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We obtain the Schwarzschild solution from thermodynamic considerations using the assumptions of a quasi local mass form (the Misner-Sharp mass) and geometric surface gravity in a spherically symmetric spacetime. The deduction is extended to other cases such as the de Sitter, anti-de Sitter, Reissner-Nordstr\"om and higher dimensional spacetimes. This paper demonstrates the simple hypotheses to obtain these known spherically symmetric solutions via thermodynamics, where essentially the Misner-Sharp mass is the mass for an adiabatic system., Comment: two new authors joined, 5pages, no figure, PRD in press

We study the semi-holographic idea in context of decaying dark components. The energy flow between dark energy and the compensating dark matter is thermodynamically generalized to involve a particle number variable dark component with non-zero chemical potential. It's found that, unlike the original semi-holographic model, no cosmological constant is needed for a dynamical evolution of the universe. A transient phantom phase appears while a non-trivial dark energy-dark matter scaling solution keeps at late time, which evades the big-rip and helps to resolve the coincidence problem. For reasonable parameters, the deceleration parameter is well consistent with current observations. The original semi-holographic model is extended and it also suggests that the concordance model may be reconstructed from the semi-holographic idea., Comment: 15pages,5figs. arXiv admin note: substantial text overlap with arXiv:1010.1362

Phase transition and critical phenomenon is a very interesting topic in thermodynamics and statistical mechanics. Gravity is believed to has deep and inherent relation to thermodynamics. Near the critical point, the perturbation becomes significant. Thus for ordinary matter (govern by interactions besides gravity) the critical behavior will become very different if we ignore the perturbations around the critical point, such as mean field theory. We find that the critical exponents for RN-AdS spacetime keeps the same values even we consider the full quantum perturbations. This indicates a key difference between gravity and ordinary thermodynamic system., Comment: 6 pages, two figures,match the published version

We exhaust the brane instanton solutions---an Einstein brane inhabiting at different positions in a 5-dimensional Einstein bulk with negative curvature. We construct a brane instanton model consisting of a brane with asymmetric bulk along two sides of the brane. And the junction condition of the resulting space-time is analyzed in the frame of induced gravity (DGP model). In spirits of quantum gravity of path integral formulism we calculate the Euclidean actions on three canonical paths and then compare the Euclidean actions of different instantons per unit 4-volume. We also compare the Euclidean actions per unit 4-volume of instantons, which consist of a brane gluing to a fixed half, with other Euclidean actions of halves possessing different cosmological constants., Comment: 20 pages, JHEP in press. It is a significantly improved version of hep-th/0412317

We derive the MOND cosmology which is uniquely corresponding to the original MOND at galaxy scales via entropic gravity method. It inherits the key merit of MOND, that is, it reduces the baryonic matter and non-baryonic dark matter into baryonic matter only. For the first time we obtain the critical parameter in MOND, i.e., the transition acceleration $a_c$ at cosmological scale. We thus solve the long-standing coincidence problem $a_c\sim cH_{0}$. More interestingly, a term like age-graphic dark energy emerges naturally. In the frame of this MOND cosmology, we only need baryonic matter to describe both dark matter and dark energy in standard cosmology., Comment: 4 pages, several references added, to appear in PLB

By assuming that a dark component (dark energy) in the universe strictly obeys the holographic principle, that is, its entropy is one fourth of the apparent horizon, we find that the existence of the other dark component (dark matter) is compulsory, as a compensation of dark energy, based on the first law of thermodynamics. By using the method of dynamical system analysis, we find that there exists a stable dark energy-dark matter scaling solution at late time, which is helpful to solve the coincidence problem. For reasonable parameters, the deceleration parameter is well consistent with current observations., Comment: 10 pages, 3 figs, comments are welcome

Recently, type Ia supernovae data appear to support a dark energy whose equation of state $w$ crosses -1, which is a much more amazing problem than the acceleration of the universe. We show that it is possible for the equation of state to cross the phantom divide by a scalar field in the gravity with an additional inverse power-law term of Ricci scalar in the Lagrangian. The necessary and sufficient condition for a universe in which the dark energy can cross the phantom divide is obtained. Some analytical solutions with $w<-1$ or $w>-1$ are obtained. A minimal coupled scalar with different potentials, including quadratic, cubic, quantic, exponential and logarithmic potentials are investigated via numerical methods, respectively. All these potentials lead to the crossing behavior. We show that it is a robust result which is hardly dependent on the concrete form of the potential of the scalar., Comment: 11 pages, 5 figs, v3: several references added, to match the published version

