Crunching, Bouncing, and Cyclical Cosmologies from Dark Sector Interactions

TL;DR

This paper explores how strong dark sector interactions can lead to cosmological phenomena like bounces, recollapses, or cyclic universes within flat FLRW models, expanding understanding of possible cosmic evolutions.

Contribution

It introduces new analytic conditions and explicit formulas for cosmological turnarounds driven by dark sector interactions, highlighting exotic behaviors in familiar dark energy models.

Findings

Dark sector interactions can cause universe recollapse or bounce.

Sign changes in dark sector densities enable violation of energy conditions.

Cyclic scenarios are possible in closed universe models.

Abstract

We present new mechanisms that produce either a future Big Crunch turnaround or a past non-singular bounce in flat FLRW cosmologies within general relativity at the background level, driven solely by non-gravitational interactions between dark matter (DM) and dark energy (DE). We study phenomenological interacting dark energy (IDE) models based on linear kernels of the form $Q = 3H(\delta_{\rm dm}\rho_{\rm dm} + \delta_{\rm de}\rho_{\rm de})$, focusing on parameter regimes with strong energy transfer from dark energy to dark matter. In this strong interacting regime, the interaction does not vanish when one component crosses zero density, allowing one of the dark-sector densities to become negative. The resulting sign changes can violate the energy conditions required for cosmological turnarounds in a flat universe, thereby enabling either (i) a maximum scale factor followed by recollapse into a big crunch, or (ii) a minimum non-zero scale factor corresponding to a bounce. We derive analytic conditions for these turnarounds and obtain closed-form expressions for the associated maximum or minimum scale factor. We also show that, in a closed universe, a special case of the same IDE framework can be tuned to yield a cyclic scenario. Although these strong interaction scenarios are unlikely to describe the observed Universe, they provide a concrete demonstration that exotic cosmological behaviour can arise naturally in underexplored regions of the parameter space of familiar IDE models.