Improved constraint on primordial gravitational waves in light of the Hubble tension and BICEP/Keck

Abstract: The Hubble tension that the standard $\Lambda$CDM model is suffering from can be resolved with pre-recombination early dark energy. We present the first constraint on the tensor-to-scalar ratio $r$ in corresponding Hubble-tension-free cosmologies using the most recent BICEP/Keck cosmic microwave background (CMB) B-mode polarization data. We find, combining BICEP/Keck with Planck18 CMB and baryon acoustic oscillation data, that the models with larger Hubble constant $H_0$ will have tighter upper bound on $r$, and resolution $H_0\sim73$ km/s/Mpc of the Hubble tension tightens the upper bound to $r<0.028\ (95\%\text{C.L.})$, $25\%$ tighter than the $\Lambda$CDM constraint $r<0.036$. We clarify the origin of this tightening bound.