Effects of the ekpyrotic mechanism on inflationary phase in loop quantum cosmologies

In bouncing cosmological models, either classical or quantum, the Big Bang singularity is replaced by a regular bounce. A challenging question in such models is how to keep the shear under control in the contracting phase, as it is well-known that the shear grows as fast as 1/a⁶ toward the bounce, where a is the expansion factor of the universe. A common approach is to introduce a scalar field with an ekpyrotic-like potential, which becomes negative near the bounce, so the effective equation of state of the scalar field will be greater than one, whereby it dominates the shear and other matter fields in the bounce region. As a result, a homogeneous and isotropic universe can be produced. In this paper, we study how the ekpyrotic mechanism affects the inflationary phase in both loop quantum cosmology (LQC) and a modified loop quantum cosmological model (mLQC-I) because in these frameworks, the inflation is generic without such a mechanism. After numerically studying various cases in which the potential of the inflaton consists of two parts, an inflationary potential and an ekpyrotic-like one, we find that, despite the fact that the influence is significant, by properly choosing the free parameters involved in the models, the ekpyrotic-like potential dominates in the bounce region, during which the effective equation of state is larger than one, so the shear problem is resolved. As the time continuously increases after the bounce, the inflationary potential grows and ultimately becomes dominant, resulting in an inflationary phase. This phase can last long enough to solve the cosmological problems existing in the Big Bang model.