Obstructions to Bell CMB Experiments

TL;DR

This paper explores the theoretical possibility of conducting Bell experiments on the cosmic microwave background to demonstrate quantum entanglement, but concludes practical obstacles are likely insurmountable, proposing alternative methods instead.

Contribution

It systematically studies Bell experiments on the CMB, introduces pseudo-spin operators, and discusses practical challenges and alternatives for revealing quantum origins of cosmological structures.

Findings

Bell inequality violations are theoretically possible with inflation-predicted states.

Practical realization faces significant, likely insurmountable obstacles.

Alternative methods are proposed to reveal quantum origins without Bell tests.

Abstract

We present a general and systematic study of how a Bell experiment on the cosmic microwave background could be carried out. We introduce different classes of pseudo-spin operators and show that, if the system is placed in a two-mode squeezed state as inflation predicts, they all lead to a violation of the Bell inequality. However, we also discuss the obstacles that one faces in order to realize this program in practice and show that they are probably insurmountable. We suggest alternative methods that would reveal the quantum origin of cosmological structures without relying on Bell experiments.