Retarded Green's Function Of A Vainshtein System And Galileon Waves

TL;DR

This paper derives the retarded Green's function for Galileon fields around a massive body, analyzing how Galileon radiation is emitted and suppressed in various regimes, relevant for testing modified gravity theories with the Vainshtein mechanism.

Contribution

It provides the first detailed calculation of the retarded Galileon Green's function in relevant limits, elucidating radiation emission characteristics within the Vainshtein radius.

Findings

Galileon radiation is suppressed at high frequencies for monopole and dipole modes.

Radiation rates are enhanced at high multipole orders when the source is close to the central mass.

At low frequencies, Galileon waves produce comparable monopole and dipole radiation, amplified by the Vainshtein radius.

Abstract

Motivated by the desire to test modified gravity theories exhibiting the Vainshtein mechanism, we solve in various physically relevant limits, the retarded Galileon Green's function (for the cubic theory) about a background sourced by a massive spherically symmetric static body. The static limit of our result will aid us, in a forthcoming paper, in understanding the impact of Galileon fields on the problem of motion in the solar system. In this paper, we employ this retarded Green's function to investigate the emission of Galileon radiation generated by the motion of matter lying deep within the Vainshtein radius r_v of the central object: acoustic waves vibrating on its surface, and the motion of compact bodies gravitationally bound to it. If \lambda is the typical wavelength of the emitted radiation, and r_0 is the typical distance of the source from the central mass, with r_0 << r_v, then, compared to its non-interacting massless scalar counterpart, we find that the Galileon radiation rate is suppressed by the ratio (r_v/\lambda)^{-3/2} at the monopole and dipole orders at high frequencies r_v/\lambda >> 1. However, at high enough multipole order, the radiation rate is enhanced by powers of r_v/r_0. At low frequencies r_v/\lambda << 1, and when the motion is non-relativistic, Galileon waves yield a comparable rate for the monopole and dipole terms, and are amplified by powers of the ratio r_v/r_0 for the higher multipoles.