| Astrophysics (Index) | About |
Diffusion damping (aka Silk damping) is a process presumed to occur during recombination, the damping being a reduction in the density of the denser regions, a mechanism that reduces some of the CMB anisotropies. The mechanism consists of the increase of diffusion of photons as the plasma grew more transparent (as their mean free path grows, the resulting energy diffusion gradually increases), and diffusion damping model calculates the slight tendency (on average) for photons to move away from dense regions. Plasma before recombination included many photons, enough to be in equilibrium with the baryons, i.e., as a group, photons were being created as much as they were absorbed and the total photon energy in the universe remained the same. The baryon acoustic oscillations constituted density waves at that time, and diffusion damping tended to reduce them. A result of diffusion damping is a reduction in the higher coefficients of the CMB-anisotropy power spectrum. The resulting reduced tail of the power spectrum is termed the damping tail.