Anisotropy dissipation in the early Universe: Finite-temperature effects reexamined

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Abstract
We reexamine the role that finite-temperature effects play in the dissipation of the initial anisotropy of the Universe. The issue was previously studied both in the zero- and finite-temperature cases. Our results show that the finite-temperature corrections do not introduce new qualitative behavior and that the anisotropy dissipation scenario is approximately the same as in the zero-temperature case. We use a formalism that is based on the extension to finite temperature of the closed-time-path method (it can be used to study systems out of thermal equilibrium). The effective equations that we obtain are real and causal. We compare our results with others obtained in the zero- and finite-temperature cases. We discuss some qualitative differences between the formalisms that are more frequently used to study real-time processes at finite temperature.