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.
Highlights
The arrow of time arises from the universe being far from equilibrium in a state of low entropy
The origin of all thermodynamic irreversibility in the real universe depends on gravitation. In this connection Leubner [6] writes: “In contrast to thermodynamic systems driven to a uniform distribution, the components of gravitating systems tend to clump, implying a gravitational arrow of time, which points in the direction of growing inhomogeneity”, and further: “Increasing inhomogeneity due to gravitational clumping reflects increasing gravitational entropy in a time evolving universe”
We shall calculate the “Weyl-entropy” change during the inflationary era in a plane-symmetric Bianchi type I universe dominated by Lorentz invariant vacuum energy (LIVE), using SG1 given in eq (34) and SG 2 PV with P given in eq (38), as measures of gravitational entropy
Summary
The arrow of time arises from the universe being far from equilibrium in a state of low entropy. Entropy 2012, 14 subsequent entropy increase likely for many billions of years.”. He notes: “When we look to cosmology for information about the actual Past State, we find early cosmological states that appear to be states of very high entropy, not very low entropy. Measurements of temperature variations in the cosmic microwave background has shown that 400,000 years after the Big Bang the universe was in a state very close to thermal equilibrium. The temperature differences became much larger, and one might wonder if the thermodynamic entropy of the universe had become smaller in conflict with the Second Law of Thermodynamics. In the present review we shall consider different aspects of this question
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