Post-Recombination Fluctuations from a Sequestered Dark Sector

TL;DR

This paper introduces a formalism to analyze how late-time energy injections from a hidden dark sector affect cosmological observations, especially the CMB, with current data constraining such injections to very small levels.

Contribution

It develops a new framework for understanding the impact of short-timescale energy injections on cosmological fluctuations and the CMB, focusing on sequestered dark sectors.

Findings

Current observations limit fractional energy injections to the permille level.

The formalism accounts for scalar, vector, and tensor perturbations affecting photon geodesics.

Instantaneous anisotropic stress injections dominate the signal in the CMB.

Abstract

We develop a formalism to characterize the imprints of late-time sources of cosmological fluctuations under the sole assumption that the injection occurs on timescales short compared to the horizon. For post-recombination injections, we derive the general modification of photon geodesics in the presence of scalar, vector, and tensor perturbations, and compute the resulting impact on the Cosmic Microwave Background through the integrated Sachs-Wolfe effect. We show that the signal is generically dominated by instantaneous injections of anisotropic stress. As an application, we consider first-order phase transitions in a sequestered dark sector and show that current observations constrain fractional energy injections at the permille level.