What existed before the singularity

interview : "Mathematics just collapses there"

Mr. Bojowald, “Universe out of nowhere” is the name of the new book by your colleague Lawrence Krauss. Has physics explained how everything could come from nothing?

I'm rather skeptical about that. It is fundamentally difficult to work with the term “nothing”. Because it is nothing, it is not described by any formula, but then you cannot do mathematics or theoretical physics either. Plato already said in a general sense: whoever thinks of nothing does not think at all.

But according to quantum theory, particles keep appearing and disappearing from nowhere. If matter can arise out of nowhere, why not a whole universe?

In fact, particles can appear to appear out of nowhere in a vacuum. But Krauss tries to apply this transition not only to matter, but to the entire universe, which also includes space and time. That is much more difficult because then you also have to show how time emerges from nothing. We can't do that yet.

If you retrace the history of the universe using general relativity, you end up in a state of infinite heat and density, a singularity, the Big Bang. Wasn't that the beginning of it all?

It has often been portrayed, including by Stephen Hawking, that this really was the beginning of the universe. But that was never valid. Mathematics just collapses there. Because of this, the singularity cannot really be used as part of the cosmological model. The big bang singularity is a problem. The big question is how to eliminate them.

What would be the alternative?

That everything has always been there. The big bang would then not be the point of origin, but a transition. A collapsing universe continued to contract until the repulsive forces became so strong that the movement was reversed: the expanding universe in which we live.

A cyclical universe that keeps collapsing and is reborn?

In the last couple of years another possibility has arisen: that there was something before, but without time. This could be seen as the beginning of our universe, but it would also not be a creation out of nothing. And that has been specifically described mathematically.

How do I have to imagine a universe without time?

Today the average density in the universe is about one atom per cubic meter. The universe I am now speaking of is the equivalent of more than a quadrillion suns concentrated on the size of a single proton. It's like the density is so extreme that even time can't get through it.

Isn't that both equally difficult to imagine? A universe that has always existed and one that arose out of nothing?

Just looking at the math makes it easier to imagine something that has always been there. The transition from nothing to something, on the other hand, is difficult. Incidentally, this applies not only to the beginning of the universe, but also to the end, because the universe will probably exist forever. An infinite time that the universe has existed and an infinite time that it will still exist is actually no difference.

Martin Bojowald (40) is a physicist at Pennsylvania State University, where he works on cosmology. The questions were asked by Kai Kupferschmidt.

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