TianQin is a proposal for a space-borne detector of gravitational waves in the millihertz frequencies. The experiment relies on a constellation of three drag-free spacecraft orbiting the Earth. Inter-spacecraft laser interferometry is used to monitor the distances between the test masses. The experiment is designed to be capable of detecting a signal with high confidence from a single source of gravitational waves within a few months of observing time. We describe the preliminary mission concept for TianQin, including the candidate source and experimental designs. We present estimates for the major constituents of the * experiment's error budget and discuss the project's overall feasibility. Given the current level of technology readiness, we expect TianQin to be flown in the second half of the next decade.

A model of holographic dark energy with an interaction with matter fields has been investigated. Choosing the future event horizon as an IR cutoff, we have shown that the ratio of energy densities can vary with time. With the interaction between the two different constituents of the universe, we observed the evolution of the universe, from early deceleration to late time acceleration. In addition, we have found that such an interacting dark energy model can accommodate a transition of the dark energy from a normal state where w D > −1 to w D < −1 phantom regimes. Implications of interacting dark energy model for the observation of dark energy transition has been discussed.

We investigate the laws of thermodynamics in an accelerating universe driven by dark energy with a time-dependent equation of state. In the case we consider that the physically relevant part of the Universe is that envelopped by the dynamical apparent horizon, we have shown that both the first law and second law of thermodynamics are satisfied. On the other hand, if the boundary of the Universe is considered to be the cosmological event horizon the thermodynamical description based on the definitions of boundary entropy and temperature breaks down. No parameter redefinition can rescue the thermodynamics laws from such a fate, rendering the cosmological event horizon unphysical from the point of view of the laws of thermodynamics.Comment: 13 pages, 2 figure

In this paper, we use the type Ia supernova data to constrain the model of holographic dark energy. For d = 1, the best fit result is Ω 0 m = 0.25, the equation of the state of the holographic dark energy w 0 Λ = −0.91 and the transition between the decelerating expansion and the accelerating expansion happened when the cosmological red-shift was z T = 0.72.If we set d as a free parameter, the best fit results are d = 0.21, Ω 0 m = 0.46, w 0 Λ = −2.67, which sounds like a phantom today, and the transition redshift is z T = 0.28.

We have investigated the thermodynamical properties of dark energy. Assuming that the dark energy temperature T ∼ a −n and considering that the volume of the Universe enveloped by the apparent horizon relates to the temperature, we have derived the dark energy entropy. For dark energy with constant equation of state w > −1 and the generalized Chaplygin gas, the derived entropy can be positive and satisfy the entropy bound. The total entropy, including those of dark energy, the thermal radiation and the apparent horizon, satisfies the generalized second law of thermodynamics. However, for the phantom with constant equation of state, the positivity of entropy, the entropy bound, and the generalized second law cannot be satisfied simultaneously. Results from numerous and complementary observations show an emerging a paradigm 'concordance cosmology' indicating that our universe is spatially flat and composed of about 70% dark energy (DE) and about 25% dark matter. The weird DE is a major puzzle of physics now. Its nature and origin have been the intriguing subject of discussions in the past years. The DE has been sought within a wide range of physical phenomena, including a cosmological constant, quintessence or an exotic field called phantom [1]. Except the known fact that DE has a negative pressure causing the acceleration of the universe, its nature still remains a complete mystery. In the conceptual set up of the DE, one of the important questions concerns its thermodynamical properties. It is expected that the thermodynamical consideration might shed some light on the properties of DE and help us understand its nature.The topic on the DE entropy, temperature and their evolution by using the first law of thermodynamics was widely discussed in the literature [2,3,4,5,6,7,8,9,10]. It was found that the entropy of the phantom might be negative [6,7,8]. The existence of negative entropy of the phantom could be easily seen from the relation T s = ρ+p between the temperature T , the entropy density s, the energy density ρ and the pressure p. Negative entropy is problematic if we accept that the entropy is in association with the measure of the number of microstates in statistical mechanics. The intuition of statistical mechanics requires that the entropy of all physical components to be positive. Besides if we consider the universe as a ther- * Electronic address: yungui˙gong@baylor.edu † Electronic address: wangb@fudan.edu.cn ‡ Electronic address: anzhong˙wang@baylor.edu modynamical system, the total entropy of the universe including DE and dark matter should satisfy the second law of thermodynamics. The generalized second law (GSL) for phantom and non-phantom DE has been explored in [8]. It was found that the GSL can be protected in the universe with DE. The GSL of the universe with DE has been investigated in [9,10] as well. In order to rescue the GSL of thermodynamics, Bekenstein conjectured that there exists an upper bound on the entropy for a weakly self-gravitating physical system [11]. Bekenstein's entropy bou...

