Quasinormal modes in de Sitter space: Plane wave method

Recently, in the context of dS/CFT correspondence, quasinormal modes have been put forward to address certain features of this conjecture. In particular, it is argued that the dual states of quasinormal modes are in fact the states of CFT₃ which are created by operator insertions. For a scalar field in dS₄, quasinormal modes which are singular on the past horizon of the south pole and decay exponentially towards the future have been considered in [G. S. Ng and A. Strominger, Classical Quantum Gravity 30, 104002 (2013); D. L. Jafferis et al., arXiv:1305.5523]; these modes lie in two complex highest-weight representations of the dS₄ isometry group. In this work, we present a simple group representation analysis of these modes so that the de Sitter invariance is obviously manifest. By making use of the so-called plane wave method, we will show that the quasinormal modes correspond to one class of the unitary irreducible representation of the de Sitter group. This consideration could be generalized straightforwardly for higher-spin fields and higher dimensions; in particular, we will study the quasinormal modes for gauge and spinor fields, and, in the case of a scalar field, the generalization to higher dimensions is also obtained.