  In a gas of $N$ interacting bosons, the Hamiltonian $H_c$, obtained by
dropping all the interaction terms between free bosons with moment
$\hbar\mathbf{k}\ne\mathbf{0}$, is diagonalized exactly. The resulting
eigenstates $|\:S,\:\mathbf{k},\:\eta\:\rangle$ depend on two discrete indices
$S,\:\eta=0,\:1,\:\dots$, where $\eta$ numerates the \emph{quasiphonons}
carrying a moment $\hbar\mathbf{k}$, responsible for transport or dissipation
processes. $S$, in turn, numerates a ladder of \textquoteleft
vacua\textquoteright$\:|\:S,\:\mathbf{k},\:0\:\rangle$, with increasing
equispaced energies, formed by boson pairs with opposite moment. Passing from
one vacuum to another ($S\rightarrow S\pm1$), results from
creation/annihilation of new momentless collective excitations, that we call
\emph{vacuons}. Exact quasiphonons originate from one of the vacua by
\textquoteleft creating\textquoteright$\:$an asymmetry in the number of
opposite moment bosons. The well known Bogoliubov collective excitations (CEs)
are shown to coincide with the exact eigenstates
$|\:0,\:\mathbf{k},\:\eta\:\rangle$, i.e. with the quasiphonons created from
the lowest-level vacuum ($S=0$). All this is discussed, in view of existing or
future experimental observations of the vacuons (PBs), a sort of bosonic Cooper
pairs, which are the main factor of novelty beyond Bogoliubov theory.

Quantum Gases