A smooth bouncing cosmology with scale invariant spectrum

TL;DR

The paper constructs a explicit, smooth bouncing cosmology within a 4D effective field theory using a ghost-condensate background to realize the bounce and a light isocurvature field with a negative exponential potential to source density perturbations. These perturbations are nearly scale-invariant during contraction and convert to adiabatic modes, yielding a slightly blue tilt (n_S ≳ 1), negligible primordial gravitational waves, and potentially large local non-Gaussianities. The bounce is shown to be stable and controllable, with predictions largely independent of the microscopic bounce details. The exponential potential is argued to be technically natural, arising from higher-dimensional or non-perturbative physics, making the model a concrete but still exploratory alternative to inflation that can be tested with future data.

Abstract

We present a bouncing cosmology which evolves from the contracting to the expanding phase in a smooth way, without developing instabilities or pathologies and remaining in the regime of validity of 4d effective field theory. A nearly scale invariant spectrum of perturbations is generated during the contracting phase by an isocurvature scalar with a negative exponential potential and then converted to adiabatic. The model predicts a slightly blue spectrum, n_S >~ 1, no observable gravitational waves and a high (but model dependent) level of non-Gaussianities with local shape. The model represents an explicit and predictive alternative to inflation, although, at present, it is clearly less compelling.

Figures (1)

• Figure 1: Schematic representation of the potential of $\phi$ during the contracting and the expanding phase.