The Effect of a Chameleon Scalar Field on the Cosmic Microwave Background

TL;DR

This paper investigates how a chameleon scalar field coupled to photons can modify the CMB via a SZ-like effect, providing constraints on the coupling strength and predicting observable polarization and intensity changes.

Contribution

It introduces a model of photon-chameleon coupling affecting the CMB and derives constraints from SZ measurements, also predicting observable polarization and intensity signatures.

Findings

Constraints on photon-chameleon coupling strength from SZ data

Predicted radial profile of chameleonic CMB intensity decrement

Potential discrepancies between X-ray and SZ observations due to chameleon effects

Abstract

We show that a direct coupling between a chameleon-like scalar field and photons can give rise to a modified Sunyaev-Zel'dovich (SZ) effect in the Cosmic Microwave Background (CMB). The coupling induces a mixing between chameleon particles and the CMB photons when they pass through the magnetic field of a galaxy cluster. Both the intensity and the polarization of the radiation are modified. The degree of modification depends strongly on the properties of the galaxy cluster such as magnetic field strength and electron number density. Existing SZ measurements of the Coma cluster enable us to place constraints on the photon-chameleon coupling. The constrained conversion probability in the cluster is P_Coma(204 GHz) < 6.2 * 10^{-5} at 95% confidence, corresponding to an upper bound on the coupling strength of g_eff < 2.2 * 10^{-8} GeV^-1 or g_eff < 7.2-32.5 * 10^{-10} GeV^-1, depending on the model that is assumed for the cluster magnetic field structure. We predict the radial profile of the chameleonic CMB intensity decrement. We find that the chameleon effect extends further towards the edges of the cluster than the thermal SZ effect. Thus we might see a discrepancy between the X-ray emission data and the observed SZ intensity decrement. We further predict the expected change to the CMB polarization arising from the existence of a chameleon-like scalar field. These predictions could be verified or constrained by future CMB experiments.