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.