  The degree of second-order coherence of the relic gravitons produced from the
vacuum is super-Poissonian and larger than in the case of a chaotic source
characterized by a Bose-Einstein distribution. If the initial state does not
minimize the tensor Hamiltonian and has a dispersion smaller than its averaged
multiplicity, the overall statistics is by definition sub-Poissonian. Depending
on the nature of the sub-Poissonian initial state, the final degree of
second-order coherence of the quanta produced by stimulated emission may
diminish (possibly even below the characteristic value of a chaotic source) but
it always remains larger than one (i.e. super-Poissonian). When the initial
statistics is Poissonian (like in the case of a coherent state or for a mixed
state weighted by a Poisson distribution) the degree of second-order coherence
of the produced gravitons is still super-Poissonian. Even though the quantum
origin of the relic gravitons inside the Hubble radius can be effectively
disambiguated by looking at the corresponding Hanbury Brown-Twiss correlations,
the final distributions caused by different initial states maintain their
super-Poissonian character which cannot be altered.
