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Paper   IPM / Astronomy / 15221
School of Astronomy
  Title:   Systematics of adiabatic modes: flat universes
1.  E. Pajer
2.  S. Jazayeri
  Status:   Published
  Journal: JCAP
  Vol.:  2018
  Year:  2018
  Supported by:            ipm IPM
Adiabatic modes are cosmological perturbations that are locally indistinguishable from a (large) change of coordinates. At the classical level, they provide model independent solutions. At the quantum level, they lead to soft theorems for cosmological correlators. We present a systematic derivation of adiabatic modes in spatially-flat cosmological backgrounds with asymptotically-perfect fluids. We find several new adiabatic modes including vector, time-dependent tensor and time-dependent scalar modes. The new vector and tensor modes decay with time in standard cosmologies but are the leading modes in contracting universes. We present a preliminary derivation of the related soft theorems. In passing, we discuss a distinction between classical and quantum adiabatic modes, we clarify the subtle nature of Weinberg's second adiabatic mode and point out that the adiabatic nature of a perturbation is a gauge dependent statement.

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