Scalar-tensor gravity is one of the most competitive gravity theory to Einstein's relativity. We reconstruct the exact de Sitter solution in scalar-tensor gravity, in which the non-minimal coupling scalar is rolling along the potential. This solution may have some relation to the early inflation and present acceleration of the universe. We investigated its primordial quantum perturbation around the adiabatic vacuum. We put forward for the first time that exact de Sitter generates non-exactly scale invariant perturbations. In the conformal coupling case, this model predicts that the tensor mode of the perturbation (gravity wave) is strongly depressed., Comment: 9 pages

An exact de Sitter solution of scalar-tensor gravity is found, in which the non-minimal coupling scalar is rolling along a non-constant potential. Based on this solution, a dust-filled FRW universe is explored in frame of scalar-tensor gravity. The effective dark energy induced by the sole non-minimal scalar can be quintessence-like, phantom-like, and more significantly, can cross the phantom divide. The rich and varied properties of scalar-tensor gravity even with only one scalar is shown in this article., Comment: 9 pages, 9figs

The dynamics of a tachyon attached to a Dvali, Gabadadze and Porrati (DGP) brane is investigated. Exponential potential and inverse power law potential are explored, respectively. The quasi-attractor behavior, for which the universe will eventually go into a phase similar to the slow-roll inflation, is discovered in both cases of exponential potential and inverse power law potential. The equation of state (EOS) of the virtual dark energy for a single scalar can cross the phantom divide in the branch $\theta=-1$ for both potentials, while the EOS of the virtual dark energy for a single scalar can not cross this divide in the branch $\theta=1$., Comment: 20 pages, 12 figs, to appear in PLB

The cosmic acceleration is one of the most significant cosmological discoveries over the last century. Following the more accurate data a more dramatic result appears: the recent analysis of the observation data (especially from SNe Ia) indicate that the time varying dark energy gives a better fit than a cosmological constant, and in particular, the equation of state parameter $w$ (defined as the ratio of pressure to energy density) crosses -1 at some low redshift region. This crossing behavior is a serious challenge to fundamental physics. In this article, we review a number of approaches which try to explain this remarkable crossing behavior. First we show the key observations which imply the crossing behavior. And then we concentrate on the theoretical progresses on the dark energy models which can realize the crossing -1 phenomenon. We discuss three kinds of dark energy models: 1. two-field models (quintom-like), 2. interacting models (dark energy interacts with dark matter), and 3. the models in frame of modified gravity theory (concentrating on brane world)., Comment: 42 pages, 17 figs, invited review for Nova Science Publisher, as a chapter of the book Dark Energy: Theories, Developments and Implications

We present a novel mechanism for the present acceleration of the universe. We find that the temperature of the Unruh radiation perceived by the brane is not equal to the inherent temperature (Hawking temperature at the apparent horizon) of the brane universe in the frame of Dvali-Gabadadze-Porrati (DGP) braneworld model. The Unruh radiation perceived by a dust dominated brane is always warmer than the brane measured by the geometric temperature, which naturally induces an energy flow between bulk and brane based on the most sound thermodynamics principles. Through a thorough investigation to the microscopic mechanism of interaction between bulk Unruh radiation and brane matter, we put forward that an energy influx from bulk Unruh radiation to the dust matter on the brane accelerates the universe., Comment: 28 pages, 4 figs, to appear in NPB; This is a joint paper of hep-th/0607166 and astro-ph/0607531, which will be withdrawn

The cosmic coincidence problem is a serious challenge to dark energy model. We suggest a quantitative criteria for judging the severity of the coincidence problem. Applying this criteria to three different interacting models, including the interacting quintessence, interacting phantom, and interacting Chaplygin gas models, we find that the interacting Chaplygin gas model has a better chance to solve the coincidence problem. Quantitatively, we find that the coincidence index C for the interacting Chaplygin gas model is smaller than that for the interacting quintessence and phantom models by six orders of magnitude., Comment: 6 pages, 3 figures, accepted for publication in Phys.Lett.B

