Time evolution of Wouthuysen-Field coupling

TL;DR

This paper investigates the time evolution of Wouthuysen-Field coupling in the early universe using numerical solutions, revealing how the coupling approaches saturation over hundreds of thousands of years and its implications for 21 cm cosmology.

Contribution

Developed a WENO-based numerical solver for the photon kinetics equation, providing new insights into the time-dependent behavior of W-F coupling during reionization.

Findings

Local Boltzmann distribution forms after ~10^4 scatterings (~10^3 years).

Shape and width of the distribution are insensitive to initial conditions.

Photon flux intensity at the Boltzmann distribution is highly time-dependent, reaching saturation in 10^5-10^6 years.

Abstract

We study the Wouthuysen-Field coupling at early universe with numerical solutions of the integrodifferential equation describing the kinetics of photons undergoing resonant scattering. The numerical solver is developed based on the weighted essentially non-oscillatory (WENO) scheme for the Boltzmann-like integrodifferential equation. We focus on the time evolution of the Wouthuysen-Field (W-F) coupling in relation to the 21 cm emission and absorption at the epoch of reionization. We show that a local Boltzmann distribution will be formed if photons with frequency \sim \nu_0 have undergone a ten thousand or more times of scattering, which corresponds to the order of 10^3 yrs for neutral hydrogen density of the concordance \Lambda CDM model. The time evolution of the shape and width of the local Boltzmann distribution actually doesn't dependent on the details of atomic recoil, photon sources, or initial conditions very much. However, the intensity of photon flux at the local Boltzmann distribution is substantially time-dependent. The time scale of approaching the saturated intensity can be as long as 10^5-10^6 yrs for typical parameters of the \Lambda CDM model. The intensity of the local Boltzmann distribution at time less than 10^5 yrs is significantly lower than that of the saturation state. Therefore, it may not be always reasonable to assume that the deviation of the spin temperature of 21 cm energy states from cosmic background temperature is mainly due to the W-F coupling if first stars or their emission/absorption regions evolved with a time scale equal to or less than Myrs.