In the Department of Gravity and Cosmology, Faculty of Physics, Isfahan University of Technology, we are studying various aspects of the universe with the help of general relativity. Standard cosmology deals with the behavior of the universe on a large scale and examines its evolution and the origin of structures. For structures to form, the density of the original universe must be heterogeneous. Inflation theory can explain the origin of fluctuations in the density of the early universe. We study different parts of inflation theories.