Primordial Gravitational Waves in Loop Quantum Cosmology: Regularization Ambiguities and Initial State Dependence

Abstract

We compute the primordial tensor power spectrum in standard loop quantum cosmology and in the modified models mLQC-I and mLQC-II, evolving the tensor perturbations across the bounce in the hybrid and dressed metric prescriptions with a Starobinsky potential. In mLQC-I, the emergent de Sitter contracting branch requires the use of dedicated initial states. We use de Sitter mode function in the dressed metric approach and a Hankel-function solution in the hybrid approach. For inflationary e-foldings $N_e = 60$ and $65$, the pivot-scale tensor-to-scalar ratio agrees with the value of standard inflation with this potential. The scale-dependent ratio $r(k)=\mathcal{P}_T/\mathcal{P}_S$ cancels effects common to the two sectors and separates features that are independent of the initial state from those that depend on it. It varies slowly in the scale-invariant regime and oscillates around $r_\star$ at smaller $k$. The oscillations next to the scale-invariant regime do not depend on the adiabatic order, unlike the peaks and spikes at lower $k$. For a given initial state, the regularization dependence survives in the infrared, where long-wavelength modes retain a record of the bounce and of the pre-bounce branch. We propagate the normalized spectra to the CMB $B$-mode spectrum and discuss observational prospects.