Dynamics of primordial fields in quantum cosmological spacetimes

Abstract

Quantum cosmological models are commonly described by means of semiclassical approximations in which a smooth evolution of the expectation values of geometry operators replaces the classical and singular dynamics. The advantage of such descriptions is that they are relatively simple and display the classical behavior for large universes. However, they may "smooth out" an important inner structure to include which a more nuanced treatment is needed. The purpose of the present work is to investigate this inner structure and its influence on primordial gravitational waves. To this end we quantize a model of the Friedmann-Lemaitre-Robertson-Walker universe filled with a "linear" barotropic cosmological fluid and with gravitational waves. The quantization yields an equation of motion for the Fourier modes of gravitational radiation, which is a quantum extension to the usual parametric oscillator equation for gravitational waves propagating in an expanding universe. The two quantum effects from the cosmological background that enter the enhanced equation of motion are (i) a repulsive potential resolving the big bang singularity and replacing it with a Big Bounce; and (ii) uncertainties in the numerical values for the background spacetime dynamical variables. First we study the former effect and its consequences for the primordial amplitude spectrum and carefully discuss the physical scales and parameters of the model. Next we investigate the latter effect, in particular the extent to which it may affect the primordial amplitude of gravitational waves.