The universe expands and contracts at the same time

What was before the universe? [Jannis, 11 years]

We know that the universe used to be very hot and very dense. Sometimes you hear that the universe exploded out of the "Big Bang". It is now known that this may not be entirely true. However, we don't yet know what really happened. The universe was once very dense, but it was already quite big then, so the Big Bang cannot have been the beginning of everything. Maybe the universe has been around a lot longer - maybe it is expanding and contracting again. In that case, the reversal from contraction to expansion would look like a big bang. Our universe may have originated from another universe. Then there could even be innumerable universes - one multiverse. That would not answer the question of where the first universe came from. Einstein's general theory of relativity shows that space, time and matter are closely linked. It seems that space and time came into being at the same time as the universe. So there may not be any “before” the universe because there was no time. It is exciting to see if there is any way to test this idea. We are researching that!

Dr. Jean-Luc Lehners heads the Theoretical Cosmology working group at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Potsdam. He researches the early universe and the big bang.

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