The universe is where it always was and always will be. At least that's what we were told and so follows from the very word "Universe". But whatever the true nature of the universe, our ability to gather information about it is fundamentally limited. 13.8 billion years have passed since the Big Bang, and the speed at which information travels - the ultimate speed, the speed of light - is limited. Therefore, while the entire universe can be truly unlimited, the observable universe is not.
According to the leading ideas of theoretical physics, our Universe can be one small region of huge multiple universes, which can be infinitely many. Some of these ideas are really scientific, and some are purely speculative, wishful thinking. Let's learn how to separate them. But first, a little background.
Are there multiple universes?
The modern Universe offers us some interesting facts that are very easy to observe and verify, at least with the help of world-class scientific objects. We know that the Universe is expanding: we can measure the properties of galaxies, find out their distance and speed of moving away from us. The further they are, the faster they are removed. In the context of general relativity, this means that the universe is expanding.
And if the universe is expanding today, that means it was smaller and denser in the past. If you go deep enough in the past, you will find that it was also more homogeneous (because gravity took time to collect everything in piles) and hotter (because shorter wavelengths of light mean higher energies and temperatures). This brings us back to the Big Bang.
But the Big Bang was not the very beginning of the universe. We can look into the past only up to a certain point in time, after which the predictions of the Big Bang cease to come true. There are several observations of things in the universe that the Big Bang does not explain, but the theory of cosmic inflation does.
Promotional video:
In the 1980s, quite a few theoretical implications of inflation were developed, including:
- what the seeding of large-scale structures should look like;
- that temperature and density fluctuations must exist on a scale exceeding the cosmic horizon;
- that all regions of space, even with fluctuations, must have constant entropy;
- should be the maximum temperature reached by the Big Bang.
In the 1990s, 2000s, and 2010s, these four predictions were observationally confirmed with high accuracy. Cosmic inflation is winning.
Inflation tells us that before the Big Bang, the universe was not filled with particles, antiparticles, and radiation. Instead, it was filled with the energy inherent in space itself, and this energy caused space to expand rapidly, inexorably and exponentially. At some point, inflation ended and all (or almost all) of this energy turned out to be converted into matter and energy, initiating a hot Big Bang. The end of inflation marked the beginning of the Big Bang. That is, there was a Big Bang, but not at the very beginning.
If this were the full story, we would have one extremely large universe in our hands. 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 it would be impossible to call them multiple universes.
That is, it would not be possible until you remember that everything that exists physically must be quantum in nature. Even inflation, with all the unknowns surrounding it, must be a quantum field.
If you need inflation to have the properties of quantum fields:
- in its properties there must be uncertainties inherent in them;
- the field must be described by a wave function;
- field values stretch over time;
then you will come to an unusual conclusion.
Inflation did not end everywhere at the same time, but rather in separate, chosen, independent locations, while the space between them continued to swell. There must be several huge regions of space where inflation ends and the Big Bang begins, but they will never meet because they are separated by regions of swelling space. Once started, inflation will continue for sure and indefinitely, at least in some places.
When inflation ends, we get a Big Bang. The part of the Universe that we observe is only a part of the region in which inflation has ended, beyond which there is a lot of the unobservable Universe. And there are a huge number of regions, divided among themselves, with exactly the same history.
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: the quantum nature of everything and the properties of cosmic inflation. There is no way to measure it, and there is no way to measure the unobservable part of the universe. But the two theories that underlie it, inflation and quantum physics, have shown their worth. If they are correct, multiple universes will be the inevitable consequence of this, and we will live in them.
So what? There are many theoretical implications that are inevitable, but which we cannot know for sure because we cannot verify them. Multiple universes are one such consequence. Not that it's helpful, it's just an interesting prediction that comes from theories.
Why are so many theoretical physicists writing papers on multiple universes? On the topic of parallel universes and their relationship with our own? Why are they claiming that multiple universes are tied to strings, a cosmological constant, and the fact that our universe is ideally tuned for life?
Because they have no better ideas.
In the context of string theory, there is a huge list of parameters that can, in principle, take on almost any value. This theory does not make any predictions for them, so we are forced to figure out their meanings in the context of string vacua. If you've heard of incredibly large numbers such as the famous 10 to the 500 power that appear in string theory, they refer to the possible meanings of string vacua. We do not yet know what they are or why they have such meanings. Nobody knows how to count them.
So instead of saying, "These are multiple universes!" People think like this:
- We do not know why the fundamental constants have the values they do.
- We don't know why the laws of physics are what they are.
- String theory is a framework that could provide our laws of physics with our fundamental constants, as well as give us other laws or constants.
- Consequently, if we have huge multiple universes, in which different regions will have different laws and constants, one of them may be ours.
The problem is that all this is not only purely speculative, but there is no reason, given inflation and quantum physics, to believe that swelling space-time has different laws or constants in different regions.
Don't like this approach to reasoning? And nobody likes it.
As we have already found out, multiple universes are not a scientific theory in and of itself. Rather, it is a theoretical consequence of the laws of physics in their fullest understanding. Even if you have an inflationary universe governed by quantum physics, you will be attached to it. But - like string theory - it has problems: it does not predict anything that we observed and could not explain without it, and it does not predict anything concrete that we could go and look at.
In this physical universe, it is important to observe everything that we can, and to collect bit by bit any knowledge that we have access to. Only from a complete set of data that we hope will be correct will it be possible to make scientific judgments about the nature of the universe. Some of these findings will have consequences that we cannot measure and prove: the existence of multiple universes, for example. But when people talk about fundamental constants, about the laws of physics, about the values of string vacuums, they are not engaged in science, they are just talking. You can gossip about multiple universes as much as you like and cite the prominent works of such theorists as an example, but you cannot make a scientific view of this.
Ilya Khel
