Diffusion damping
![The power spectrum of the cosmic microwave background radiation temperature anisotropy in terms of the angular scale (or multipole moment). Diffusion damping can be easily seen in the suppression of power peaks when l ≳ 1000.[1]](/uploads/202502/11/WMAP_TT_power_spectrum1650.png)
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. Around 300,000 years after the Big Bang, during the epoch of recombination, diffusing photons travelled from hot regions of space to cold ones, equalising the temperatures of these regions. This effect is responsible, along with baryon acoustic oscillations, the Doppler effect, and the effects of gravity on electromagnetic radiation, for the eventual formation of galaxies and galaxy clusters, these being the dominant large scale structures which are observed in the universe. It is a damping by diffusion, not of diffusion.