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The evolution of nonlinear subhorizon-scale entropy fluctuations in the early universe

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We examine the damping of nonlinear subhorizon-scale entropy fluctuations in early epochs of the universe (T ≈ 100 GeV to T ≈ 1 keV) by neutrino-, baryon-, and photon-induced dissipative processes. Results of numerical evolution calculations are presented for broad ranges of initial fluctuation amplitudes and length scales. These calculations include a detailed treatment of neutrino inflation, neutron and proton diffusion, photon diffusive heat transport, and hydrodynamic expansion with photon-electron Thomson drag. Neutrino inflation is treated both in the diffusive heat transport regime where neutrinos are optically thick on the length scales of the fluctuations, and in the homogeneous heating regime where neutrinos are optically thin on these scales. We find considerable convergence in amplitude evolution for appreciable ranges in initial fluctuation length scales and amplitudes. Fluctuations produced with the right characteristics at very early times (T ≳ 100 GeV) are found to survive through the nucleosynthesis epoch.

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