A cosmic shadow on CSL

The Continuous Spontaneous Localisation (CSL) model solves the measurement problem of standard quantum mechanics, by coupling the mass density of a quantum system to a white-noise field. Since the mass density is not uniquely defined in general relativity, this model is ambiguous when applied to cosmology. We however show that some well-motivated choices of the density contrast already make current measurements of the cosmic microwave background incompatible with other laboratory experiments