Note on gauge invariance of second order cosmological perturbations

Understand

We study the gauge invariant cosmological perturbations up to second order.

• We show that there are infinite families of gauge invariant variables at both of the first and second orders.
• The conversion formulae among different families are shown to be described by a finite number of bases that are gauge invariant.
• For the second order cosmological perturbations induced by the first order scalar perturbations, we explicitly represent the equations of motion of them in terms of the gauge invariant Newtonian, synchronous and hybrid variables, respectively.