Entropy production in a lepton-photon universe

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• 作者：Lars Husdal ; Iver Brevik
• 关键词：Viscous cosmology ; Bulk viscosity ; Lepton era ; Relativistic kinetic theory ; Entropy ; Entropy production
• 刊名：Astrophysics and Space Science
• 出版年：2017
• 出版时间：February 2017
• 年：2017
• 卷：362
• 期：2
• 全文大小：
• 刊物类别：Physics and Astronomy
• 刊物主题：Astrophysics and Astroparticles; Astronomy, Observations and Techniques; Cosmology; Space Sciences (including Extraterrestrial Physics, Space Exploration and Astronautics); Astrobiology;
• 出版者：Springer Netherlands
• ISSN：1572-946X
• 卷排序：362

文摘

We look at the entropy production during the lepton era in the early universe by using a model where we exclude all particles except the leptons and photons. We assume a temperature dependent viscosity as calculated recently by one of us (Husdal in Astrophys. Space Sci. 361(8):1, 2016b) with the use of relativistic kinetic theory. We consider only the bulk viscosity, the shear viscosity being omitted because of spatial isotropy. The rate of entropy production is highest just before the neutrinos decouple. Our results show that the increase in entropy during the lepton era is quite small, about 0.071 % at a decoupling temperature of \(T=10^{10}~\mbox{K}\). This result is slightly smaller than that obtained earlier by Caderni and Fabbri (Phys. Lett. B 69:508, 1977). After the neutrino decoupling, when the Universe has entered the photon era, kinetic theory arguments no longer support the appearance of a bulk viscosity. At high temperatures and a stable particle ratio, entropy production (\(\mathrm {d}{\sigma }/\mathrm {d}{T}\)) goes as \(T^{-8}\), with the total entropy (\(\Delta \sigma\)) increasing as \(T^{-7}\). These rates go slightly down just before the neutrinos decouple, where \(\Delta\sigma\propto T^{-6.2}\).