Diffusion Damping - Introduction

Introduction

Diffusion damping took place about 13.8 billion years ago, during the stage of the early universe called recombination or matter-radiation decoupling. This period occurred about 380,000 years after the Big Bang. This is equivalent to a redshift of around z = 1090. Recombination was the stage during which simple atoms, e.g. hydrogen and helium, began to form in the cooling, but still very hot, soup of protons, electrons and photons that composed the universe. Prior to the recombination epoch, this soup, a plasma, was largely opaque to the electromagnetic radiation of photons. This meant that the permanently excited photons were scattered by the protons and electrons too often to travel very far in straight lines. During the recombination epoch, the universe cooled rapidly as free electrons were captured by atomic nuclei; atoms formed from their constituent parts and the universe became transparent: the amount of photon scattering decreased dramatically. Scattering less, photons could diffuse (travel) much greater distances. There is no significant diffusion damping for electrons, which cannot diffuse nearly as far as photons can in similar circumstances. Thus all damping by electron diffusion is negligible when compared to photon diffusion damping.

Acoustic perturbations of initial density fluctuations in the universe made some regions of space hotter and denser than others. These differences in temperature and density are called anisotropies. Photons diffused from the hot, overdense regions of plasma to the cold, underdense ones: they dragged along the protons and electrons: the photons pushed electrons along, and these, in turn, pulled on protons by the Coulomb force. This caused the temperatures and densities of the hot and cold regions to be averaged and the universe became less anisotropic (characteristically various) and more isotropic (characteristically uniform). This reduction in anisotropy is the damping of diffusion damping. Diffusion damping thus damps temperature and density anisotropies in the early universe. With baryonic matter (protons and electrons) escaping the dense areas along with the photons; the temperature and density inequalities were adiabatically damped. That is to say the ratios of photons to baryons remained constant during the damping process.

Photon diffusion was first described in Joseph Silk's 1968 paper entitled "Cosmic Black-Body Radiation and Galaxy Formation", which was published in The Astrophysical Journal. As such, diffusion damping is sometimes also called Silk damping, though this term may apply only to one possible damping scenario. Silk damping was thus named after its discoverer.