  In previous papers we argued that mesons and baryons can be described as
rotating open strings in holographic backgrounds. Now we turn to closed
strings, which should be the duals of glueballs. We look at the rotating folded
closed string in both flat and curved backgrounds.
  A basic prediction of the closed string model is that the slope of Regge
trajectories is half that of open strings. We propose that a simple method to
identify glueballs is to look for resonances that belong to trajectories with a
slope of approximately 0.45 GeV^-2, half the meson slope. We therefore look at
the experimental spectra of flavorless light mesons to see if such a scheme,
where some of the states are placed on open string trajectories and some on
closed ones, can fit known experimental data. We look at the f_0 (J^PC = 0^++)
and f_2 (2^++) resonances. As there is no preference for a single scheme of
sorting the different states into meson and glueball trajectories, we present
several possibilities, each identifying a different state as the glueball. We
supplement each scheme with predictions for the masses of excited glueballs.
  We show that the width of the decay into two mesons is different for
glueballs and mesons thus providing a supplementary tool to distinguish between
them. In addition, we look at some lattice QCD results for glueball spectra and
check their compatibility with the closed string model.
  One of the main conclusions of this paper is that an extension of
experimental data on the spectrum of flavorless hadrons is needed, in
particular in the region between around 2.4 GeV and 3 GeV.
