[Review] Dark Energy

We review the problem of dark energy, including a survey of theoretical models and some aspects of numerical studies.     

Holographic Dark Energy in Induced Gravity

Many astrophysics data show that our universe has a critical energy density, and 73% of it is dark energy, which drives the accelerating expansion of the universe. We consider the holographic dark energy in induced gravity by taking the Hubble scale, particle horizon and event horizon as the infrared cutoff. We find that only the event horizon can give accelerating expansion of our universe.     

Dark Energy Accretion onto a Black Hole in an Expanding Universe

By using the solution describing a black hole embedded in the FLRW universe, we obtain the evolving equation of the black hole mass expressed in terms of the cosmological parameters. The evolving equation indicates that in the phantom dark energy universe the black hole mass becomes zero before the Big Rip is reached.     

Effects of Low Anisotropy on Generalized Ghost Dark Energy in Galileon Gravity

The definition of the Galileon gravity form is extended to the Brans-Dicke theory. Given, the framework of the Galileon theory, the generalized ghost dark energy model in an anisotropic universe is investigated. We study the cosmological implications of this model. In particular, we obtain the equation of state and the deceleration parameters and a differential equation governing the evolution of this dark energy in Bianchi type I model. We also probe observational constraints by using the latest observational data on the generalized ghost dark energy models as the unification of dark matter and dark energy. In order to do so, we focus on observational determinations of the Hubble expansion rate(namely, the expansion history) H(z). As a result, we show the influence of the anisotropy(although low) on the evolution of the universe in the statefinder diagrams for Galileon gravity.     

New Agegraphic Dark Energy in Brans-Dicke Theory

In this paper, we investigate the new agegraphic dark energy model in the framework of Brans-Dicke theory, which is a natural extension of the Einstein's general relativity. In this framework the form of the new agegraphic dark energy density takes as ρ_q =3n² φ(t) η^-2, where η is the conformalage of the universe and φ(t) is the Brans-Dicke scalar field representing the inverse of the time-variable Newton's constant. We derive the equation of state of the new agegraphic dark energy and the deceleration parameter of the universe in the Brans-Dicke theory. It is very interesting to find that in the Brans-Dicke theory the agegraphic dark energy realizes quintom-like behavior,i.e., its equation of state crosses the phantom divide w=-1 duringthe evolution. We also compare the situation of the agegraphic darkenergy model in the Brans-Dicke theory with that in the Einstein'stheory. In addition, we discuss the new agegraphic dark energy modelwith interaction in the framework of the Brans-Dicke theory.     

Reconstruction of f(R) Gravity with Ordinary and Entropy-Corrected (m,n)-Type Holographic Dark Energy Model

In this assignment we will present a reconstruction scheme between f(R) gravity with ordinary and entropy corrected (m, n)-type holographic dark energy. The correspondence is established and expressions for the reconstructed f(R) models are determined. To study the evolution of the reconstructed models plots are generated. The stability of the calculated models are also investigated using the squared speed of sound in the background of the reconstructed gravities.     

Dark energy versus modified gravity: Impacts on measuring neutrino mass

In this paper,we make a comparison for the impacts of smooth dynamical dark energy,modified gravity,and interacting dark energy on the cosmological constraints on the total mass of active neutrinos.For definiteness,we consider theΛCDM model,the w CDM model,the f(R)model,and two typical interacting vacuum energy models,i.e.,the IΛCDM1 model with Q=βHρc and the IΛCDM2 model with Q=βHρΛ.In the cosmological fits,we use the Planck 2015 temperature and polarization data,in combination with other low-redshift observations including the baryon acoustic oscillations,the type Ia supernovae,the Hubble constant measurement,and the large-scale structure observations,such as the weak lensing as well as the redshift-space distortions.Besides,the Planck lensing measurement is also employed in this work.We find that,the w CDM model favors a higher upper limit on the neutrino mass compared to theΛCDM model,while the upper limit in the f(R)model is similar with that in theΛCDM model.For the interacting vacuum energy models,the IΛCDM1 model favors a higher upper limit on neutrino mass,while the IΛCDM2 model favors an identical neutrino mass with the case ofΛCDM.     

New Agegraphic Dark Energy in Brans-Dicke Theory

In this paper, we investigate the new agegraphic dark energy model in the framework of Brans-Dicke theory, which is a natural extension of the Einstein＇s general relativity. In this framework the form of the new agegraphic dark energy density takes as pq = 3n^2Ф（t）η^-2, where η is the conformal age of the universe and Ф（t） is the Brans-Dicke scalar field representing the inverse of the time-variable Newton＇s constant. We derive the equation of state of the new agegraphic dark energy and the deceleration parameter of the universe in the Brans-Dicke theory. It is very interesting to find that in the Brans-Dicke theory the agegraphic dark energy realizes quintom-like behavior, i.e., its equation of state crosses the phantom divide ω= -1 during the evolution. We also compare the situation of the agegraphic dark energy model in the Brans-Dicke theory with that in the Einstein＇s theory. In addition, we discuss the new agegraphic dark energy model with interaction in the framework of the Brans-Dicke theory.     

理解暗能量和暗物质之间的相互作用

从热力学角度研究了暗能量和暗物质之间的相互作用. 假设相互作用是平衡态上的涨落并考虑此涨落导致的熵的修正, 导出了相互作用的物理表述, 把我们模型和观测结果作了比较.     

