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Article
Report number	arXiv:2401.08269
Title	Spontaneous collapse models lead to the emergence of classicality of the Universe
Author(s)	Gaona-Reyes, José Luis (Trieste U. ; INFN, Trieste) ; Menéndez-Pidal, Lucía (Madrid U.) ; Faizal, Mir (British Columbia U., Okanagan ; CQRC, Vernon ; CERN) ; Carlesso, Matteo (Trieste U. ; INFN, Trieste ; Queen's U., Belfast)
Publication	2024-02-23
Imprint	2024-01-16
Number of pages	17
In:	JHEP 2402 (2024) 193
DOI	10.1007/JHEP02(2024)193 (publication)
Subject category	quant-ph ; General Theoretical Physics ; gr-qc ; General Relativity and Cosmology
Abstract	Assuming that Quantum Mechanics is universal and that it can be applied over all scales, then the Universe is allowed to be in a quantum superposition of states, where each of them can correspond to a different space-time geometry. How can one then describe the emergence of the classical, well-defined geometry that we observe? Considering that the decoherence-driven quantum-to-classical transition relies on external physical entities, this process cannot account for the emergence of the classical behaviour of the Universe. Here, we show how models of spontaneous collapse of the wavefunction can offer a viable mechanism for explaining such an emergence. We apply it to a simple General Relativity dynamical model for gravity and a perfect fluid. We show that, by starting from a general quantum superposition of different geometries, the collapse dynamics leads to a single geometry, thus providing a possible mechanism for the quantum-to-classical transition of the Universe. Similarly, when applying our dynamics to the physically-equivalent Parametrised Unimodular gravity model, we obtain a collapse on the basis of the cosmological constant, where eventually one precise value is selected, thus providing also a viable explanation for the cosmological constant problem. Our formalism can be easily applied to other quantum cosmological models where we can choose a well-defined clock variable.
Copyright/License	preprint: (License: arXiv nonexclusive-distrib 1.0) publication: © 2024-2025 The Authors (License: CC-BY-4.0)

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