Adiabatic initial conditions for perturbations in interacting dark energy models

TL;DR

This paper derives adiabatic initial conditions for perturbations in an interacting dark energy model during the early radiation era, showing conditions for viability and automatic adiabaticity of dark energy.

Contribution

It provides a systematic analysis of initial conditions in interacting dark energy models with a time-varying equation of state, extending previous work to early universe conditions.

Findings

Adiabatic initial conditions are possible if early dark energy w_e > -4/5.

Dark energy automatically satisfies adiabaticity when cold dark matter, baryons, neutrinos, and photons are adiabatic.

Viable cosmologies require a time-varying dark energy equation of state.

Abstract

We present a new systematic analysis of the early radiation era solution in an interacting dark energy model to find the adiabatic initial conditions for the Boltzmann integration. In a model where the interaction is proportional to the dark matter density, adiabatic initial conditions and viable cosmologies are possible if the early-time dark energy equation of state parameter is $w_e > -4/5$. We find that when adiabaticity between cold dark matter, baryons, neutrinos and photons is demanded, the dark energy component satisfies automatically the adiabaticity condition. As supernovae Ia or baryon acoustic oscillation data require the recent-time equation of state parameter to be more negative, we consider a time-varying equation of state in our model. In a companion paper [arXiv:0907.4987] we apply the initial conditions derived here, and perform a full Monte Carlo Markov Chain likelihood analysis of this model.