Dark energy from dark radiation in strongly coupled cosmologies with no fine tuning

TL;DR

This paper proposes a cosmological model with coupled dark matter and scalar fields that naturally explains dark energy and dark radiation without fine-tuning, matching observational data and addressing small-scale structure issues.

Contribution

It introduces a dual component model with strong coupling that naturally reaches an attractor solution, providing a novel explanation for dark energy and dark radiation in strongly coupled cosmologies.

Findings

The model predicts a constant density parameter for dual components during radiation era.

It explains the transition from radiation to matter dominance at z~3-5000.

The scalar field can be tuned to fit dark energy observations.

Abstract

A dual component made of non-relativistic particles and a scalar field, exchanging energy, naturally falls onto an attractor solution, making them a (sub)dominant part of the cosmic energy during the radiation dominated era, provided that the constant \beta, measuring the coupling, is strong enough. The density parameters of both components are then constant, as they expand as a^{-4}. If the field energy is then prevalently kinetic, as is expected, its energy is exactly half of the pressureless component; the dual component as a whole, then, has a density parameter \Omega_{cd} = 3/4\beta^2 (e.g., for \beta~2.5, \Omega_{cd}~0.1, in accordance with Dark Radiation expectations). The stationary evolution can only be broken by the rising of other component(s), expanding as a^{-3}. In a realistic scenario, this happens when z~3-5x10^3. When such extra component(s) become(s) dominant, the densities of the dual components also rise above radiation. The scalar field behavior can be easily tuned to fit Dark Energy data, while the coupled DM density parameter becomes O(10^{-3}). This model however requires that, at present, two different DM components exist. The one responsible for the break of the stationary regime could be made, e.g., by thermally distributed particles with mass even >>1$-2 keV (or non-thermal particles with analogous average speed) so accounting for the size of observed galactic cores; in fact, a fair amount of small scale objects is however produced by fluctuation re-generated by the coupled DM component, in spite of its small density parameter, after the warm component has become non-relativistic.