Is space really infinite?
What is behind the universe?
Nothing, because there is no "behind it". In the word "universe", as well as in the word "universe", there is the meaning "everything". The universe includes everything. There is only one universe. A limit beyond which something else could begin is logically impossible! Because this "other" would also belong to the universe again. As we have known since Einstein, space and time are inextricably linked with cosmic matter, the substance of which the universe is made. So physically speaking, there is no space outside.
Even if the universe is limitless, it does not have to be infinitely large. To illustrate this, imagine a spherical surface, i.e. something two-dimensional. A spherical surface is unlimited, but still finite. In such a world I could always walk straight ahead, at some point I would come back to the starting point without, however, having encountered a limit on the way. The universe can be finite, but it doesn't have to be. The astronomical observations are also compatible with the assumption of an infinitely large universe.
You can never prove infinity. There are also the limits of observability. The universe began 13 billion years ago with an extremely dense and hot phase. Hundreds of thousands of years after this so-called "Big Bang" it was completely opaque. Astronomers are the only people who can look back in time. Because the light that is reaching us now has been on the move for a long time, sometimes for billions of years. Since it could not have been sent out until 13 billion years ago at the earliest - after the universe had become transparent - we have not been able to look further than what corresponds to a distance that light has covered during this time. Our horizons are gradually expanding. In a billion years we will see a billion light years ahead. The volume of the manageable part of the universe is increasing.
The question was answered by Dr. Hans-Erich Fröhlich, Astrophysical Institute Potsdam
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