Everyone knows that the universe is expanding. But where? What is this extension? By observing how the nuclear mushroom grows, we can definitely limit the space in which it grows. The question can be very stupid, on the one hand, but very interesting on the other.
So, the Universe has been expanding or contracting (respectively, exhibiting redshifts and blueshifts) since the Big Bang. But where will its expansion end? It cannot be in infinity, after all. Why should we say that the universe is expanding as if it were the most normal and natural thing in the world?
First, a few simple truths
1. You are not expanding now
The earth is also not. Neither the solar system nor the Milky Way. The expansion of the universe depends on gravity, which means only that local gravity dominance effects are observed in high density regions. It turns out that not all galaxies are moving away from the Milky Way. Our closest neighbor, the Andromeda galaxy, is rushing towards us at a speed of 80 km / s and will collide with us within several billion years.
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2. Don't believe metaphors
It may seem to you that the universe is expanding like a balloon being pumped with air. “Look, exactly like the universe!” A fashionable British scientist will tell you. But you, being smart, will notice that there is space outside of the ball, and that the 2-dimensional surface of the ball is expanding into 3-dimensional space. However, our universe has three dimensions.
3. The universe has no end or end
We're not really sure if the universe is infinitely large or just very large, but even so, traveling in one direction long enough will still return you to square one. Think of Pacman, but without fruit and ghosts. As for the center of the universe, here's where the ball analogy helps us. It seems to us that all galaxies are moving away from us, but from their point of view, they will also be the center of the universe. This is just an illusion.
So where is the universe actually expanding? Yes to nowhere. There is no space closet full of things. But to understand this, let's see what general relativity has to say about spacetime.
In general relativity (as professionals say), the most important property of space (and time) is the distance (and time interval) between two points. In fact, distance fully defines space. The evolution of the distance scale is determined by the amount of matter and energy in space, and as time goes on, the scale increases, so does the distance between galaxies. However - and this is the oddity - it happens without actually moving the galaxies.
Perhaps at this point your intuition has faltered. But that won't stop us from sorting out the oddities.
We have already said that galaxies are moving away from us. Not really. It's just that it's easier for scientists to explain what is really happening. They are deceiving you.
“But wait!” The most scientifically savvy of you will say. - "We are measuring the Doppler shift of distant galaxies." This so-called "redshift", which you know about, is fixed on the Earth, and like the siren of a passing ambulance, it lets us know that there is movement. But this is not something that happens on a cosmological scale. It's just that since the distant galaxies emitted light and it reached us, the scale of space has seriously changed, increased. As space has expanded, the wavelength of the photons has increased, so the light gives off red.
Another question follows from this approach: "Is the Universe really expanding faster than the speed of light?" It is absolutely true that most distant galaxies are increasing their distance in relation to us faster than the speed of light, so what? They do not move faster than light (they generally stand still). Moreover, knowing this will not help you in any way: information is not transmitted. If you send a package of food to another galaxy, faster than the speed of light, you cannot do this (and even here, in principle, you will have to try). The speed of light remains the universal speed limiter.
We have given the most widespread (well, or accepted in the sphere of relativists) opinion about the cosmological expansion, but it would be logical to end on what we do not understand at all. All of the above works great if you have room to step forward and stretch. But what happened at the very beginning that made space literally out of nothing? Physics has no answer to this question. And you have to wait until the theory of quantum gravity appears and sheds light on this issue.
Ilya Khel