THERMODYNAMICS IN MODIFIED GRAVITY THEORIES

We demonstrate that there exists an equilibrium description of thermodynamics on the apparent horizon in the expanding cosmological background for a wide class of modified gravity theories with the Lagrangian density f(R,ϕ,X), where R is the Ricci scalar and X is the kinetic energy of a scalar field ϕ. This comes from a suitable definition of an energy momentum tensor of the "dark" component obeying the local energy conservation law in the Jordan frame. It is shown that the equilibrium description in terms of the horizon entropy S is convenient because it takes into account the contribution of the horizon entropy Ŝ in non-equilibrium thermodynamics as well as an entropy production term.

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Thermodynamic properties of modified gravity theories

International Journal of Geometric Methods in Modern Physics ◽

10.1142/s0219887816300075 ◽

2016 ◽

Vol 13 (06) ◽

pp. 1630007 ◽

Cited By ~ 19

Author(s):

Kazuharu Bamba

Keyword(s):

Thermodynamic Properties ◽

Modified Gravity ◽

Equations Of Motion ◽

Apparent Horizon ◽

Second Law Of Thermodynamics ◽

Second Law ◽

Modified Gravity Theories ◽

Gravity Theories ◽

Clausius Relation ◽

The Universe

We review thermodynamic properties of modified gravity theories, such as [Formula: see text] gravity and [Formula: see text] gravity, where [Formula: see text] is the scalar curvature and [Formula: see text] is the torsion scalar in teleparallelism. In particular, we explore the equivalence between the equations of motion for modified gravity theories and the Clausius relation in thermodynamics. In addition, thermodynamics of the cosmological apparent horizon is investigated in [Formula: see text] gravity. We show both equilibrium and nonequilibrium descriptions of thermodynamics. It is demonstrated that the second law of thermodynamics in the universe can be met, when the temperature of the outside of the apparent horizon is equivalent to that of the inside of it.

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Modified gravity from an entropy functional

International Journal of Modern Physics D ◽

10.1142/s0218271814500734 ◽

2014 ◽

Vol 23 (09) ◽

pp. 1450073 ◽

Cited By ~ 2

Author(s):

Fayçal Hammad

Keyword(s):

Modified Gravity ◽

Higher Order ◽

Functional Formalism ◽

Entire Series ◽

Entropy Functional ◽

Modified Gravity Theory ◽

Horizon Entropy ◽

Modified Gravity Theories ◽

Gravity Theories ◽

Higher Order Corrections

We extend Padmanabhan's entropy functional formalism to show that, in addition to the Gauss–Bonnet (GB) or the entire series of Lanczos–Lovelock Lagrangians already obtained, more general higher-order corrections to General Relativity, i.e. the so-called modified gravity theories, also emerge naturally from this formalism. This extension shows that the formalism constitutes a valuable tool to investigate, at each order in the curvature, the possible structure the higher-order modified gravity theories might have. As an application, the extended formalism is used to evaluate the horizon entropy in a modified gravity theory of the second-order in the curvature. Our findings are in agreement with previous results from the literature.

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Conformal (In)Equality

EPJ Web of Conferences ◽

10.1051/epjconf/201816808001 ◽

2018 ◽

Vol 168 ◽

pp. 08001

Author(s):

Young-Hwan Hyun ◽

Yoonbai Kim ◽

Seokcheon Lee

Keyword(s):

Dark Energy ◽

Modified Gravity ◽

Large Scale ◽

Growth Index ◽

Gravity Theory ◽

Jordan Frame ◽

Model Parameters ◽

Modified Gravity Theories ◽

Gravity Theories ◽

The Universe

The current accelerating expansion of the Universe is explained either by dark energy or by modified gravity theories. Both of them can explain exactly the same background evolution of the Universe, however this degeneracy may be broken when the observation of large scale structure formation is taken into account. Two observables are parameterized by the so-called dark energy equation of state, ω and the growth index parameter, γ. From these observed parameters, one may reconstruct the model parameters of the so-called scalar-tensor gravity theory, one of the modified gravity theories. Especially, the scalar-tensor gravity theory is described both in Jordan frame and in Einstein frame. If cosmological observations are interpreted in one frame, then all of the observables should also be interpreted in that frame. This explicitly shows conformal inequality of cosmological observables.

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