# Adiabatic vs. Isocurvature cosmological perturbations

1. Mar 17, 2015

### yancey

Dear everyone,
Now, I'm reading some papers on the "Curvaton Mechanism"(eg.http://arxiv.org/abs/hep-ph/0110002v2 as the original literature.) Here I have some confusion on this topic.

(1) Why the primordial perturbation of curvaton field is initially isocurvature-type? When the inflation driven by the inflaton field ends, the cosmological fluid is consist of radiation and curvaton, with radiation dominating the universe. I think it's meaningless to characterize a single constituent with adiabatic or isocurvature perturbation. So when people say the curvaton perturbation is initially isocurvature, dose they mean $$\delta{\rho}_{\sigma} =-\delta{\rho}_{r}?$$ i.e. the density pertubation of radiations cancel with the one of curvaton at each point in space. I wonder why this is the truth.

(2) When dose the transformation from isocurvature perturbation to adiabatic perturbation begin? The idea of the literature is that the generation of the curvature perturbation begins at the point when the curvaton field starts to oscillate. Why cannot the transformation starts earliar?

(3) When dose the generation of the curvature perturbation end? The literature argue that "It ends when the pressure perturbation again becomes adiabatic, which is at the epoch of curvaton matter domination, or the epoch of curvaton decay, whichever is earlier." Why the moment of curvaton matter domination or the curvaton dacay should be the moment when the pressure perturbation again becomes adiabatic?

Can anybody give me some hint on these? Maybe I should do some calculation to understand these problem. Thanks for your time.

2. Mar 22, 2015