The theory that the recycled universe was based on, called loop quantum cosmology (LQC), had managed to illuminate the very birth of the universe – something even Einstein’s general theory of relativity fails to do.
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LQC is in fact the first tangible application of another theory called loop quantum gravity, which cunningly combines Einstein’s theory of gravity with quantum mechanics. We need theories like this to work out what happens when microscopic volumes experience an extreme gravitational force, as happened near the big bang, for example.
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If LQC turns out to be right, our universe emerged from a pre-existing universe that had been expanding before contracting due to gravity. As all the matter squeezed into a microscopic volume, this universe approached the so-called Planck density, 5.1 × 1096 kilograms per cubic metre. At this stage, it stopped contracting and rebounded, giving us our universe.

In classical cosmology, a phenomenon called inflation caused the universe to expand at incredible speed in the first fractions of a second after the big bang. This inflationary phase is needed to explain why the temperature of faraway regions of the universe is almost identical, even though heat should not have had time to spread that far – the so-called horizon problem. It also explains why the universe is so finely balanced between expanding forever and contracting eventually under gravity – the flatness problem. Cosmologists invoke a particle called the inflaton to make inflation happen, but precious little is known about it.