How to use retarded Green's functions in de Sitter spacetime

TL;DR

This paper demonstrates the correct application of covariant retarded Green's functions in electromagnetism and linearized gravity within de Sitter spacetime through explicit examples, clarifying their use in spacetimes with spacelike past infinity.

Contribution

It clarifies how to properly use retarded Green's functions in de Sitter spacetime and provides explicit examples confirming their correct application in electromagnetism and gravity.

Findings

Green's functions reproduce fields accurately in de Sitter spacetime.

Constructed gravitational field from point masses matches Green's function predictions.

Method applies to fields with specified initial data and sources in de Sitter.

Abstract

We demonstrate in examples that the covariant retarded Green's functions in electromagnetism and linearized gravity work as expected in de Sitter spacetime. We first clarify how retarded Green's functions should be used in spacetimes with spacelike past infinity such as de Sitter spacetime. In particular, we remind the reader of a general formula which gives the field for given initial data on a Cauchy surface and a given source (a charge or stress-energy tensor distribution) in its future. We then apply this formula to three examples: (i) electromagnetism in the future of a Cauchy surface in Minkowski spacetime, (ii) electromagnetism in de Sitter spacetime, and (iii) linearized gravity in de Sitter spacetime. In each example the field is reproduced correctly as predicted by the general argument. In the third example we construct a linearized gravitational field from two equal point masses located at the "North and South Poles" which is non-singular on the cosmological horizon and satisfies a covariant gauge condition and show that this field is reproduced by the retarded Green's function with corresponding gauge parameters.