Galileon Radiation from Binary Systems

TL;DR

The paper analyzes scalar radiation from binary systems in the most general Galileon EFT with a conformal $\pi T$ coupling, using a perturbative split around a static, spherically symmetric background to study the Vainshtein mechanism. It finds that when quartic or quintic Galileon terms dominate, the classical perturbative expansion breaks down due to an effectively one-dimensional fluctuation dynamics that equidistributes power among many multipoles, challenging straightforward radiation calculations. Perturbation theory remains reliable only in regimes with a large mass hierarchy or when the quartic interactions become relevant only at scales smaller than the inverse pulsar frequency, revealing additional suppression beyond existing static Vainshtein intuition. The work highlights the need for full, time-dependent Galileon calculations that account for non-spherical dynamics and Vainshtein screening in realistic astrophysical systems, with implications for tests of modified gravity using binary pulsars.

Abstract

We calculate the power emitted in scalar modes for a binary systems, including binary pulsars, with a conformal coupling to the most general Galileon effective field theory by considering perturbations around a static, spherical background. While this method is effective for calculating the power in the cubic Galileon case, here we find that if the quartic or quintic Galileon dominate, for realistic pulsar systems the classical perturbative expansion about spherically symmetric backgrounds breaks down (although the quantum effective theory is well-defined). The basic reason is that the equations of motion for the fluctuations are then effectively one dimensional. This leads to many multipoles radiating with equal strength, as opposed to the normal Minkowski spacetime and cubic Galileon cases, where increasing multipoles are suppressed by increasing powers of the orbital velocity. We consider two cases where perturbation theory gives trust-worthy results: (1) when there is a large hierarchy between the masses of two orbiting objects, and (2) when we choose scales such that the quartic Galileon only begins to dominate at distances smaller than the inverse pulsar frequency. Implications for future calculations with the full Galileon that account for the Vainshtein mechanism are considered.