title: Silk damping at a redshift of a billion: new limit on small-scale adiabatic perturbations.
abstract: We study the dissipation of small-scale adiabatic perturbations at early times when the Universe is hotter than T≃0.5  keV. When the wavelength falls below the damping scale k(D)(-1), the acoustic modes diffuse and thermalize, causing entropy production. Before neutrino decoupling, k(D) is primarily set by the neutrino shear viscosity, and we study the effect of acoustic damping on the relic neutrino number, primordial nucleosynthesis, dark-matter freeze-out, and baryogenesis. This sets a new limit on the amplitude of primordial fluctuations of Δ(R)(2)<0.007 at 10(4)  Mpc(-1)≲k≲10(5)  Mpc(-1) and a model-dependent limit of Δ(R)(2)≲0.3 at k≲10(20-25)  Mpc(-1).
authors.affiliation: Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
authors.forename: Donghui
authors.initials: D
authors.lastname: Jeong
authors.affiliation: Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
authors.forename: Josef
authors.initials: J
authors.lastname: Pradler
authors.affiliation: Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
authors.forename: Jens
authors.initials: J
authors.lastname: Chluba
authors.affiliation: Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
authors.forename: Marc
authors.initials: M
authors.lastname: Kamionkowski