Has the Universe always expanded ?

TL;DR

This paper argues that in Einstein gravity with hydrodynamical fluids, a bouncing universe scenario leads to singular perturbations incompatible with observations, implying the universe has always been expanding.

Contribution

It demonstrates that bouncing cosmologies with smooth joins to radiation era are incompatible with linear perturbation theory under Einstein gravity and hydrodynamics.

Findings

Scalar perturbations become singular at the bounce

Bouncing models conflict with nucleosynthesis constraints

Universe likely always expanded under these conditions

Abstract

We consider a cosmological setting for which the currently expanding era is preceded by a contracting phase, that is, we assume the Universe experienced at least one bounce. We show that scalar hydrodynamic perturbations lead to a singular behavior of the Bardeen potential and/or its derivatives (i.e. the curvature) for whatever Universe model for which the last bounce epoch can be smoothly and causally joined to the radiation dominated era. Such a Universe would be filled with non-linear perturbations long before nucleosynthesis, and would thus be incompatible with observations. We therefore conclude that no observable bounce could possibly have taken place in the early universe if Einstein gravity together with hydrodynamical fluids is to describe its evolution, and hence, under these conditions, that the Universe has always expanded.