Constraints on String Cosmology

TL;DR

The work tackles whether intrinsically stringy, higher-derivative corrections in the heterotic string can realize accelerating cosmologies by violating the strong energy condition. It derives and analyzes the ten-dimensional effective action up to alpha-prime corrections, rewrites the equations in the Einstein frame, and evaluates the curvature contributions from the torsionful connection R_plus for Minkowski, (A)dS, and general FLRW backgrounds. The main results are that de Sitter solutions remain forbidden at this order, Minkowski vacua require an unwarped string-frame metric, and AdS4 backgrounds can arise as an alpha-prime effect due to the R_plus^2 corrections; broader FLRW analyses reinforce that the effective cosmological constant is non-positive. These findings show that stringy corrections can evade certain supergravity no-go theorems and delineate a region of the heterotic landscape that admits AdS backgrounds, informing both holographic considerations and future explorations of dynamical scalars.

Abstract

String theory contains sources like orientifold planes that support higher derivative interactions. These interactions make possible static flux compactifications which are forbidden in supergravity. They can also lead to violations of the strong energy condition (SEC) which is needed for an accelerating universe. We examine how large a violation is possible in the context of the heterotic string compactified to four dimensions. We find that de Sitter solutions are still not possible but that classically forbidden anti-de Sitter solutions are possible.