Yungui Gong

Transition of the dark energy equation of state in an interacting holographic dark energy model

Abstract 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.

Supernova constraints on a holographic dark energy model

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 Ωm0 = 0.25, the equation of the state of the holographic dark energy wΛ0 = −0.91 and the transition between the decelerating expansion and the accelerating expansion happened when the cosmological red-shift was zT = 0.72. If we set d as a free parameter, the best fit results are d = 0.21, Ωm0 = 0.46, wΛ0 = −2.67, which sounds like a phantom today, and the transition red-shift is zT = 0.28.

Thermodynamics of an accelerated expanding universe

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 enveloped 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.

Extended holographic dark energy

The idea of relating the infrared and ultraviolet cutoffs is applied to the 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.

Friedmann equations and thermodynamics of apparent horizons.

With the help of a masslike function which has a dimension of energy and is equal to the Misner-Sharp mass at the apparent horizon, we show that the first law of thermodynamics of the apparent horizon dE=T(A)dS(A) 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.

Testing the viability of the interacting holographic dark energy model by using combined observational constraints

Using the data coming from the new 182 gold type Ia supernova samples, the shift parameter of the cosmic microwave background given by the three-year Wilkinson Microwave Anisotropy Probe observations, and the baryon acoustic oscillation measurement from the Sloan digital sky survey, H(z) and lookback time measurements, we have performed a statistical joint analysis of the interacting holographic dark energy model. Consistent parameter estimations show us that the interacting holographic dark energy model is a viable candidate for explaining the observed acceleration of our universe.

Reconstruction of the deceleration parameter and the equation of state of dark energy

The new 182 gold supernova Ia data, the baryon acoustic oscillation measurement and the shift parameter determined from the Sloan Digital Sky Survey, and the three-year Wilkinson Microwave Anisotropy Probe data are combined to reconstruct the dark energy equation of state parameter

Phase-space analysis of interacting phantom cosmology

w(z)

The generalized second law of thermodynamics in the accelerating universe

and the deceleration parameter

Current constraints on interacting holographic dark energy

q(z)