With the help of a masslike function which has dimension of energy and equals to the Misner-Sharp mass at the apparent horizon, we show that the first law of thermodynamics of the apparent horizon dE = TAdSA can be derived from the Friedmann equation in various theories of gravity, including the Einstein, Lovelock, nonlinear, and scalar-tensor theories. This result strongly suggests that the relationship between the first law of thermodynamics of the apparent horizon and the Friedmann equation is not just a simple coincidence, but rather a more profound physical connection.PACS numbers: 04.20.Cv,04.70.Dy The derivation of the thermodynamic laws of black holes from the classical Einstein equation suggests a deep connection between gravitation and thermodynamics [1]. The discovery of the quantum Hawking radiation [2] and black hole entropy which is proportional to the area of the event horizon of the black hole [3] further supports this connection and the thermodynamic (physical) interpretation of geometric quantities. The interesting relation between thermodynamics and gravitation became manifest when Jacobson derived Einstein equation from the first law of thermodynamics by assuming the proportionality of the entropy and the horizon area for all local acceleration horizons [4].In cosmology, like in black holes, for the cosmological model with a cosmological constant (called de Sitter space), there also exist Hawking temperature and entropy associated with the cosmological event horizon, and thermodynamic laws of the cosmological event horizon [5]. In de Sitter space, the event horizon coincides with the apparent horizon (AH). For more general cosmological models, the event horizon may not exist, but the AH always exists, so it is possible to have Hawking temperature and entropy associated with the AH. The connection between the first law of thermodynamics of the AH and the Friedmann equation was shown in [6]. Now, we must ask if this interesting relation between gravitation and thermodynamics exists in more general theories of gravity, like Brans-Dicke (BD) theory and nonlinear gravitational theory. In [7], the gravitational field equations for the nonlinear theory of gravity were derived from the first law of thermodynamics by adding some nonequilibrium corrections. In this Letter, we show that equilibrium thermodynamics indeed exists for more general theories of gravity, provided that a new masslike function is introduced.To show our claim, we begin by reviewing the thermodynamics of the AH with the use of the MisnerSharp (MS) mass in Einstein and BD theories of gravity, whereby we find the equilibrium thermodynamics fails to hold for the BD theory. The Einstein equation can be rewritten as the mass formulas with the help of the MS mass M. The energy flow through the AH dE is related with the MS mass. Since the MS mass M, the Hawking temperature T A , and the entropy S A of the AH are geometric quantities, the first law of thermodynamics of the AH can be thought of as a geometric relation. Therefore, we exp...

The growth rate of matter perturbation and the expansion rate of the Universe can be used to distinguish modified gravity and dark energy models in explaining the cosmic acceleration. The growth rate is parametrized by the growth index γ. We discuss the dependence of γ on the matter energy density Ω and its current value Ω 0 for a more accurate approximation of the growth factor.The observational data, including the data of the growth rate, are used to fit different models.

The idea of relating the infrared and ultraviolet cutoffs is applied to Brans-Dicke theory of gravitation. We find that the Hubble scale or the particle horizon as the infrared cutoff will not give accelerating expansion. The dynamical cosmological constant with the event horizon as the infrared cutoff is a viable dark energy model.The Type Ia supernova (SN Ia) observations suggest that the expansion of our universe is accelerating and dark energy contributes 2/3 to the critical density of the present universe [1,2]. SN Ia observations also provide the evidence of a decelerated universe in the recent past with the transition redshift z q=0 ∼ 0.5 [3,4]. The cosmic background microwave (CMB) observations support a spatially flat universe as predicted by the inflationary models [5,6]. The simplest candidate of dark energy is the cosmological constant. However, the unusual small value of the cosmological constant leads to the search for dynamical dark energy models [7,8]. For a review of dark energy models, see, for example and references therein [8]. Cohen, Kaplan and Nelson proposed that for any state in the Hilbert space with energy E, the corresponding Schwarzschild radius R s ∼ E is less than the infrared (IR) cutoff L [9]. Therefore, the maximum entropy is S 3/4 BH . Under this assumption, a relationship between the ultraviolet (UV) cutoff and the infrared cutoff is derived, i.e., 8πGL 9,10]. So the holographic cosmological constant isHsu found that the holographic cosmological constant model based on the Hubble scale as IR cutoff won't give an accelerating universe [11]. Li showed that the holographic dark energy model based on event horizon gave an accelerating universe, this model was also found to be consistent with current observations [12,13]. Einstein's theory of gravity may not describe gravity at very high energy. The simplest alternative to general relativity is Brans-Dicke scalar-tensor theory. The recent interest in scalar-tensor theories of gravity arises from inflationary cosmology, supergravity and superstring theory. The dilaton field appears naturally in the low energy effective bosonic string theory. Scalar degree of freedom arises also upon compactification of higher dimensions. In this note, we apply the holographic dark energy idea to Brans-Dicke cosmology.The Brans-Dicke Lagrangian in the Jordan frame is given by(2) * Electronic address: gongyg@cqupt.edu.cnIn the Jordan frame, the matter minimally couples to the metric and there is no interaction between the scalar field φ and the matter field ψ. Here we work on the Jordan frame so that test particles follow geodesic motion. The gravitational part of the above Lagrangian (2) is conformal invariant under the conformal transformationsNote that the matter Lagrangian L m (ψ, g µν ) in Eq. (2) is not conformal invariant under the above conformal transformations. For the case λ = 1/2, we make the following transformations:where κ 2 = 8πG, α = βκ, and β 2 = 2/(2ω + 3). Remember that the Jordan-Brans-Dicke Lagrangian is not invariant under the ab...