Extended Chameleons

TL;DR

This paper extends chameleon scalar field models with density-dependent potentials, analyzing their effects on gravity tests from Solar System to galactic scales, and exploring potential observable deviations in galaxy rotation curves.

Contribution

It introduces a quadratic scalar-fluid coupling with density-dependent minima and masses, and studies its implications across different astrophysical scales.

Findings

Models can satisfy Solar System constraints while modifying gravity on galactic scales.

The fifth force can cause significant deviations in galaxy cores, especially with a constant scalar mass and linearly varying potential minimum.

Cosmologically, the models do not affect large-scale structure growth, making galactic observations key for testing.

Abstract

We extend the chameleon models by considering Scalar-Fluid theories where the coupling between matter and the scalar field can be represented by a quadratic effective potential with density-dependent minimum and mass. In this context, we study the effects of the scalar field on Solar System tests of gravity and show that models passing these stringent constraints can still induce large modifications of Newton's law on galactic scales. On these scales we analyse models which could lead to a percent deviation of Newton's law outside the virial radius. We then model the dark matter halo as a Navarro-Frenk-White profile and explicitly find that the fifth force can give large contributions around the galactic core in a particular model where the scalar field mass is constant and the minimum of its potential varies linearly with the matter density. At cosmological distances, we find that this model does not alter the growth of large scale structures and therefore would be best tested on galactic scales, where interesting signatures might arise in the galaxy rotation curves.