Observations of non-Gaussianities in the cosmic microwave background increased interest in second-order cosmological perturbation theory, which induces non-linearities in the equations of motion in contrast to linear perturbation theory. In the latter, working with the truncated action or with the truncated Hamiltonian is equivalent, in the sense that truncation an Legendre transformation of the Lagrangian commute. We will prove that this also holds for the third-order truncation in full generality. Moreover, a complete scalar-vector-tensor decomposition is carried out on the third order Hamiltonian.
We close with an outlook on the next steps in this programme.