Systematics of Adiabatic Modes in Cosmology

We present a systematic discussion of adiabatic modes in FLRW cosmologies, i.e. cosmological perturbations that are locally approximated by a change of coordinates. We find new scalar, vectorial and tensorial adiabatic modes. We also show that not only the constant tensor mode but also the tensor time dependent mode is adiabatic on large scales. Our new modes decay with time in standard cosmologies but grow in contracting universes. Also we discuss that why some of these adiabatic modes are absent for the special case of a scalar field. We make the connection between these adiabatic modes and soft theorems. In light of the new time dependent adiabatic tensor modes, we discuss a distinction between classical and quantum adiabatic modes.