Stability of black holes based on horizon thermodynamics

On the basis of horizon thermodynamics we study the thermodynamic stability of black holes constructed in general relativity and Gauss-Bonnet gravity. In the framework of horizon thermodynamics there are only five thermodynamic variables $E,P,V,T,S$. It is not necessary to consider concrete matter fields, which may contribute to the pressure of black hole thermodynamic system. In non-vacuum cases, we can derive the equation of state, $P=P(V,T)$. According to the requirements of stable equilibrium in conventional thermodynamics, we start from these thermodynamic variables to calculate the heat capacity at constant pressure and Gibbs free energy and analyze the local and global thermodynamic stability of black holes. It is shown that $P>0$ is the necessary condition for black holes in general relativity to be thermodynamically stable, however this condition cannot be satisfied by many black holes in general relativity. For black hole in Gauss-Bonnet gravity negative pressure can be feasible, but only local stable black hole exists in this case.Comment: 6 pages, 7 figure

Noncommutative geometry inspired black holes in Rastall gravity

Under two different metric ansatzes, the noncommutative geometry inspired black holes (NCBH) in the framework of Rastall gravity are derived and analyzed. We consider the fluid-type matter with the Gaussian-distribution smeared mass density. Taking a Schwarzschild-like metric ansatz, it is shown that the noncommutative geometry inspired Schwarzschild black hole (NCSBH) in Rastall gravity, unlike its counterpart in general relativity (GR), is not a regular black hole. It has at most one event horizon. After showing a finite maximal temperature, the black hole will leave behind a point-like massive remnant at zero temperature. Considering a more general metric ansatz and a special equation of state of the matter, we also find a regular NCBH in Rastall gravity, which has a similar geometric structure and temperature to that of NCSBH in GR.Comment: 12 pages, 5 figures. to match the published versio

Tachyon field inspired dark energy and supernovae constraints

The tachyon field in cosmology is studied by applying the generating function method to obtain exact solutions. The equation of state parameter of the tachyon field is $w=-1+\epsilon\dot{\phi^2}$, which can be expressed as a function in terms of the redshift $z$. Based on these solutions, we propose some tachyon-inspired dark energy models to explore the properties of the corresponding cosmological evolution. The explicit relations between Hubble parameter and redshift enable us to test the models with SNe Ia data sets easily. In the current work we employ the SNe Ia data with the parameter $\mathcal{A}$ measured from the SDSS and the shift parameter $\mathcal{R}$ from WMAP observations to constrain the parameters in our models.Comment: 6 pages, 2 figures; v2: accepted by IJMP