If one assumes Quantum Mechanics to be valid at all scales, then one can apply it to describe the evolution of the whole Universe as a system. Since we describe the structure of space-time observed today in terms of a classical theory, then a quantum-to-classical transition must have occurred at some point in the evolution of the Universe. The usually invoked decoherence-driven quantum-to-classical transition mechanisms cannot account for the classical Universe that we see today as there are no physical external entities of the Universe that can make it decohere or collapse. In this work, we construct a quantum-to-classical transition mechanism by the incorporation of models of spontaneous wavefunction collapse into the Wheeler-DeWitt equation that governs the wavefunction of the Universe. Starting from a quantum superposition of different geometries, we show that the collapse dynamics leads to a single geometry, and thus explains the quantum-to-classical transition of the structure of spacetime. As a byproduct of our approach, in the particular context of Unimodular gravity, such a collapse dynamics would also be responsible for the selection of a value of the cosmological constant, thus providing a viable explanation for the cosmological constant problem.

Our research was published on JHEP: https://link.springer.com/article/10.1007/JHEP02(2024)193