The theory of multiple universes. Where science ends and fiction begins?
The universe where there was always and will always be. At least that's what we were told and so it follows from the word "universe". But whatever were the true nature of the universe, our ability to collect information about it is fundamentally limited. From the moment of the Big Bang took place 13, 8 billion years, and the speed at which information travels - the speed limit, the speed of light - is limited. Therefore, even though the whole universe can be truly limitless, the observable universe - no.
According to the leading ideas of theoretical physics, our universe may be one small region of vast multiverse, which can be infinite. Some of these ideas really are scientific, and some - a purely speculative, wishful thinking. Let us learn to share them. But first, a little background.
Are there multiple universes?
Modern Universe offers us some interesting facts that are very easy to observe and check, in any case, with the help of world-class research facilities. We know that the universe is expanding: we can measure properties of galaxies, their distance and rate of removal from us. The farther away they are, the more quickly removed. In the context of general relativity, which means that the universe is expanding.
And if the universe is expanding today, it means that in the past it was smaller and denser. If you delve far enough into the past, you may find that it was also more homogeneous (because gravity took time to collect all of piles) and hotter (because the smaller light wavelengths mean 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 at which the Big Bang predictions come true no longer. There are some things in the universe of observations, which the Big Bang does not explain, however, explains the theory of cosmic inflation. quite a lot of the theoretical effects of inflation have been developed in the 1980s, including:
- should look like the sowing of large-scale structures;
- that temperature fluctuations and density must be found above the horizon of the space;
- that all regions of space, even with the fluctuations should have constant entropy;
- must be a maximum temperature reached by the Big Bang.
In the 1990s, 2000s and 2010s, these four predictions were observationally confirmed with high precision. Cosmic inflation wins.
Inflation tells us that before the Big Bang, the universe was not filled with particles and antiparticles and radiation. Instead, it was filled with the energy inherent in space itself, and this energy is led to the fact that space is expanding rapidly, inexorably and exponentially. At some point, inflation is over and all (or almost all), this energy was transformed into matter and energy, setting off a hot Big Bang. The end of inflation initiated the Big Bang. That is, the Big Bang was not the beginning.
If it was a full story, we would have been in the hands of a very large universe. Its properties would be the same everywhere, the laws are the same, but the parts that were beyond the visible horizon, would be similar to the place where we are, but it would be impossible to name them multiple universes.
That is, it would be impossible as long as you do not think that all that exists physically be quantum in nature. Even inflation with all the unknowns surrounding it, should be a quantum field.
If you need to get inflation have properties of quantum fields:
- in its properties should be the uncertainty inherent in them;
- field should be described by a wave function;
- values of the field stretched over time;
Then you will come to the extraordinary conclusion.
Inflation is not over everywhere at once, but rather in a separate, independently selected places, while the space between them continued to swell. There must be some huge regions of space, where inflation ends and begins the Big Bang, but they will never meet, because the divided regions swell space. After the start of inflation will continue indefinitely and guaranteed, at least in some places.
When inflation is over, we get a big bang. That part of the universe that we see is only a part of the region where inflation is over, beyond which many unobservable universe. And there are plenty 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 all things, and properties of cosmic inflation. There is no way to measure it, and no way to measure non-observed part of the universe. But these two theories that lie at its base, inflation and quantum physics have shown their worth. If they are correct, multiple universes will be the inevitable consequence of this, and we shall live in them.
So what? There are many theoretical consequences that are inevitable, but of which we can not know for sure, because we can not test them. Multiple universes - one of those consequences. Not that it was useful, it's just an interesting prediction that follows from the theory. Why do so many theoretical physicists write papers on the topic of multiple universes? On the theme of parallel universes and their connections with our own? Why do they say that the multiple universes tied to the strings, the cosmological constant, and the fact that our Universe is ideally set up 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 have to estimate their value in the context of string vacua. If you've heard about the incredibly large numbers, such as the famous 10 500, which appear in string theory, they refer to the possible values of the string vacuums. We do not know what they are or why have these values. No one knows how to count.
So, instead of saying: "It is multiple universes," people think of as follows:
- We do not know why the fundamental constants possess the values they have.
- We do not know why the laws of physics are the same, what are.
- String theory - a framework that would provide our physical laws of our fundamental constants, as well as give us other laws or constants.
- Therefore, if we have a huge multiple universes in which different regions will have different laws and constants, one of those could be ours.
The problem is that all this is not only a purely speculative, but there is no reason, given inflation and quantum physics, to believe that ballooning of the space-time different laws or constants in the different regions. I do not like this approach to reasoning? Yes, and no one likes.
As we have seen, multiple universes - this is not a scientific theory itself. Rather, it is a theoretical consequence of the laws of physics in their fullest sense. Even if you will have an inflationary universe is governed by quantum physics, you'll be tied to it. But - like string theory - it is a problem: it does not predict anything from what we saw could not be explained without it, and it does not predict anything specific on that, we could go and have a look.
In this physical universe is important to observe all that we can, and collect the crumbs of any knowledge to which they have access. Only the full set of data, which we hope will be true, it will be possible to extract scientific statements about the nature of the universe. Some of these findings will have consequences that we can not measure and prove that the existence of multiple universes, for example. But when people talk about the fundamental constants, laws of physics, about the values of string vacuums, they are not doing science, they just talk. One may gossip about multiple universes, and cite as an example the work of such eminent theorists, but to do this scientific opinion - no.