The Models Of Dark Energy For The State Equation Crossing-1

The recent cosmological observations strongly support that the expansion of the universespeeds up rather than slows down, obtained by studying type Ia supernova, cosmic microwavebackground and large scale structure. Theoreticians think the existence of the mysteriouscomponent with negative pressure called "dark energy" is certain, which violates the basicconcept of the gravitational effect (gravitation makes any two bodies interattraction forever).The research shows that dark energy comprises about 2/3 of the total energy density of theuniverse whose nature and evolution decide the destiny of the universe. This is why darkenergy is regarded as the one of the most important research subjects of the recent cosmology.It is unknown that what is dark energy on earth. It is recognized that dark energyhas enough sufficient negative pressure to drive the accelerating expansion. Otherwise, darkenergy has been not detected on the scale of galaxies and galaxy clusters, so dark energyevenly distributes in the whole universe or on the observable scales at least and it has nointeraction with the photons.On the base of the above characters and the astronomical observations, the theoreticianspromoted many models of dark energy. The simplest model is a cosmological constant. Thereare many more complex models proposed attempting to explain dark energy either as adynamic substance: slow-rolling scalar field for example Quintessence, Phantom and Quintomand so on. The difference of these candidates for dark energy is the size of the state equationω_de=p_de/ρ_de and for the cosmological constantω_de= -1, for quintessence -1＜ω_de＜-1/3,for phantomω_de＜-1 and for quintom the crossing -1, namely, the state equation of darkenergy being below-1 around the present epoch evolved fromω_de＜-1 in the past. Thesemodels show various degrees of consistency with the observational data in describing theevolution of the universe. Given the plethora of models, choosing the models of the best fitto the observational data and investigating the properties of this mysterious component isone of the most exciting problems of modern cosmology.In this thesis, first the basic knowledge of cosmology is reviewed including cosmologicalprinciple, big-bang cosmology and inflationary cosmology. In chapter 2, the observationalevidences for the accelerating expansion of the universe and the existence of dark energy arediscussed: type Ia supernova and cosmic microwave background. Many candidates of darkenergy are introduced, but lay an emphasis on the observations for the state equation of dark energy crossing -1 and introduce several models of dark energy which can realize the stateequation of dark energy being below -1 around the present epoch evolved fromω_de＜-1 inthe past. This is also the basic of this thesis.In chapter 3, a generalized Quintom dark energy is studied. In the model of Quintom darkenergy, the weights of the negative-kinetic scalar field (Phantom) and that of the normal scalarfield (Quintessence) are equal. So they influence the evolution of dark energy comparably.The model of the generalized Quintom dark energy is developed by adding a parameter g tochange the situation of their equal weight and to increase the degree of freedom, which canbetter describe the evolution of the university.In chapter 4, a unified equation for dark energy and dark matter is presented: P_dm+P_de=B(z)(ρ_dm+ρ_de). In terms of the evolution of the universe, some constraints for B(z) areproposed. Based on the constraints, given a concrete form of function B(z), the past evolutionof the universe can be obtained which is in consistence with what is recognized and the futureevolution of the universe can be forecasted.In chapter 5, the phase transition of dark energy is discussed. The change rate of thefractional density and the density of dark energy can be obtained from the conservationcondition which gives some foundations to choose the models of dark energy. Combining thestate equation of dark energyω_de crossing -1(which can be regarded as the phase transitionof dark energy from Quintessence phase to Phantom phase), the conclusion can be obtainedthat the density of dark energy first decreases to a minimum then increases. Taken the densityof dark energy to be a quadratic function, the cosmological parameters corresponding to thetransformation point are analyzed and the conclusions show that dark energy of the phasetransition accords with the evolution of the universe.In chapter 6, the possible evolution of dark energy is analyzed by using the powerseries. The parameters are constrained from the newly released Gold sample of the supernovadataset. Two kinds of evolutional trend of dark energy are obtained. Expect the study cangive some clues in probing into the nature of dark energy.In summary, the models of dark energy for the state equation crossing -1 are analyzedwhichrealize that the universe transited from deceleration to acceleration. But These workcan not answer what is dark energy on earth, what is the influence of dark energy on thestructure formation, etc. Expect that the future work can well answer these questions.