Chameleon Dark Energy

Abstract

Chameleons are scalar fields whose mass depends on the environment, specifically on the ambient matter density. While nearly massless in the cosmos, where the matter density is tiny, their mass is of order of an inverse millimeter on Earth, where the density is high. In this note, we review how chameleons can satisfy current experimental constraints on deviations from General Relativity (GR). Moreover, we study the cosmological evolution with a chameleon field and show the existence of an attractor solution, akin to the tracker solution in quintessence models. We discuss how chameleons can naturally drive the observed acceleration of the universe

Figures (2)

• Figure 1: The effective potential for the chameleon (solid line) is the sum of the "bare" potential, $V(\phi)$, which is of the runaway form (dashed line), and a density-dependent term (dotted line). Here we choose the exponential coupling: $A(\phi) = e^{\beta\phi/M_{Pl}}$
• Figure 2: The effect of kicks on the chameleon. Ignoring the kicks (dashed curve), the chameleon remains frozen at its initial value during the RD era due to Hubble damping. Including the kicks (solid curve), however, results in a total displacement of order $M_{Pl}$.