Spinfoams, $\gamma$-duality and parity violation in primordial gravitational waves

Abstract

The Barbero-Immirzi parameter $\gamma$ appears as a coupling constant in the spinfoam dynamics of loop quantum gravity. In this work, we highlight that $\gamma$ can be understood as a measure of gravitational parity violation via a duality rotation for the EPRL spinfoam model. We call this property $\gamma$-duality, and we investigate an effective field theory for gravity and a scalar field with the same degree of parity violation. The resulting relation between the coupling constants of parity-even and parity-odd higher-curvature terms in the effective action is determined by $\gamma$, opening the possibility of its measurement in the semiclassical regime. For a choice of $\gamma$-dual effective action, we study cosmic inflation and show that the observation of a primordial tensor polarization, together with the tensor tilt and the tensor-to-scalar ratio, provides a measurement of the Barbero-Immirzi parameter and, therefore, of the scale of discreteness of the quantum geometry of space.