  We present a calculation of the low lying spectrum of a rotating
Bose-Einstein condensate. We show that in a cylindrical geometry, there exist
two linear branches, one associated to usual acoustic excitation, the other
corresponding to a twisting mode of the vortex lattice. Using a hydrodynamical
approach we derive the elasticity coefficient of the vortex lattice and
calculate the spectrum of condensate in a three dimensionnal harmonic trap with
cylindrical symmetry.
