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Assuming that Quantum Mechanics is universal and that it can be applied overall scales, then the Universe is allowed to be in a quantum superposition ofstates, where each of them can correspond to a different space-time geometry.How can one then describe the emergence of the classical, well-defined geometrythat we observe? Considering that the decoherence-driven quantum-to-classicaltransition relies on external physical entities, this process cannot accountfor the emergence of the classical behaviour of the Universe. Here, we show howmodels of spontaneous collapse of the wavefunction can offer a viable mechanismfor explaining such an emergence. We apply it to a simple General Relativitydynamical model for gravity and a perfect fluid. We show that, by starting froma general quantum superposition of different geometries, the collapse dynamicsleads to a single geometry, thus providing a possible mechanism for thequantum-to-classical transition of the Universe. Similarly, when applying ourdynamics to the physically-equivalent Parametrised Unimodular gravity model, weobtain a collapse on the basis of the cosmological constant, where eventuallyone precise value is selected, thus providing also a viable explanation for thecosmological constant problem. Our formalism can be easily applied to otherquantum cosmological models.

Spontaneous collapse models lead to the emergence of classicality of the Universe