Cosmology at the top of the $\alpha'$ tower

TL;DR

This paper explores how an infinite series of string theory corrections ($5'$) influence cosmological models, enabling non-singular bounces and de Sitter solutions even with matter that obeys energy conditions.

Contribution

It demonstrates that the full $5'$-corrected duality-invariant action can support non-singular cosmologies and fixed-dilaton de Sitter solutions, expanding understanding of string cosmology.

Findings

Infinite $5'$ corrections enable non-singular bounces.

De Sitter solutions are possible with fixed dilaton.

Matter obeying null energy condition can still allow bouncing cosmologies.

Abstract

The cosmology of the fully $\alpha'$-corrected duality-invariant action for the Neveu-Schwarz sector of string theory is revisited, with special emphasis on its coupling to matter sources. The role of the duality covariant pressure and dilatonic charge of the matter sector is explored in various contexts, from the low-curvature regime to non-perturbative solutions in $\alpha'$. We comment on how an infinite tower of $\alpha'$ corrections allows for fixed-dilaton de Sitter solutions, even in vacuum. We further investigate the necessary conditions for accelerated expansion in the Einstein frame, as well as for non-singular bounces that could resolve the big bang singularity. In particular, explicit examples are constructed, which show that the tower of $\alpha'$ corrections may support an Einstein-frame non-singular cosmological bouncing background, even when the matter sector respects the null energy condition.