Existence of parametrically scale-separated string compactifications

Establish whether there exist string compactifications whose low-energy effective theories exhibit a cosmological constant that is parametrically separated from the Kaluza–Klein scale, thereby realizing genuine scale separation in string theory.

Background

The paper investigates the longstanding issue of scale separation in string compactifications, namely whether the cosmological constant can be made parametrically smaller than the Kaluza–Klein scale in a controlled setting. This topic is closely tied to swampland considerations and holography, where the general viability of such vacua is debated.

The work contributes by constructing scale-separated AdS3 vacua in massless type IIA with only O6-planes (obtained via double T-duality from massive IIA G2 compactifications), but emphasizes that the overarching question of the existence of parametrically scale-separated vacua in string theory remains unresolved in full generality.

References

Understanding whether string compactifications can give rise to effective theories in which the cosmological constant is parametrically separated from the Kaluza--Klein scale remains an open question in string theory.

— T-dualities and scale-separated AdS$_3$ in massless IIA on $(X_6 \times S^1)/\mathbb{Z}_2$  (2603.26615 - Tringas, 27 Mar 2026) in Section 1, Introduction

Nevertheless, whether parametrically scale-separated AdS vacua can actually arise in string theory remains an open and actively debated question.

— Low-energy brane decoupling in AdS flux vacua  (2609.17342 - Apers et al., 15 Sep 2026) in Section 1, Introduction

However, whether this conclusion must also apply to the neutral sector at fixed electric charge is not clear to us, and we leave this question for future work.

— Scale-separated AdS$_2$ flux vacua from type II  (2609.02814 - Tringas et al., 2 Sep 2026) in Section 7, “Geometric scale separation and obstruction”; reiterated in Section 9, “Conclusion”