Standard inflationary models predict a background of primordial gravitational waves (tensor modes) generated by quantum vacuum fluctuations during the exponential expansion. The amplitude of these waves is parametrized by the tensor-to-scalar ratio $r$. Recent results from BICEP/Keck combined with Planck have tightened the upper limit to $r < 0.036$ (95% C.L.), ruling out many classic "high-scale" inflation models ($V^{1/4} \sim 10^{16}$ GeV). As experimental sensitivity pushes toward $r \sim 10^{-3}$, a continued non-detection would challenge the generic predictions of simple single-field inflation, forcing theorists to fine-tune potentials or abandon the standard paradigm (BICEP/Keck Collaboration 2021; Ye & Piao 2022).