Quantum coherence of relic gravitons and Hanbury Brown-Twiss
  interferometry
  The coherence of the relic gravitons is investigated within a quantum
mechanical perspective. After introducing the notion and the properties of the
generalized Glauber correlators valid in the tensor case, the degrees of first-
and second-order coherence are evaluated both inside and beyond the effective
horizon. The inclusive approach (encompassing the polarizations of the
gravitons) is contrasted with the exclusive approximation where the total
intensity is calculated either from a single polarization or even from a single
mode of the field. While the relic gravitons reentering the effective horizon
after the end of a quasi-de Sitter stage of expansion are first-order coherent,
the Hanbury Brown-Twiss correlations always exhibit a super-Poissonian
statistics with different quantitative features that depend on the properties
of their initial states and on the average over the tensor polarizations.
