A gauge-invariant approach to interactions in the dark sector

TL;DR

This paper develops a gauge-invariant framework to analyze interactions between dark energy and dark matter, revealing subtle effects on cosmological observables and emphasizing the significance of momentum exchanges.

Contribution

It introduces a gauge-invariant method for studying dark sector interactions, providing detailed calculations of power spectra and ISW effect for various models.

Findings

Dark-sector interactions can enhance matter power spectrum at large scales.

Interactions can boost low multipoles in CMB anisotropies.

Effects are subtler than previous simple equation of state models.

Abstract

We outline a gauge-invariant framework to calculate cosmological perturbations in dark energy models consisting of a scalar field interacting with dark matter via energy and momentum exchanges. Focusing on three well-known models of quintessence and three common types of dark-sector interactions, we calculate the matter and dark energy power spectra as well as the Integrated Sachs-Wolfe (ISW) effect in these models. We show how the presence of dark-sector interactions can produce a large-scale enhancement in the matter power spectrum and a boost in the low multipoles of the cosmic microwave background anisotropies. Nevertheless, we find these enhancements to be much more subtle than those found by previous authors who model dark energy using simple ansatz for the equation of state. We also address issues of instabilities and emphasise the importance of momentum exchanges in the dark sector.