What we know is that the universe is expanding. But what expands inside? And is there a center of this expansion?
One of the things we know for sure is that the universe is expanding. We understood this almost a century ago thanks to the observations of the American astronomer Edwin Hubble. He discovered that galaxies were moving away from ours with a speed proportional to their distance from the Milky Way. This doesn’t mean there’s anything special about our position in the universe: the distance between any two galaxies grows as time passes. Einstein’s relativity tells us why, which presupposes that space is not static, but that it can deform, dilate and curve. One of the questions you ask us most often in the comments is: what does the universe expand into? And is there a center of this expansion? Let’s try to understand it together with Professor Amedeo Balbi of the University of Tor Vergata in Rome ().
This is why the universe does not have a center
We assume that the universe does not expand into something, because the universe itself is all there is. There is no external space in which expansion occurs. The reason many ask this question is that they view the expansion incorrectly, as if the galaxies were hurled away by the explosion of the Big Bang. In reality things, because it is space that expands, the galaxies are still in space but are dragged away by the expansion. This happens the same way throughout space. Imagine the panettone dough rising and the raisins inside the dough moving away from each other as time passes. They stay still, it’s the dough that rises and makes them move away.
Well, if you visualize things this way you not only understand that we don’t have to imagine something within which expansion occurs, but that there isn’t even a center of expansion. Every point in the universe is equivalent to all the others. If we looked at things, we would see other galaxies moving away from us exactly as they do from our point of view. So there are no privileged positions in the universe and there is no center of expansion. The expansion occurs in exactly the same way throughout space.
