Thermodynamical Properties Of Interacting Holographic Dark Energy

From the cosmological observations, it is generally accepted in the astrophysics commu-nity that the universe is expanding in an accelerated manner due to some kind of negative-pressure form of matter known as dark energy. Therefore, in order to explain this bizarre phe-nomenon, various models of dark energy have been proposed.In the first chapter, we briefly review the standard cosmological model and the modified model after the discovery of dark energy.In the second chapter, we discuss several existing models of dark energy. The most naive dark energy candidate is the cosmological constant, conventionally associated with the energy of the vacuum with constant energy density and pressure, and an equation of state ω=—1. However, it is confronted with two fundamental problems:the fine-tuning problem and the cosmic coincidence problem. To alleviate the drawbacks, cosmologists and particle physicists have proposed dynamical dark energy models, including minimally coupled scalar fields, vec-tor fields or even modifications to General Relativity on cosmological scales, such as those associated with extra-dimensions and f(R) theories.In the third chapter we have investigated the thermodynamical properties of the universe with dark energy. It is demonstrated that in a universe with spacial curvature the natural choice for IR cutoff could be the apparent horizon radius. We shown that any interaction of pressureless dark matter with holographic dark energy, whose infrared cutoff is set by the apparent horizon radius, implying a constant effective equation of state of dark component in a universe. In addition we found that for the static observer in space, the comoving distance has a faster expansion than the apparent horizon radius with any spatial curvature. We also verify that in some conditions the modified first law of thermodynamics could return to the classic form at apparent horizon for a universe filled with dark energy and dark matter. Besides, the generalized second law of thermodynamics is discussed in a region enclosed by the apparent horizon.