  We explore the possibility of detecting entangled photon pairs from cosmic
microwave background or other cosmological sources coming from two patches of
the sky. The measurements use two detectors with different photon polarizer
directions. When two photon sources are separated by a large angle relative to
the earth, such that each detector has only one photon source in its field of
view, a null test of unentangled photons can be performed. The deviation from
this unentangled background is, in principle, the signature of photon
entanglement. To confirm whether the deviation is consistent with entangled
photons, we derive a photon polarization correlation to compare with, similar
to that in a Bell inequality measurement. However, since photon coincidence
measurement cannot be used to discriminate unentangled cosmic photons, it is
unlikely that the correlation expectation value alone can violate Bell
inequality to provide the signature for entanglement.
