The initial conditions of the universe: how much isocurvature is   allowed?
  We investigate the constraints imposed by the current data on correlated mixtures of adiabatic and non-adiabatic primordial perturbations. We discover subtle flat directions in parameter space that tolerate large (~60%) contributions of non-adiabatic fluctuations. In particular, larger values of the baryon density and a spectral tilt are allowed. The cancellations in the degenerate directions are explored and the role of priors elucidated. 
M. Bucher, J. Dunkley, P. G. Ferreira, K. Moodley, C. Skordis