Influence Phase of a dS Observer II: Electromagnetism

We extend our proposal for static patch holography made in part I (arXiv:2309.07290) to the case of electromagnetism. Using the on-shell action on de Sitter Schwinger-Keldysh (dS-SK) geometry, we derive the influence phase of an observer in dS$_{d+1}$ interacting with bulk electromagnetic fields. This influence phase, computed with appropriate boundary conditions and counterterms, encodes the physics of electromagnetic radiation reaction and corresponding dS Hawking radiation. The self-force is rendered finite through holographic renormalisation. In the short-time limit, we reproduce electromagnetic flat space radiation reaction ala Abraham-Lorentz-Dirac, along with cosmological corrections. We give a fully covariantised expression for this radiation reaction in several even spacetime dimensions. In this process, we also extend many existing results to cosmological spacetimes: multipole expansions including smearing effects, non-relativistic expansion for electromagnetism, and classical renormalisation in odd spacetime dimensions. Further, we review and extend many properties of vector spherical harmonics(VSHs) in arbitrary dimensions, explain the relation between spherical and cartesian VSH, and derive an addition theorem for VSHs.