How CMB and large-scale structure constrain chameleon interacting dark energy

We explore a chameleon type of interacting dark matter-dark energy scenario in which a scalar field adiabatically traces the minimum of an effective potential sourced by the dark matter density. We discuss extensively the effect of this coupling on cosmological observables, especially the parameter degeneracies expected to arise between the model parameters and other cosmological parameters, and then test the model against observations of the cosmic microwave background (CMB) anisotropies and other cosmological probes. We find that the chameleon parameters $\alpha$ and $\beta$, which determine respectively the slope of the scalar field potential and the dark matter-dark energy coupling strength, can be constrained to $\alpha < 0.17$ and $\beta < 0.19$ using CMB data alone. The latter parameter in particular is constrained only by the late Integrated Sachs-Wolfe effect. Adding measurements of the local Hubble expansion rate $H_0$ tightens the bound on $\alpha$ by a factor of two, although this apparent improvement is arguably an artefact of the tension between the local measurement and the $H_0$ value inferred from Planck data in the minimal $\Lambda$CDM model. The same argument also precludes chameleon models from mimicking a dark radiation component, despite a passing similarity between the two scenarios in that they both delay the epoch of matter-radiation equality. Based on the derived parameter constraints, we discuss possible signatures of the model for ongoing and future large-scale structure surveys.