Into the Gompverse: A robust Gompertzian reionization model for CMB analyses

TL;DR

This paper introduces a physically motivated Gompertzian reionization model for CMB analyses, improving parameter constraints and enabling better joint analysis with reionization data.

Contribution

The authors develop a new Gompertzian reionization model with three parameters, reducing uncertainties in optical depth and enhancing constraints on neutrino masses.

Findings

Reduced uncertainty in optical depth by a factor of three.

Tighter constraints on the sum of neutrino masses.

Synergistic potential with 21 cm reionization experiments.

Abstract

Cosmic reionization is driven by the formation of sources of ultraviolet photons, and hence it is an intrinsically asymmetric process, where its earlier stages occur at a slower pace relative to its later stages. Yet most modern cosmic microwave background (CMB) analyses rely on a hyperbolic tangent template, i.e. a symmetric sigmoid, that is not well suited for joint fitting of CMB and reionization observations. In this work, we introduce a physically motivated Gompertzian reionization model with three astrophysical (nuisance) parameters, designed to enable joint analyses of CMB and reionization data and to be applicable to a wide range of datasets and cosmological models. This robust Gompertzian model leverages the connection between cosmology and reionization, typically ignored in standard CMB analyses, to demote the optical depth ($\tau_{\rm reio}$) to derived parameter, reducing its uncertainty by approximately a factor of three compared to the conventional $\tanh$ prescription. The $\tau_{\rm reio}$ improvement enables tighter constraints on the sum of the neutrino masses, revealing potential tension with neutrino oscillation experiments even after accounting for the known relaxation of neutrino mass bounds in $w_0w_a$CDM models -- a tension that is partially obscured by the conventional treatment of reionization. In addition, the inferred constraints on the astrophysical parameters governing reionization naturally synergize with current and upcoming 21 cm experiments, providing physically informed parameter ranges for future 21 cm studies.