Revisiting the Lyth bound constraints on inflation from ACT DR6 results

Abstract The Lyth bound asserts that the field excursion of inflaton must be sub-Planckian, thereby imposing an upper bound on the amplitude of the tensor power spectrum in inflationary scenario. This bound is conventionally derived assuming a scale-invariant curvature power spectrum, i.e., $n_s = 1$. However, astrophysical observations confirm a red-tilted spectrum with $n_s < 1$. In light of recent results from the Atacama Cosmology Telescope (ACT) DR6, we revisit these constraints using the newly implied scalar spectral index of $n_s \simeq 0.9743$. Incorporating the ACT data yields a different upper bound on the tensor-to-scalar ratio $r$, and clarifies whether non-canonical inflationary scenarios can remain compatible with both CMB observations and a sub-Planckian field excursion. Our result highlights the importance of combining theoretical Lyth bound considerations with up to date astrophysical data.