Abstract
In this article, the perturbation theory is used to study the behaviour of the Universe after the Big Rip appears in the linearly varying deceleration parameter model. The study postulates that the pressure in the fluid has a huge impact on the evolution of the Universe from the beginning to the end as well as beyond. The flat model can reshape itself after a moment of the Big Rip due to the pressure change in the fluid, or perhaps the Universe will restructure itself and move to another Universe under the conditions of quantum physics. On the other hand, in the closed model, the moment of the Big Rip is continuous and cannot go back again to build its physical nature. Also, the physical behaviour of the open model cannot be accepted due to the special behaviour of both the energy density of the matter and the pressure of the fluid. The effect of pressure on the fluid of the Big Rip in the cosmological models, with linearly varying deceleration parameters and the periodic Universe of varying deceleration parameters of the second degree, is investigated. The study also extends the time of the Universe to be twice that of the first model in such a way as to be consistent with the second model when exploring the physical behaviour of the evolution of the Universe. Additionally, it gives us a visualisation of what happened to the Universe after the moment of the Big Rip. The two models help in explaining the positive pressure at the beginning of the Universe, as well as the negative pressure at the Big Rip. They expand the Universe to the moment of the Big Rip, and then the opposite pressure that forces the Universe to return to its beginning state.