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Entropy 2012, 14(12), 2456-2477; doi:10.3390/e14122456
Entropy and Gravity
Received: 26 October 2012; in revised form: 22 November 2012 / Accepted: 23 November 2012 / Published: 4 December 2012
(This article belongs to the Special Issue Modified Gravity: From Black Holes Entropy to Current Cosmology)
Abstract: The effect of gravity upon changes of the entropy of a gravity-dominated system is discussed. In a universe dominated by vacuum energy, gravity is repulsive, and there is accelerated expansion. Furthermore, inhomogeneities are inflated and the universe approaches a state of thermal equilibrium. The difference between the evolution of the cosmic entropy in a co-moving volume in an inflationary era with repulsive gravity and a matter-dominated era with attractive gravity is discussed. The significance of conversion of gravitational energy to thermal energy in a process with gravitational clumping, in order that the entropy of the universe shall increase, is made clear. Entropy of black holes and cosmic horizons are considered. The contribution to the gravitational entropy according to the Weyl curvature hypothesis is discussed. The entropy history of the Universe is reviewed.
Keywords: entropy; gravity; gravitational contraction; cosmological constant; black hole; horizon; Weyl curvature hypothesis; inflationary era
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