We present a wholly self-consistent, complete cyclic bouncing cosmology based on components drawn from string theory and constructed in a way that is under perturbative control throughout (e.g., with temperatures well below the string scale and a small string coupling at all times).

The cyclic evolution is governed by standard dilaton-gravity in 3+1 dimensions with a perturbatively generated potential and a coupling between the dilaton and a second field that becomes massless at an enhanced symmetry point (ESP), which prevents the dilaton from running all the way to zero coupling.

A central role is played by Instant Folded Strings (IFSs) - fundamental strings with the unusual property of being much lighter than the string mass while extending far beyond the string length, and violating the Null Energy Condition (NEC).

IFSs are produced classically when the string coupling grows with time, which occurs at two critical points in each cycle.

In…