CRRfast: An emulator for the Cosmological Recombination Radiation with effects from inhomogeneous recombination

TL;DR

CRRfast is a new emulator that accurately models the Cosmological Recombination Radiation across various cosmologies, incorporating effects of inhomogeneous recombination and enabling enhanced testing of cosmological models with future CMB observations.

Contribution

We developed CRRfast, a fast and accurate emulator for the CRR that includes second-order effects from inhomogeneous recombination, extending existing tools and enabling new cosmological tests.

Findings

CRRfast accurately reproduces the CRR spectrum for diverse cosmologies.

Second-order effects from inhomogeneous recombination broaden and shift CRR features.

The emulator can evaluate the impact of primordial magnetic fields on the CRR.

Abstract

The Cosmological Recombination Radiation (CRR) is one of the guaranteed $\Lambda$CDM Spectral Distortion (SD) signals. Even if very small in amplitude, it provides a direct probe of the three recombination eras, opening the path for testing one of the key pillars in our cosmological interpretation of the measured CMB anisotropies. Here we develop a new emulator, CRRfast, to quickly and accurately represent the CRR for a wide range of cosmologies, using the state-of-the-art CosmoSpec code as a reference. CRRfast has been made publicly available both as stand-alone code and as part of CLASS, thereby completing the set of $\Lambda$CDM sources of SDs that can be modeled with CLASS. With this newly-developed pipeline we investigate the full constraining power of SDs within $\Lambda$CDM and highlight possible future applications to experimental design optimization. Furthermore, we show that the inhomogeneous evolution of the recombination process imprints second-order contributions to the CRR spectrum, leading to a broadening and shifting of the CRR features. These second-order terms are naturally captured by the emulator and allow us to evaluate the $\Lambda$CDM contributions to the average CRR as well as to illustrate the effect of perturbed recombination due to Primordial Magnetic Fields (PMFs). As it turns out, while the $\Lambda$CDM variance effects can be neglected, they could be significantly enhanced in the beyond-$\Lambda$CDM models. In particular in the case of PMFs we demonstrate that through these non-linear terms the parameter space relevant to the Hubble tension could be tested with future CMB spectrometers.