Emergent Scenario In The Einstein-Cartan Theory

One of the most basic and ancient questions in cosmology is whether our universe has a beginning,the answer for this question is still far from clear.General Relativity has withstood the test of various observations after it was presented by Einstein,and it has got great achievements when it was used to describe the evolution of our universe.Standard cosmology which is based on the General Relativity assume our universe stem from a big bang singularity with extremely high energy density and temperature,and all kinds of physical theories are invalid at this point.In order to solve the singularity problem,Ellis et al put forward the so-called emergent scenario,in which our universe stems from the Einstein static universe,i.e.the universe stays in an Einstein static past eternally and then naturally evolves into an inflationary era.Thus,the emergent scenario can accommodate with the hot big bang model in the General Relativity.A successful emergent scenario not only require our universe can stay in a stable Einstein static state past eternally but also need it can exit this state at a given time and then evolve into an inflationary era.However,there is no stable Einstein static solution in the General Relativity,the original emergent scenario does not successful.In order to obtain a successful emergent scenario,the work to analyze the stability of Einstein static model in the other theory becomes a necessary one.Recently,it has been found that the Einstein static universe is stable in various theories including loop quantum gravity theory,branemodel gravity theory,f(T)gravity theory,and so on.Einstein-Cartan theory is a modification of General Relativity by introducingspacetime torsion.In this theory,torsion stems from the intrinsic spin of the fermion matter,it can be expressed completely in terms of the Weyssenhoff spin fluid which is used to describe the fermion according to this relation,and it is not a dynamical quantity.Thus,we can incorporate the spinor field into the torsionfree General Relativity.Einstein-Cartan theory can be expressed as the General Relativity with an exotic stiff perfect fluid with the equation of state w = 1 in a modified energy-momentum tensor.We discuss the existence and stability of the static universe in the EinsteinCartan theory.We find that there is no stale Einstein static solution in the flat and open universes.While,the stable Einstein static solution exists in the spatially closed universe,and the universe has a naturally way to exit the stable static state.Thus,we can use the emergent scenario to solve the big bang singularity.