Correlated isocurvature perturbations from mixed inflaton-curvaton decay

We study cosmological perturbations in the case that present-day matter consists of a mixture of inflaton and curvaton decay products. We calculate how the curvaton perturbations are transferred to its decay products in the general case when it does not behave like dust. Taking into account that the decay products of the inflaton can also have perturbations results in an interesting mixture of correlated adiabatic and isocurvature perturbations. In particular, negative correlation can improve the fit to the CMB data by lowering the angular power in the Sachs-Wolfe plateau without changing the peak structure. We do an 11-parameter fit to the WMAP data. We find that the best-fit is not the 'concordance model', and that well-fitting models do not cluster around the best-fit, so that cosmological parameters cannot be reliably estimated. We also find that in our model the mean quadrupole (l=2) power is l(l+1) C_l/2pi = 1081 muK^2, much lower than in the pure adiabatic LCDM model, which gives 1262 muK^2.