Abstract
The observed temperature fluctuations in the cosmic microwave background can be traced back to primordial curvature modes that are sourced by adiabatic and/or entropic matter perturbations. In this paper, we explore the entropic mechanism in the context of non-singular bouncing cosmologies. We show that curvature modes are naturally generated during `graceful exit,' i.e., when the smoothing slow contraction phase ends and the universe enters the bounce stage. Here, the key role is played by the kinetic energy components that come to dominate the energy density and drive the evolution towards the cosmological bounce.
Highlights
Background evolutionSlow contraction is a ‘super-smoother,’ meaning that it rapidly homogenizes, isotropizes, and flattens the universe according to the classical equations of motion and when quantum effects are included, even when starting from initial conditions that lie far outside the perturbative regime of FRW spacetimes [8, 15]
Our goal in this paper was to explore the ‘entropic mechanism’ for inducing the generation of co-moving curvature perturbations on large scales when the mode wavelengths are much larger than the Hubble radius (k/aH 1)
The entropic mechanism is of particular importance in the context of bouncing and cyclic cosmologies in which the generation of a nearly scaleinvariant spectrum of curvature perturbations during a slow contraction smoothing phase is not possible through a purely adiabatic mechanism
Summary
According to our current understanding, the generation of primordial curvature perturbations is closely tied with the classical mechanism that smooths and flattens the cosmological background and drives it to a homogeneous and isotropic Friedmann-Robertson-Walker (FRW). The curvature fluctuations as observed in the CMB originate from quantum fluctuations of the stress-energy that occur during the last 60 e-folds of smoothing. We shall employ a (macroscopic) hydrodynamic ansatz [30, 35] to characterize the background evolution as well as the corresponding adiabatic and entropic fluctuations generated by the quantum fluctuations. The main result of the section is a master equation relating the co-moving curvature perturbation R to its possible sources, the adiabatic and entropy perturbations. It is minimalist in that it suffices to reduces the description of the stressenergy to just four relativistic fluid parameters: the equation of state, the sound speed of the adiabatic modes, the non-adiabatic pressure and the anisotropic stress. The hydrodynamic ansatz can be used to identify generic predictions for different classes of stressenergy without reference to specific microphysical realizations
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