Spectator stresses and CMB observables
  The large-scale curvature perturbations induced by spectator anisotropic
stresses are analyzed across the matter-radiation transition. It is assumed
that the anisotropic stress is associated with a plasma component whose energy
density is subdominant both today and prior to photon decoupling. The
enforcement of the momentum constraint and the interplay with the neutrino
anisotropic stress determine the regular initial conditions of the
Einstein-Boltzmann hierarchy. The Cosmic Microwave Background observables have
shapes and phases which differ both from the ones of the conventional adiabatic
mode as well as from their non-adiabatic counterparts.