A new braneworld in the sourced-Taub background is proposed. The gravity field equations in the internal source region and external vacuum region are investigated, respectively. We find that the equation of state for the effective dark energy of a dust brane in the source region can cross the phantom divide $w=-1$. Furthermore, there is a drop on $H(z)$ diagram, which presents a possible mechanism for the recent direct data of $H(z)$., Comment: 10 pages, 3 figures, v2: a section to review the previous results on the source of Taub space added, PLB in press

A new class of plane symmetric solution sourced by a perfect fluid is found in our recent work. An n-dimensional ($n\geq 4$) global plane symmetric solution of Einstein field equation generated by a perfect fluid source is investigated, which is the direct generalization of our previous 4-dimensional solution. One time-like Killing vector and $(n-2)(n-1)/2$ space-like Killing vectors, which span a Euclidean group $G_{(n-2)(n-1)/2}$, are permitted in this solution. The regions of the parameters constrained by weak, strong and dominant energy conditions for the source are studied. The boundary condition to match to n-dimensional Taub metric and Minkowski metric are analyzed respectively., Comment: 9 pages, 1 table; v2: 2 references added

A new class of static plane symmetric solution of Einstein field equation, which is judged as the source of Taub solution, was presented in our previous work. In this letter the properties of geodesics of this solution are explored. It is found that this solution can be an appropriate simulation to the field of a uniformly accelerated observer in Newton mechanics. The essence of the source is investigated. A phantom with dust and photon is suggested as the substance of the source matter., Comment: 8 pages, no figure, v2: 2 refences added

The cosmic acceleration is one of the most significant cosmological discoveries over the last century. The two categories of explanation are exotic component (dark energy) and modified gravity. We constrain the two types of model by a joint analysis with perturbation growth and direct $H(z)$ data. Though the minimal $\chi^2$ of the $\Lambda$CDM is almost the same as that of DGP, in the sense of consistency we find that the dark energy ($\Lambda$CDM) model is more favored through a detailed comparison with the corresponding parameters fitted by expansion data., Comment: 10 pages, 6 figures, typo corrected

A new class of static plane symmetric solution of Einstein field equation generated by a perfect fluid source is put forward. A special family of this new solution is investigated in detail. The constraints on the parameters by different energy conditions are studied. The classical stability of this solution is discussed. The junction conditions matching to Minkowski metric and Taub metric are analyzed respectively., Comment: 10 pages, v2: a typo in equation (37) corrected, v3: equation (68) corrected, PLB V663, Pages 291-296

Hybrid Chaplygin gas model is put forward, in which the gases play the role of dark energy. For this model the coincidence problem is greatly alleviated. The effective equation of state of the dark energy may cross the phantom divide $w=-1$. Furthermore, the crossing behaviour is decoupled from any gravity theories. In the present model, $w<-1$ is only a transient behaviour. There is a de Sitter attractor in the future infinity. Hence, the big rip singularity, which often afflicts the models with matter whose effective equation of state less than -1, is naturally disappear. There exist stable scaling solutions, both at the early universe and the late universe. We discuss the perturbation growth of this model. We find that the index is consistent with observations., Comment: 11 pages, 4 figures, V3: discussions on the perturbation growth added, V4: minor corrections, to match the published version

Recent direct $H(z)$ data indicate that the parameter $H(z)$ may wiggle with respect to $z$. On the other hand the luminosity distance data of supernovae flatten the wiggles of $H(z)$ because of integration effect. It is expected that the fitting results can be very different in a model permitting a wiggling $H(z)$ because the data of supernovae is highly degenerated to such a model. As an example the natural phantom dark energy is investigated in this paper. The dynamical property of this model is studied. The model is fitted by the direct $H(z)$ data set and the SNLS data set, respectively. And the results are quite different, as expected. The quantum stability of this model is also shortly discussed. We find it is a viable model if we treat it as an effective theory truncated by an upperbound., Comment: 14 pages, 2 figures, discussions on the stability added, conclusions not changed

Recent type Ia supernovae data seem to favor a dark energy model whose equation of state $w(z)$ crosses -1, which is a much more amazing problem than the acceleration of the universe. Either the case that $w(z)$ evolves from above -1 to below -1 or the case that $w(z)$ runs from below -1 to above -1, is consistent with present data. In this paper we show that it is possible to realize the crossing behaviours of both of the two cases by only a single scalar field in frame of Dvali-Gabadadze-Porrati braneworld. At the same time we prove that there does not exist scaling solution in a universe with dust., Comment: 8 pages, 4 figs, several references added, minor correction