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

Abstract

Abstract In bouncing cosmological models – whether classical or quantum – the big bang singularity is replaced by a regular bounce. A well-known challenge is controlling the growth of shear during the contracting phase, as the shear scales as $$a^{-6}$$ a - 6 toward the bounce, where a is the average expansion factor of the universe. A common solution is to introduce a scalar field with an ekpyrotic-like potential that becomes negative near the bounce, giving an effective equation of state greater than one, thereby dominating the shear and allowing a homogeneous, isotropic universe to emerge after the bounce. In this paper, we investigate how the ekpyrotic mechanism affects the inflationary phase in both loop quantum cosmology (LQC) and a modified loop quantum cosmology model (mLQC-I) – frameworks in which inflation is otherwise generic. We consider a potential consisting of an inflationary part and an ekpyrotic-like component. Through numerical studies of various models, we find that the ekpyrotic mechanism can significantly influence the subsequent inflation, even though, for appropriately chosen parameters, it dominates near the bounce (with equation of state $$w > 1$$ w > 1 ) and successfully resolves the shear problem. As time increases beyond the bounce, the inflationary potential eventually dominates, producing an inflationary phase that can be long enough to address standard hot big bang cosmology problems. Nevertheless, our numerical findings suggest that fine-tuning may be required. Because the results are numerical, our conclusions are not definitive, and a more systematic analysis will be necessary to fully assess the generality of the observed behavior.