Ellipticity Weakens Chameleon Screening

TL;DR

This paper investigates how the shape of sources, specifically ellipsoids, affects the chameleon fifth force, revealing that non-spherical shapes can enhance the force compared to spherical sources.

Contribution

It provides a detailed analysis of the shape dependence of the chameleon force, extending previous studies beyond spherical symmetry to ellipsoidal sources.

Findings

Chameleon force can be up to 40% stronger for ellipsoidal sources.

Shape deformation from sphere to ellipsoid significantly impacts the force.

Ellipticity around 0.99 yields notable force enhancement.

Abstract

The chameleon mechanism enables a long range fifth force to be screened in dense environments when non-trivial self interactions of the field cause its mass to increase with the local density. To date, chameleon fifth forces have mainly been studied for spherically symmetric sources, however the non-linear self interactions mean that the chameleon responds to changes in the shape of the source differently to gravity. In this work we focus on ellipsoidal departures from spherical symmetry and compute the full form of the chameleon force, comparing it's shape dependence to that of gravity. Enhancement of the chameleon force by up to 40% is possible when deforming a sphere to an ellipsoid of the same mass, with an ellipticity $\simeq 0.99$.