Evolution of Cosmological Perturbations in the Universe dominated by Multiple Scalar Fields

TL;DR

This paper analyzes the evolution of superhorizon cosmological perturbations in a universe dominated by multiple interacting scalar fields, demonstrating that the Bardeen parameter remains constant for adiabatic modes and decays for others, using canonical perturbation theory.

Contribution

It introduces a novel application of canonical perturbation theory to study superhorizon perturbations with multiple incommensurable scalar fields during reheating.

Findings

Bardeen parameter remains constant for adiabatic modes.

Bardeen parameter decays for non-adiabatic modes.

Probabilistic singularities in the Bardeen parameter are discussed.

Abstract

By efforts of several authors, it is recently established that the dynamical behavior of the cosmological perturbation on superhorizon scales is well approximated in terms of that in the long wavelength limit, and the latter can be constructed from the evolution of corresponding exactly homogeneous universe. Using these facts, we investigate the evolution of the cosmological perturbation on superhorizon scales in the universe dominated by oscillating multiple scalar fields which are generally interacting with each other, and the ratio of whose masses is incommensurable. Since the scalar fields oscillate rapidly around the local minimum of the potential, we use the action angle variables. We found that this problem can be formulated as the canonical perturbation theory in which the perturbed part appearing as the result of the expansion of the universe and the interaction of the scalar fields is bounded by the negative power ot time. We show that by constructing the canonical transformations properly, the transformed hamiltonian becomes simple enough to be solved. As the result of the invetigation using the long wavelength limit and the canonical perturbation theory, under the sufficiently general conditions, we prove that for the adiabatic growing mode the Bardeen parameter stays constant and that for all the other modes the Bardeen parameter decays. From the viewpoint of the ergodic theory, it is discussed that as for the Bardeen parameter, the sigularities appear probabilistically. This analysis serves the understanding of the evolution of the cosmological perturbations on superhorizon scales during reheating.