An Etymological Dictionary of Astronomy and Astrophysics

The smoothing of primordial → density fluctuations at high frequencies caused by photon → diffusion. Before the → decoupling era, photons and → baryons were tightly coupled to each other by
→ Compton scattering. However, the transition to a transparent → Universe was not instantaneous. As the → opacity of the Universe dropped, the photons started diffusing away from the positions they had while opaque, hence undergoing a → random walk. Since the → acoustic waves in the decoupling era were driven by photon pressure, the photon diffusion also led to damping of the → baryon acoustic oscillations. Silk damping suppresses all perturbations with masses smaller than about 10¹³ Msun. The implication for a theory of → structure formation is that individual galaxies must have formed in a → top-down structure formation mechanism, i.e. by the fragmentation of larger objects. However, in theories of structure formation which include → non-baryonic dark matter galaxies can form from smaller objects in a → bottom-up scenario.

See also: Joseph Silk (1942-); → damping.