Can someone explain Silk-damping in a conceptual way?

I understand that it is the effect causing the power spectrum to decrease in amplitude at smaller angular sizes because the effect is apparently more significant for small diffusion length(small regions), but I do not understand why that is so. How does silk-damping really work?


2 Answers 2


Think of Silk damping as a consequence of heat conduction in the primordial plasma. In the initial conditions, there are temperature variations potentially down to arbitrarily small scales. However, heat spreads over time from hot regions to cold regions, and this effect washes out small-scale temperature variations. Large-scale temperature variations still persist because the heat does not have enough time to conduct across those scales.

(Silk damping is usually described as a consequence of photon diffusion, but that's just the underlying mechanism for the thermal conduction.)


How I understand this question is following; as we know from the CMB power-spectrum at the large l power spectrum does go to zero (i.e., we're looking in the past or say early epochs of the universe) because that time, photon and matter (i.e., Baryons) were tightly coupled and moved together as a single fluid. see the below power-spectrum enter image description here

In reality recombination doesn't happen instantaneously and photons execute a random walk during this phase. That means photons travel a finite distance between scatterings and after each collision the photon will continue in some arbitrary direction. The resulting effect is that these diffused photons travel from hot to cold regions. Perturbations with wavelengths which are shorter than the photon mean path are damped that means as the photons random walk through the baryons, hot & cold regions are mixed. So in simple words we can say that the damping is the process that reduces some of the CMB-Anisotropies.

I hope this will helpful.




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