous 10500that appear in string theory, they refer to the possible values of string vacuums. We don’t know what they are or why have such values. No one knows what to expect.

So, instead of saying, “there are multiple universes!”, people think as follows:

  • We don’t know why the fundamental constants have such values that they have.
  • We don’t know why the laws of physics are as they are.
  • String theory is the only framework that would ensure our laws our fundamental constant of physics, and also give us other laws or constants.
  • Therefore, if we have a huge multiple universes in which different regions will have different laws and permanent, one of those could be ours.

The problem is that it is not only purely speculative, but there is no reason, given inflation and quantum physics, to believe that the ballooning of space-time different laws or constants in different regions.

Don’t like this approach to reasoning? Yes and no one likes it.

As we have seen, multiple universes is not a scientific theory itself. Rather, it is a theoretical consequence of the laws of physics in the most complete sense. Even if you have the inflationary universe governed by quantum physics, you will be bound to that. But — like string theory — it problems: it does not predict anything of what we saw and couldn’t explain without it, and it does not predict anything specific that we could go and have a look.

In this physical Universe it is important to observe all we can, and to glean any knowledge to which you have access. Only from the full dataset, which we hope will be true, it will be possible to extract scientific judgment about the nature of the Universe. Some of these insights will have consequences that we cannot measure and prove the existence of multiple universes, for example. But when people talk about fundamental constants, the physical laws, the values of string vacuums, they are not doing science, they just argue. One may speculate about multiple universes and give an example of a prominent work of such theorists, but to do this the scientific mind.

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