Obstructions to Bell CMB Experiments
  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.
