What happened before the Big Bang?

We’ve all heard that the universe began with the Big Bang — a colossal explosion that ushered it into existence.

But what if the Big Bang wasn’t the beginning? Could there have been an earlier version of the universe, with its own stars and galaxies? A universe populated, perhaps, by its own creatures, themselves wondering what came before their universe?

Or might there be an infinite number of universes, endlessly branching out to create more and more universes? If so, does it even make sense to ask how that whole array of universes came into being?

In trying to answer such questions, scientists bump up against the limits of the laws of physics. Existing theories can account for the evolution of the universe from its earliest moments — from a fraction of a second after the Big Bang — but the question of what came before has been among the most vexing in all of science.

“I’ve been to seminars where we fight about it for hours and hours,” says Renée Hložek, a physicist and cosmologist at the University of Toronto.

Physicists are most confident when discussing things they can see — and the oldest stuff visible with our telescopes is the Cosmic Microwave Background. A sort of “echo” of the Big Bang, the CMB is an all-sky glow that can be measured and mapped with radio telescopes. It came into being 380,000 years after the Big Bang. Before that, we can’t see what the universe was like; it was simply too full of matter for light to flow freely.

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