Galileon Radiation from Binary Systems

TL;DR

This paper investigates scalar radiation from binary systems within Galileon theories, revealing breakdowns in classical perturbation methods for higher-order Galileons and proposing conditions where reliable calculations are possible.

Contribution

It analyzes scalar radiation in binary systems for general Galileon theories, highlighting limitations of perturbation theory for quartic and quintic cases and identifying regimes for valid calculations.

Findings

Classical perturbation theory breaks down for quartic/quintic Galileons in realistic pulsar systems.

Higher multipoles radiate with similar strength in these theories, unlike in cubic Galileons.

Perturbation theory is reliable when there's a large mass hierarchy or at scales below the inverse pulsar frequency.

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.