Exploring Affine Interacting Models of Dark Energy as a Replacement for ΛCDM

Adam Williams; Marco Bruni

doi:10.59973/emjsr.389

Authors

Adam Williams School of Mathematics and Physics, University of Portsmouth, PO1 3QL Portsmouth, UK https://orcid.org/0009-0003-4394-3500
Marco Bruni Institute of Cosmology and Gravitation, Dennis Sciama Building, Burnaby Rd, Portsmouth PO1 3FX, UK

DOI:

https://doi.org/10.59973/emjsr.389

Keywords:

Dark Energy, Affine Equation of State, Interacting Dark Sector

Abstract

The ΛCDM model provides a rudimentary but adequate description of the Universe governed by the Friedmann-Lemaˆıtre-Robertson-Walker metric. However, increasing observational precision has revealed tensions between observations and model. In particular, ΛCDM struggles to accurately explain DESI type Ia supernova observations, Cosmic-Microwave-Background anisotropies and Baryon-acoustic-oscillation distance scales; whilst it’s model of constant dark energy offers no explanation for cosmic coincidence or the Hubble tension. Therefore, in this work we model the dark energy component as an interacting affine fluid coupled across the dark sector, with dynamical and Λ-like constant components that evolve independently, naturally alleviating many of these issues. Using dynamical system theory techniques, we study the evolution of four models of linear and non-linear interactions between dark matter and energy, across a range of interaction strengths. By treating the model as an autonomous closed differential system, we redefine our model in terms of dimensionless variables that are solved using python packages, ensuring ΛCDM-like dynamics at present time via PLANCK observations as well as enhancing the dynamical freedom of the system from which evolving equations of state can be plotted. Furthermore, by analysing the system in phase space we observe natural scaling solutions as well as Fixed Point solutions that alleviate coincidence without the explicit need for fine tuned initial conditions; along with altered cosmological histories mitigating the Hubble tension. Finally, we assess the physical viability of each interaction model with parameter ranges by comparing the predicted expansion history with DESI supernova constraints and PLANCK density fraction observations. All models produce the necessary ΛCDM late time solutions whilst displaying a wide range of dynamical flexibility across expansion history that helps to reconcile the Hubble tension; therefore proving that interacting affine models of dark energy provide necessary extensions to the standard cosmological model.

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