A quantum bounce before inflation leaves subtle marks on the CMB

Two competing loop quantum cosmology frameworks — the hybrid and dressed metric approaches — both predict a suppression of primordial power at large scales, stemming from a quantum bounce that preceded inflation. By distilling this bounce physics into just two extra parameters (e-folds during the bounce and an energy-density-set suppression scale), the authors produce a clean, testable primordial spectrum. Crucially, they find that the tensor-to-scalar ratio remains identical to standard ΛCDM for observed scales above the suppressed region. For roughly 140 total e-folds of expansion, both approaches fit Planck high-multipole data well and appear to reduce tension at low multipoles.