Cosmological Inflation and the Quantum Measurement Problem

TL;DR

This paper explores how the quantum measurement problem in cosmological inflation can be addressed using the Continuous Spontaneous Localization (CSL) theory, analyzing its effects on the power spectrum of primordial perturbations.

Contribution

It applies the CSL model to inflationary cosmology, deriving the modified power spectrum and constraining the model parameters based on observational data.

Findings

The power spectrum acquires a universal shape with a scale-invariant branch and a non-scale-invariant branch.

The non-scale-invariant branch has a spectral index of nS=4, conflicting with CMB observations.

Stringent constraints are placed on CSL parameters to align with observational data.

Abstract

According to cosmological inflation, the inhomogeneities in our universe are of quantum mechanical origin. This scenario is phenomenologically very appealing as it solves the puzzles of the standard hot big bang model and naturally explains why the spectrum of cosmological perturbations is almost scale invariant. It is also an ideal playground to discuss deep questions among which is the quantum measurement problem in a cosmological context. Although the large squeezing of the quantum state of the perturbations and the phenomenon of decoherence explain many aspects of the quantum to classical transition, it remains to understand how a specific outcome can be produced in the early universe, in the absence of any observer. The Continuous Spontaneous Localization (CSL) approach to quantum mechanics attempts to solve the quantum measurement question in a general context. In this framework, the wavefunction collapse is caused by adding new non linear and stochastic terms to the Schroedinger equation. In this paper, we apply this theory to inflation, which amounts to solving the CSL parametric oscillator case. We choose the wavefunction collapse to occur on an eigenstate of the Mukhanov-Sasaki variable and discuss the corresponding modified Schroedinger equation. Then, we compute the power spectrum of the perturbations and show that it acquires a universal shape with two branches, one which remains scale invariant and one with nS=4, a spectral index in obvious contradiction with the Cosmic Microwave Background (CMB) anisotropy observations. The requirement that the non-scale invariant part be outside the observational window puts stringent constraints on the parameter controlling the deviations from ordinary quantum mechanics... (Abridged).