Coupling structure of multi-field primordial perturbations

Understand

We investigate the coupling relations among perturbations in general multi-field models.

• We derived the equations of motion for both background and perturbations in a general basis.
• Within this formalism, we revisit the construction of kinematic orthogonal normal vectors using the successive time derivatives of the background field velocity.
• We show that the coupling relations among modes in this kinematic basis can be reduced, by employing the background equations of motion for the scalar fields and their high order time derivatives.