Spontaneous collapse models lead to the emergence of classicality of the Universe
José Gaona-Reyes (Istituto Nazionale di Fisica Nucleare, Trieste Section, Via Valerio 2, Trieste, 34127, Italy, Department of Physics, University of Trieste, Strada Costiera 11, Trieste, 34151, Italy); Lucía Menéndez-Pidal (Departamento de Física Teórica, Universidad Complutense de Madrid, Parque de Ciencias 1, Madrid, 28040, Spain); Mir Faizal (CERN, Theoretical Physics Department, Geneva 23, CH-1211, Switzerland, Irving K. Barber School of Arts and Sciences, University of British Columbia Okanagan, Kelowna, BC, V1V 1V7, Canada, Canadian Quantum Research Center, 204-3002, 32 Ave, Vernon, BC, V1T 2L7, Canada); Matteo Carlesso (Istituto Nazionale di Fisica Nucleare, Trieste Section, Via Valerio 2, Trieste, 34127, Italy, Department of Physics, University of Trieste, Strada Costiera 11, Trieste, 34151, Italy, Centre for Quantum Materials and Technologies, School of Mathematics and Physics, Queens University, Belfast, BT7 1NN, United Kingdom)
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.