Conformal Wave Expansions for Flat Space Amplitudes

Understand

The extended BMS algebra contains a conformal subgroup that acts on the celestial sphere as SO(3,1).

• It is of interest to perform mode expansions of free fields in Minkowski spacetime that realize this symmetry in a simple way.
• In the present work we perform such a mode expansion for massive scalar fields using the unitary principal series representations of SO(3,1) with a view to developing a holographic approach to gravity in asymptotically flat spacetime.
• These mode expansions are also of use in studying holography in three-dimensional de Sitter spacetime.