Investigation of particle and event horizon in universe models with variable cosmological constant

The concepts of dark energy and dark matter, which have an important role in the work of the modern universe models are issues that should be given priority in cosmology researches. It is an undeniable fact and necessity to take into account the dark energy for the construction of the correct models that match the observations. The magnitude of the cosmological constant that can be associated with the dark energy, and its temporal behavior in homogeneous and isotropic universe models are the issues that need to be illuminated in the light of all these facts. In this thesis, by using Friedmann equations which are the solutions of Einstein's field equations for an expanding homogeneous and isotropic universe, the behavior of particles and event horizons, which have an important place in understanding the structure of the universe, and also the change of these horizons with respect to time is examined. In addition, three different models are proposed using Friedmann equations in order to examine the behavior of the universe from the beginning to the end. The basis of this thesis is that the cosmological constant added to Friedmann equations by Einstein is variable. There are many recent studies considering the possibility of a varying cosmological constant and it is commonly accepted in scientific community. In this thesis, the varying cosmological constant model is firstly used in calculations of particle and event horizons, and also in their evolutions. As a result of the calculations in the thesis, it is obtained that the varying cosmological constant assumption can associate two concepts, such as big bang and big crunch, which are the subject of different debates in cosmology. Moreover, it is found that the proposed model is consistent with the observations because the cosmological density parameters used in the model are taken as the values that are the results of observations as the present day values ,0=0,70, m,0=0,25 ve k,0=0,05. In the conclusion section of the thesis, each universe model corresponding to each varying cosmological constant model is discussed and it is found that the two of the models considered gives the prediction of a big crunch scenario. It is expected that the results will shed light on the studies of spatial and temporal behavior of dark energy which has an important place in cosmology researches.