Shortcomings in the Understanding of Why Cosmological Perturbations Look Classical

TL;DR

This paper critiques the common assumption that decoherence explains the classical appearance of cosmological perturbations, arguing that the measurement problem in quantum mechanics remains unresolved in this context.

Contribution

It challenges recent claims that decoherence alone accounts for classicality of cosmological perturbations during inflation, emphasizing unresolved measurement issues.

Findings

Decoherence does not fully solve the measurement problem in cosmology.

The cosmological measurement problem is more severe than in standard quantum mechanics.

Addressing the classical appearance of perturbations requires tackling foundational quantum issues.

Abstract

There is a persistent state of confusion regarding the account of the quantum origin of the seeds of cosmological structure during inflation. In fact, a recent article (C. Kiefer & D. Polarski, ArXiv: 0810.0087 [astro-ph]) addresses the question "Why do the Cosmological Perturbations look Classical?" and offers an answer based on unitary quantum mechanics (i.e., without reference to the projection postulate) relying on the decoherence type of analysis. The argument is, thus, implicitly assuming that decoherence offers a satisfactory solution to the measurement problem in quantum mechanics. We will review here, why do we, together with various other researchers in the field, consider that this is not the case, in general, and particularly not at all in the situation at hand. In fact, as has been previously discussed (A. Perez, H. Sahlmann, and D. Sudarsky, CQG 23, 2317, (2006);[arXiv: gr-qc/0508100]), we will argue that the cosmological situation is one where the measurement problem of quantum mechanics appears in a particular exacerbated form, and that, it is this, even sharper conondrum, the one that should be addressed when dealing with the inflationary account of the origin of the seeds of cosmic structure in the early universe.