Non-equilibrium H$_2$ formation in the early Universe: energy exchanges, rate coefficients and spectral distortions

TL;DR

This paper investigates how non-thermal distributions of molecular hydrogen in the early Universe influence energy exchange, chemical reactions, and spectral distortions, highlighting the importance of non-equilibrium processes in cosmological models.

Contribution

It provides new calculations of excitation temperatures, reaction rate modifications, and spectral distortions due to non-equilibrium H$_2$ in the early Universe.

Findings

Non-equilibrium processes significantly affect chemical reaction rates.

Spectral distortions from H$_2$ formation are below current detection limits.

New fits for heat transfer functions are provided.

Abstract

Energy exchange processes play a crucial role in the early Universe, affecting the thermal balance and the dynamical evolution of the primordial gas. In the present work we focus on the consequences of a non-thermal distribution of the level populations of H$_2$: first, we determine the excitation temperatures of vibrational transitions and the non-equilibrium heat transfer; second, we compare the modifications to chemical reaction rate coefficients with respect to the values obtained assuming local thermodynamic equilibrium; third, we compute the spectral distortions to the cosmic background radiation generated by the formation of H$_2$ in vibrationally excited levels. We conclude that non-equilibrium processes cannot be ignored in cosmological simulations of the evolution of baryons, although their observational signatures remain below current limits of detection. New fits to the equilibrium and non-equilibrium heat transfer functions are provided.