### Abstract

We demonstrate that it is possible to obtain a picture of equilibrium 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 = -(∇ φ)^{2}/2 is the kinetic energy of a scalar field φ. In this framework, the horizon entropy S corresponding to equilibrium thermodynamics is proportional to the horizon area A with a constant coefficient, as in the Einstein gravity. We show that for a flat cosmological background with a decreasing Hubble parameter, S globally increases with time.

Original language | English |
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Pages | 109-112 |

Number of pages | 4 |

Publication status | Published - 2009 |

Externally published | Yes |

Event | 19th Workshop on General Relativity and Gravitation in Japan, JGRG 2009 - Tokyo, Japan Duration: 2009 Nov 30 → 2009 Dec 4 |

### Conference

Conference | 19th Workshop on General Relativity and Gravitation in Japan, JGRG 2009 |
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Country | Japan |

City | Tokyo |

Period | 09/11/30 → 09/12/4 |

### ASJC Scopus subject areas

- Atomic and Molecular Physics, and Optics

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## Cite this

Bamba, K., Geng, C. Q., & Tsujikawa, S. (2009).

*Equilibrium description of thermodynamics in modified gravitational theories*. 109-112. Paper presented at 19th Workshop on General Relativity and Gravitation in Japan, JGRG 2009, Tokyo, Japan.