The Effective Field Theory of nonsingular cosmology

In this paper, we explore the nonsingular cosmology within the framework of the E ff ective Field Theory (EFT) of cosmological perturbations. Due to the recently proved no-go theorem, any nonsingular cosmological models based on the cubic Galileon su ff er from pathologies. We show how the EFT could help us clarify the origin of the no-go theorem, and o ff er us solutions to break the no-go. Particularly, we point out that the gradient instability can be removed by using some spatial derivative operators in EFT. Based on the EFT description, we obtain a realistic healthy nonsingular cosmological model, and show the perturbation spectrum can be consistent with the observations.