The Dawn of Chemistry

TL;DR

This paper reviews the chemical evolution of the early universe, focusing on molecule formation, their role in star formation, and the potential for observational detection of primordial molecules.

Contribution

It provides a comprehensive overview of early universe chemistry, highlighting key reactions, uncertainties, and prospects for detecting primordial molecular signatures.

Findings

Primordial molecules influenced early star formation and gas cooling.

Uncertainties remain in reaction rates affecting chemical models.

Potential observational signatures include spectral distortions and anisotropies.

Abstract

Within the precise cosmological framework provided by the Lambda-Cold Dark Matter model and standard Big Bang nucleosynthesis, the chemical evolution of the pregalactic gas can now be followed with accuracy limited only by the uncertainties on the reaction rates. Starting during the recombination era, the formation of the first molecules and molecular ions containing hydrogen, deuterium, helium, and lithium was severely hindered by the low density of the expanding universe, the intensity of the cosmic radiation field, and the absence of solid catalyzers. Molecular hydrogen and deuterated hydrogen, the most abundant species formed in the gas phase prior to structure formation, played a fundamental role in the cooling of the gas clouds that gave birth to the first stellar generation, contributing to determine the scale of fragmentation. Primordial molecules also interacted with the photons of the cosmic background via resonant scattering, absorption and emission. In this review we examine the current status of the chemistry of the early universe and discuss the most relevant reactions for which uncertainties still exist from theory or laboratory experiments. The prospects for detecting spectral distortions or spatial anisotropies due to the first atoms and molecules are also addressed.