R+S2R+S2 theories of gravity without big-bang singularity

The R+S2R+S2 theories of gravity, where S2S2 denotes the quadratic torsion terms, are analyzed under three cases. In the first two cases, the matter fields are described by two different spin fluids which are not homogeneous and isotropic. In the third case, a homogeneous and isotropic torsion field is used. It is found that under all the three cases, the R+S2R+S2 theories may avert the big-bang singularity of the Robertson–Walker universe, with three corresponding constraints on the parameters.