The universe once expanded so rapidly that the speed of movement of its parts exceeded the speed of light. The same is happening now, many focus on the fact that at the stage of inflation, immediately after its birth, our universe expanded faster than the speed of light. And, although one might think that such behavior is not inherent in the universe, it is happening now. That is, if you choose 2 points that are sufficiently distant from each other, it turns out that they diverge faster than the speed of light. This is due to the expansion of the space between these points.
So the universe has expanded and continues to expand faster than the speed of light. But doesn't this contradict the special theory of relativity, according to which nothing can exceed the speed of light? Not really. An object cannot move in space faster than the speed of light, however, space itself can expand as desired. Through the study of the night sky with the hubble telescope, we learned that our universe is expanding. He found that the farther from the earth, the faster they move away from it.
Imagine a point that is so far away that it moves away at approximately the speed of light, and similar points are all around us at the same distance. This forms a sphere called the hubble sphere. Everything outside of it is moving away from us faster than the speed of light. It would seem obvious that we will never be able to see the light emanating from these objects due to the high speed of their movement. However, in reality this is not the case, we see these objects.
To understand how this works, imagine a galaxy. Outside the hubble sphere, it moves away from us faster than the speed of light and from our perspective is in the region of superluminal motion. So, the light emanating towards us will actually move away from us over time. It does not sound very encouraging, however, due to the accelerating expansion of space, the Hubble sphere itself will increase, and if it grows faster than it is moving away, at some point the light will be able to leave the region of superluminal motion, end up within the sphere and begin to approach to us. So we can see it. We will see a distant galaxy, which by this time, of course, will be even further than the hubble sphere, but after seeing its light we will know that it is there. It is amazing! And some more of those photonswhat we are now seeing appeared in the first five billion years of the universe. Those areas of space where they came from were already moving faster than the speed of light, moving away from us. The sources of these photons are receding and have always receded from us faster than the speed of light. Their light reached our hubble sphere and eventually even ourselves, so we can see them.
Thus, the observable universe is larger than our hubble sphere. It is limited by what we call the particle horizon. The distance to it depends on the time that the light had to reach us since the beginning of time, which is, according to our calculations, 13.8 billion years. As the universe expands, and is constantly accelerating, the distance between some objects exceeds 13.8 billion light years. By the way, the radius of the observable universe is 46 billion light years, its diameter is approximately 93 billion light years. It includes a huge number of objects visible to us, and 13.8 billion years ago all objects, including invisible ones, were compressed into an infinitely small point, which we call a singularity.
But all this would be so if the universe had a limit. Since, if the universe is infinite, then it has always been infinite. This means that the big bang happened literally everywhere. So, since the universe has always been infinite, where is it expanding? It doesn't have to expand anywhere, it just expands in itself. This is the essence of infinity, the universe is limitless.