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Inflation models selected by the swampland distance conjecture with the Lyth bound

Yuma S. Furuta1,2, Yuta Hamada1,2, and Kazunori Kohri3,4,2,5

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Phys. Rev. D 112, 103517 – Published 12 November, 2025

DOI: https://doi.org/10.1103/lpd1-mbmm

Abstract

We investigate the extent to which the swampland conjecture can be employed to constrain large-field inflationary models from the perspective of quantum gravity consistency. In particular, we focus on the swampland distance conjecture, which imposes an upper bound on the amplitude of primordial gravitational waves predicted by large-field inflation scenarios. This provides a striking contrast with the well-known Lyth bound, which yields a lower bound on the tensor-to-scalar ratio in such models. The two bounds thus play complementary roles in assessing the viability of inflationary scenarios. We demonstrate that, for certain representative large-field inflation models, the swampland distance conjecture alone can impose more stringent upper limits on the tensor-to-scalar ratio than current observational constraints from the cosmic microwave background. These findings highlight the utility of swampland criteria as a theoretical discriminator among competing inflationary models, independent of empirical data.

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Phys. Rev. D 112, 103517– Published 12 November, 2025
  • Received 11 August 2025
  • Accepted 21 October 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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