[Paper Review] Tracker behaviour of quintom dark energy and the Hubble tension

Nandan Roy, L. Arturo Ureña–López|arXiv (Cornell University)|Dec 7, 2023

Cosmology and Gravitation Theories4 citations

TL;DR

This paper investigates a general quintom dark energy model with tracker behavior using cosmological observations, showing that the model can reduce the Hubble tension to 1.6–2.6σ and exhibits a late-time attractor solution dominated by a phantom field with ~15% contribution, outperforming ΛCDM in Bayesian model comparison.

ABSTRACT

We study the dynamics of the quintom dark energy model using state-of-the-art cosmological observations. The set of equations has been converted into an autonomous system using suitable transformations of the variables. We have discussed the fixed points of the model and the general phase-space behavior, in particular, in finding the existence of the tracker solutions for this model. The observations suggest that at late times the phantom field should dominate the dark energy sector with an approximately 15% share to the quintessence counterpart, and with both fields tracking the background at early times. A Bayesian model comparison with LambdaCDM has also been done by computing the Bayes factor and a positive preference has been obtained for the quintom model. Although not fully resolved, the Hubble tension can be reduced to 2.6σ when compared with the value of H0 reported in [1] and to 1.6σ when compared with that of [2].

Motivation & Objective

• To investigate the tracker behavior of a general quintom dark energy model with non-specific potential forms.
• To analyze the phase-space dynamics and fixed points of the model using an autonomous system formulation.
• To assess the model's ability to alleviate the Hubble tension using recent cosmological data.
• To perform Bayesian model comparison between the quintom model and ΛCDM.
• To determine the late-time dominance of phantom versus quintessence fields in the dark energy sector.

Proposed method

• Transformed the quintom model into an autonomous dynamical system using polar and hyperbolic variable transformations for the quintessence and phantom fields, respectively.
• Identified and classified fixed points into matter-dominated, quintessence-dominated, and phantom-dominated categories based on phase-space analysis.
• Computed eigenvalues of fixed points to assess stability, applying the center manifold theorem for nonhyperbolic cases.
• Constrained model parameters using state-of-the-art cosmological observations, including CMB, BAO, and SNe Ia data.
• Calculated the Bayes factor for model comparison between the quintom model and ΛCDM to evaluate Bayesian preference.
• Updated the Hubble tension status by comparing predicted $H_0$ values with recent measurements from Riess et al. (2022) and Scolnic et al. (2023).

(a)

Experimental results

Research questions

• RQ1 Does the general quintom dark energy model support tracker solutions that alleviate fine-tuning and coincidence problems?
• RQ2 What is the stability and late-time behavior of fixed points in the phase space of the quintom model?
• RQ3 To what extent can the quintom model reduce the Hubble tension compared to ΛCDM?
• RQ4 What is the relative contribution of the phantom and quintessence fields to the dark energy density at late times?
• RQ5 Does the quintom model show a Bayesian preference over the standard ΛCDM model?

Key findings

• The model supports tracker solutions for the quintom field, with the energy density of the scalar fields tracking the background energy density at early times.
• Observations favor a late-time phantom-dominated universe, with the phantom field contributing approximately 15% to the dark energy density.
• The Hubble tension is reduced to 2.6σ when compared to Riess et al. (2022) and to 1.6σ when compared to Scolnic et al. (2023).
• The quintom model shows a positive Bayes factor in favor of the model over ΛCDM, indicating Bayesian preference for the quintom scenario.
• The fixed point $p_1$ is stable for $0 < eta_0 ightarrow 9/2$, suggesting a viable late-time attractor in the phantom-dominated regime.
• Matter-dominated fixed points are unstable, while quintessence-dominated points $q_1$ and $q_2$ can act as late-time attractors under specific parameter ranges.

(b)