Diffusion damping
In modern cosmological theory, diffusion damping, also called photon diffusion damping, is a physical process which reduced density inequalities (anisotropies) in the early universe, making the universe itself and the cosmic microwave background radiation (CMB) more uniform.
[4] Diffusion damping took place about 13.8 billion years ago,[6] during the stage of the early universe called recombination or matter-radiation decoupling.
[8] 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.
[9] 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.
[11] Acoustic perturbations of initial density fluctuations in the universe made some regions of space hotter and denser than others.
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.
With baryonic matter (protons and electrons) escaping the dense areas along with the photons; the temperature and density inequalities were adiabatically damped.
[3][13][14][15][16] Photon diffusion was first described in Joseph Silk's 1968 paper entitled "Cosmic Black-Body Radiation and Galaxy Formation",[17] which was published in The Astrophysical Journal.
That is, the mean free path of the photons is inversely proportional to the electron ionisation fraction and the baryon number density (
That means that the more baryons there were, and the more they were ionised, the shorter the average photon could travel before encountering one and being scattered.
[3] This dependence on the baryon density by photon diffusion allows scientists to use analysis of the latter to investigate the former, in addition to the history of ionisation.
[23] The effect of diffusion damping is greatly augmented by the finite width of the surface of last scattering (SLS).
[24] The finite width of the SLS means the CMB photons we see were not all emitted at the same time, and the fluctuations we see are not all in phase.
[25] It also means that during recombination, the diffusion length changed dramatically, as the ionisation fraction shifted.
This means that without an accurate model of diffusion damping, scientists cannot judge the relative merits of cosmological models, whose theoretical predictions cannot be compared with observational data, this data being obscured by damping effects.
For example, the peaks in the power spectrum due to acoustic oscillations are decreased in amplitude by diffusion damping.
[16] Photon diffusion is not dependent on the causes of the initial fluctuations in the density of the universe.
[23] Damping occurs at two different scales, with the process working more quickly over short ranges than over longer distances.
A long distance is one that is greater than the mean free path, if still less than the diffusion length.
On the larger scale, anisotropies are decreased more slowly, with significant degradation happening within one unit of Hubble time.
[4][11] Scientists say diffusion damping affects small angles and corresponding anisotropies.
[31] Scientists study photon diffusion damping (and CMB anisotropies in general) because of the insight the subject provides into the question, "How did the universe come to be?".
Specifically, primordial anisotropies in the temperature and density of the universe are supposed to be the causes of later large-scale structure formation.
Diffusion damping made the universe isotropic within distances on the order of the Silk Scale.
However, these cosmic magnetic fields may have been damped by radiative diffusion: just as acoustic oscillations in the plasma were damped by the diffusion of photons, so were magnetosonic waves (waves of ions travelling through a magnetised plasma).
This process began before the era of neutrino decoupling and ended at the time of recombination.
