  The accurate calculation of excited state properties of interacting electrons
in the condensed phase is an immense challenge in computational physics. Here,
we use state-of-the-art equation-of-motion coupled-cluster theory with single
and double excitations (EOM-CCSD) to calculate the dynamic structure factor,
which can be experimentally measured by inelastic x-ray and electron
scattering. Our calculations are performed on the uniform electron gas at
densities corresponding to Wigner-Seitz radii of $r_s=5$, 4, and 3
corresponding to the valence electron densities of common metals. We compare
our results to those obtained using the random-phase approximation, which is
known to provide a reasonable description of the collective plasmon excitation
and which resums only a small subset of the polarizability diagrams included in
EOM-CCSD. We find that EOM-CCSD, instead of providing a perturbative
improvement on the RPA plasmon, predicts a many-state plasmon resonance, where
each contributing state has a double-excitation character of 80\% or more. This
finding amounts to an ab initio treatment of the plasmon linewidth, which is in
good quantitative agreement with previous diagrammatic calculations, and
highlights the strongly correlated nature of lifetime effects in
condensed-phase electronic structure theory.
