Classical radiation by free-falling charges in de Sitter spacetime

TL;DR

This paper investigates classical radiation emitted by free-falling charges in de Sitter spacetime across various fields, revealing late-time field configurations, flux behaviors, and special cases like massless minimally coupled scalars.

Contribution

It provides explicit static solutions for charges in de Sitter space, analyzes late-time field behavior, and explores flux growth for non-conformal scalar fields, including special treatment of massless minimally coupled scalars.

Findings

Charges are surrounded by negligible influence fields at late times.

A static solution exists that extends beyond the horizon, with other solutions decaying to it.

Non-conformal scalar fields exhibit flux that grows with distance, indicating parametric amplification.

Abstract

We study the classical radiation emitted by free-falling charges in de Sitter spacetime coupled to different kinds of fields. Specifically we consider the cases of the electromagnetic field, linearized gravity and scalar fields with arbitrary mass and curvature coupling. Given an arbitrary set of such charges, there is a generic result for sufficiently late times which corresponds to each charge being surrounded by a field zone with negligible influence from the other charges. Furthermore, we explicitly find a static solution in the static patch adapted to a charge (implying no energy loss by the charge) which can be regularly extended beyond the horizon to the full de Sitter spacetime, and show that any other solution decays at late times to this one. On the other hand, for non-conformal scalar fields the inertial observers naturally associated with spatially flat coordinates will see a non-vanishing flux far from the horizon, which will fall off more slowly than the inverse square of the distance for sufficiently light fields (m^2 + \xi R < 5H^2/4) and give rise to a total integrated flux that grows unboundedly with the radius. This can be qualitatively interpreted as a consequence of a classical parametric amplification of the field generated by the charge due to the time-dependent background spacetime. Most of these results do not hold for massless minimally coupled scalar fields, whose special behavior is analyzed separately.