Dynamics of entropy perturbations in assisted dark energy with mixed kinetic terms

TL;DR

This paper investigates how entropy perturbations evolve in assisted dark energy models with two scalar fields, showing they can influence cosmic microwave background observations, especially through the ISW effect, depending on initial conditions and field interactions.

Contribution

It demonstrates that entropy perturbations in two-field assisted dark energy models tend to remain constant on large scales and can significantly impact the ISW effect, especially in k-essence scenarios.

Findings

Entropy perturbations remain constant on large scales during radiation and matter eras.

Entropy perturbations can enhance or suppress the ISW effect depending on their correlation with curvature perturbations.

In k-essence models, initial entropy perturbations can be large enough to leave observable imprints on the CMB.

Abstract

We study dynamics of entropy perturbations in the two-field assisted dark energy model. Based on the scenario of assisted dark energy, in which one scalar field is subdominant compared with the other in the early epoch, we show that the entropy perturbations in this two-field system tend to be constant on large scales in the early epoch and hence survive until the present era for a generic evolution of both fields during the radiation and matter eras. This behaviour of the entropy perturbations is preserved even when the fields are coupled via kinetic interaction. Since, for assisted dark energy, the subdominant field in the early epoch becomes dominant at late time, the entropy perturbations can significantly influence the dynamics of density perturbations in the universe. Assuming correlations between the entropy and curvature perturbations, the entropy perturbations can enhance the integrated Sachs-Wolfe (ISW) effect if the signs of the contributions from entropy perturbations and curvature perturbations are opposite after the matter era, otherwise the ISW contribution is suppressed. For canonical scalar field the effect of entropy perturbations on ISW effect is small because the initial value of the entropy perturbations estimated during inflation cannot be sufficiently large. However, in the case of k-essence, the initial value of the entropy perturbations can be large enough to affect the ISW effect to leave a significant imprint on the CMB power spectrum.