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Entropy, Volume 4, Issue 3 (March 2002), Pages 49-127

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Research

Open AccessArticle Conformal Symmetry and Cosmological Entropy Production
Entropy 2002, 4(3), 49-64; doi:10.3390/e4030049
Received: 16 August 2001 / Accepted: 3 March 2002 / Published: 10 March 2002
Cited by 5 | PDF Full-text (196 KB)
Abstract
Introducing an effective refraction index of an isotropic cosmic medium, we investigate the cosmological fluid dynamics which is consistent with a conformal, timelike symmetry of a corresponding "optical" metric. We demonstrate that this kind of symmetry is compatible with the existence of [...] Read more.
Introducing an effective refraction index of an isotropic cosmic medium, we investigate the cosmological fluid dynamics which is consistent with a conformal, timelike symmetry of a corresponding "optical" metric. We demonstrate that this kind of symmetry is compatible with the existence of a negative viscous pressure and, consequently, with cosmological entropy production. We establish an exactly solvable model according to which the viscous pressure is a consequence of a self-interacting one-particle force which is self-consistently exerted on the microscopic particles of a relativistic gas. Furthermore, we show that a suficiently high decay rate of the refraction index of an ultrarelativistic cosmic medium results in an in ationary expansion of the universe. Full article
Open AccessArticle Extremal Black Holes in Supergravity and the Bekenstein-Hawking Entropy
Entropy 2002, 4(3), 65-127; doi:10.3390/e4030065
Received: 19 February 2002 / Accepted: 6 March 2002 / Published: 10 March 2002
PDF Full-text (563 KB)
Abstract
We review some results on the connection among supergravity central charges, BPS states and Bekenstein-Hawking entropy. In particular, N = 2 super-gravity in four dimensions is studied in detail. For higher N supergravities we just give an account of the general theory [...] Read more.
We review some results on the connection among supergravity central charges, BPS states and Bekenstein-Hawking entropy. In particular, N = 2 super-gravity in four dimensions is studied in detail. For higher N supergravities we just give an account of the general theory specializing the discussion to the N = 8 case when one half of supersymmetry is preserved. We stress the fact that for extremal supergravity black holes the entropy formula is topological, that is the entropy turns out to be a moduli independent quantity and can be written in terms of invariants of the duality group of the supergravity theory. Full article

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