The Complex Symmetry Gravitational Theory as a New Alternative of Dark Energy

We propose that complex symmetry gravitational theory (CSGT) explain the accelerating expansion of universe. In this paper, universe is taken as the double complex symmetric space. Cosmological solution is obtained within CSGT. The conditions of the accelerating expansion of universe are discussed within CSGT. Moreover, the range of equation of state of matter ω_ε is given in the hyperbolic imaginary space. PACS: 98.80Es, 04.20Fy     

Entropy-Corrected Holographic Dark Energy

The holographic dark energy (HDE) is now an interesting candidate of dark energy, which has been studied extensively in the literature. In the derivation of HDE, the black hole entropy plays an important role. In fact, the entropy-area relation can be modified due to loop quantum gravity or other reasons. With the modified entropy-area relation, we propose the so-called ``entropy-corrected holographic dark energy' (ECHDE) in the present work. We consider many aspects of ECHDE and find some interesting results. In addition, we briefly consider the so-called ``entropy-corrected agegraphic dark energy' (ECADE).     

Holography, UV/IR Relation, Causal Entropy Bound, and Dark Energy

The constraint on the total energy in a given spatial region is given from holography by the mass of a black hole that just fits in that region, which leads to an UV/IR relation： the maximal energy density in that region is proportional to Mp^2/L^2, where Mp is the Planck mass and L is the spatial scale of that region under consideration. Assuming the maximal black hole in the universe is formed through gravitational collapse of perturbations in the universe, then the ＂Jeans＂ scale of the perturbations gives a causal connection scale RCC. For gravitational perturbations, RCC^-2= Max （H＋ 2H^2, -H） for a fiat universe. We study the cosmological dynamics of the corresponding vacuum energy density by choosing the causal connection scale as the IR cutoff in the UV/IR relation, in the cases of the vacuum energy density as an independently conserved energy component and an effective dynamical cosmological constant, respectively. It turns out that only the case with the choice RCC^-2 = H＋ 2H^2, could be consistent with the current cosmological observations when the vacuum density appears as an independently conserved energy component. In this case, the model is called holographic Ricci scalar dark energy model in the literature.     

Entropy-Corrected Holographic Dark Energy

The holographic dark energy （HDE） is now an studied extensively in the literature. In the derivation of HDE, interesting candidate of dark energy, which has been the black hole entropy plays an important role. In fact, the entropy-area relation can be modified due to loop quantum gravity or other reasons. With the modified entropyarea relation, we propose the so-called ＂entropy-corrected holographic dark energy＂ （ECHDE） in the present work. We consider many aspects of ECHDE and tlnd some interesting results. In addition, we briefly consider the so-called ＂entropy-eorreeted agegraphic dark energy＂ （ECADE）.     

Analysis of Generalized Ghost Dark Energy in LQC and Galileon Gravity

A so-called ghost dark energy was recently proposed to explain the present acceleration of the universe.The energy density of ghost dark energy,which originates from Veneziano ghost of Quantum Chromodynamics(QCD),in a time dependent background,can be written in the form,ρD=αH + βH~2 where H is the Hubble parameter.We investigate the generalized ghost dark energy(GGDE) model in the setup of loop quantum Cosmology(LQC) and Galileon Cosmology.We study the cosmological implications of the models.We also obtain the equation of state and the deceleration parameters and differential equations governing the evolution of this dark energy model for LQC and Galileon Cosmology.     

Holographic Gas as Dark Energy

We investigate the statistical nature of holographic gas, which may represent the quasi-particle excitations of a strongly correlated gravitational system. We find that the holographic entropy can be obtained by modifying degeneracy. We calculate thermodynamical quantities and investigate stability of the holographic gas. When applying to cosmology, we find that the holographic gas behaves as holographic dark energy, and the parameter c in holographic dark energy can be calculated from our model. Our model of holographic gas generally predicts c 〈 1, implying that the fate of our universe is phantom-like.     

Reconstruction of Scalar Field Dark Energy Models in Kaluza-Klein Universe

This paper is devoted to study the modified holographic dark energy model by taking its different aspects in the flat Kaluza-Klein universe.We construct the equation of state parameter which evolutes the universe from quintessence region towards the vacuum.It is found that the modified holographic model exhibits instability against small perturbations in the early epoch of the universe but becomes stable in the later times.We also develop its correspondence with some scalar field dark energy models.It is interesting to mention here that all the results are consistent with the present observations.     

Oscillating Flat FRW Dark Energy Dominated Cosmology from Periodic Functional Approach

I discuss the modification of Einstein＇s Theory of General Relativity based on a periodic functional approach. In this new approach, a corrected periodic gravitational coupling constant arises and plays the role of periodic damping term acting on the theory. It is found that it is achievable to have an oscillating universe dominated by dark energy and expanding aceeleratedly in time.     

Oscillating Flat FRW Dark Energy Dominated Cosmology from PeriodicFunctional Approach

I discuss the modification of Einstein's Theory of General Relativity based on a periodic functional approach. In this new approach, a corrected periodic gravitational coupling constant arises and plays the role of periodic damping term acting on the theory. It is found that it is achievable to have an oscillating universe dominated by dark energy and expanding acceleratedly in time.