The Role of Quark-Gluon Plasma in the Early Universe

In an era of the early Universe at a time estimated to be a millionth of a second after the Big Bang, the Universe was filled with quark-gluon plasma. In this plasma and due to the high temperature the strong coupling constant, that characterizes the magnitude of the strong force acting on quarks and gluons, becomes so small. As a consequence quarks and gluons inside this plasma can be considered as an ideal gas of gluons and massless quarks that weakly interact with each others. Thus, for this plasma, one can describe its characteristics by the equations of states that relate both energy density and pressure to its temperature. This has been done in several models in the literature with the recent information about the properties of the quark-gluon plasma provided by relativistic heavy-ion collision experiments and some astrophysical measurement. In this article we review three of these models namely the MIT bag model, Model 1 and Model 2. Moreover we solve Einstein's field equations of the general relativity,that describe our universe, to show the time evolution of energy density, pressure and temperature in the early universe in these three models. This kind of study is important as our present universe evolved from a universe filled with quark-gluon plasma.