# Effective low energy actions in string cosmology

I wonder why can SUGRA actions be used in string cosmology, since in some cases they are used to model big bang scenarios (for example the pre big bang scenario in chapter 4 of Gasperini's "Elements of string cosmology"). Shouldn't the energy scale be so high that actual superstring theory needs to be used, instead of its low energy description?

For instance inflationary energy scale (by which I mean mass of an inflaton) is around $$M_I\sim 10^{14}$$ GeV, give or take depending on the model. Clearly $$M_I\ll M_P$$.