Hitchhiker's Guide to the Swampland: The Cosmologist's Handbook to the String-Theoretical Swampland Programme

Abstract

String theory has strong implications for cosmology, implying the absence of a cosmological constant, ruling out single-field slow-roll inflation, and that black holes decay. The origins of these statements are elucidated within the string-theoretical swampland programme. The swampland programme is generating a growing body of insights that have yet to be incorporated into cosmological models. Taking a cosmologist's perspective, the relevance of swampland conjectures to black holes, dark matter, dark energy, and inflation is highlighted, including their implications for scalar fields such as quintessence and axions. The aim is to inspire cosmological model builders to examine the compatibility of effective field theories with quantum gravitational UV completions and to address outstanding cosmological tensions such as the Hubble tension. This comprehensive literature review presents clear definitions, cosmological implications, and the current status — including evidence and counterexamples — of the following swampland conjectures: the anti–de Sitter distance conjecture, the completeness conjecture, the cobordism conjecture, the de Sitter conjecture, the swampland distance conjecture, the emergence proposal, the Festina Lente Bound, the finite number of massless fields conjecture (or finite flux vacua conjecture), the no global symmetries conjecture, the no non-supersymmetric theories conjecture, the non-negative null energy condition conjecture, the positive Gauss–Bonnet term conjecture, the species scale conjecture, the gravitino swampland conjecture, the tadpole conjecture, the tameness conjecture, the trans–Planckian censorship conjecture, the unique geodesic conjecture, and the weak gravity conjecture, including the repulsive force conjecture.