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

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...