The Universe began with a big bang. Not an explosion from a single point, but rather an early dense state. Of course an obvious question this raises is what came before the big bang? While it’s possible that the answer is “nothing,” that hasn’t stopped some theorists from postulating an earlier cause for the Universe. One of these ideas is known as the big bounce. 

The basic idea of the big bounce is that the Universe goes through a series of expansions and contractions. Right now we live in an expanding Universe, but at some point, the model argues, the Universe will start to contract. Eventually it will contract to a dense fireball again, and this will trigger a new big bang. This solves the “what came before” problem of the big bang by postulating an infinite series of big bangs, but it’s not without problems. For one, as we currently understand dark energy the Universe will likely continue to expand forever. For another, if the Universe did re-collapse into a dense state, we have no idea how it would trigger a new big bang.

A new work in Physical Review Letters proposes a solution to this second problem. The key to the idea is to introduce quantum theory into the mix. In a purely classical model, a shrinking universe will eventually collapse into a singularity. It’s long been thought that quantum theory could provide a solution to this problem, but the devil is in the details. To prevent the formation of a singularity, the work introduces a symmetry known as conformal invariance. As long as the Universe has this symmetry during its dense period, it could enter the dense period at the end of one “universe” and re-expand to form a new “universe.” The authors call this a perfect bounce.

So with the right conditions it’s possible that our Universe could simply be the period between bounces.

Paper: Steffen Gielen et al. Perfect Quantum Cosmological Bounce. Phys. Rev. Lett. 117, 021301 (2016). DOI: 10.1103/PhysRevLett.117.021301

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