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arXiv · 2509.25007

Gravitational entropy of fluids with energy flux

Roberto A. Sussman·Sebastián Nájera·Fernando A. Pizaña·Juan Carlos Hidalgo
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Abstract

We examine gravitational entropy growth within the formalism of Clifton, Ellis and Tavakol (CET) applied to a class of spherically symmetric exact solutions whose source is a shear-free fluid with energy flux in a comoving frame. By considering these solutions as potential cosmological models, we update previous literature that considered them only as restricted toy models of radiating spheres collapsing in a Vaidya background. In the present paper we examine the integrability of the CET entropy form in connection with Einstein's equations in the fluid flow approach, proving as well that all expanding configurations comply with the growth of CET gravitational entropy. Finally, we examine the connection between the CET gravitational entropy and the notion of a gravitational ``arrow of time'' based on the ratio of Weyl to Ricci curvature. Some of the solutions also provide potentially useful and viable inhomogeneous generalizations of FLRW models, thus suggesting an appealing potential for applications to current cosmological research.

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Roberto A. Sussman, Sebastián Nájera, Fernando A. Pizaña, Juan Carlos Hidalgo. 2025-09-29. Gravitational entropy of fluids with energy flux. https://arxiv.org/abs/2509.25007

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