Cosmological implications of tracker scalar fields: Testing the evidence for dynamical dark energy with recent data

TL;DR

The paper assesses non-phantom tracker scalar field models as dynamical dark energy by studying inverse axionlike (IAX) and inverse steep exponential (ISE) potentials, including their background evolution and perturbations. It combines CMB, DESI DR1/DR2 BAO, Pantheon Plus, H(z), and RSD data, using AIC/BIC for model comparison to ΛCDM. The results show a mild suppression of the matter power spectrum and fσ8 relative to ΛCDM, while the reduced bispectrum remains similar to ΛCDM, yielding no strong evidence for dynamical dark energy in the non-phantom regime; the IAX model is somewhat favored over ISE but neither outperforms ΛCDM. The findings suggest that current non-phantom tracker models do not alleviate existing tensions and that phantom-crossing scenarios, which some studies have found to fit certain datasets better, remain an important direction for future work.

Abstract

We investigate non phantom tracker scalar field models as dynamical dark energy scenario. These models can alleviate the cosmic coincidence problem and transition to a cosmological constant-like behaviour at late times. Focusing on the inverse axionlike and inverse steep exponential potentials, we study their background evolution and perturbations, finding a mild suppression in the matter power spectrum compared to $Λ$CDM but no distinguishing features in the bispectrum. Using combined datasets of ${\rm CMB}+{\rm BAO\; (DESI~DR1\; \&\; DR2)}+{\rm Pantheon~Plus}+{\rm Hubble\; parameter}+{\rm RSD}$, we perform a statistical comparison based on the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC). Our results indicate that, within the framework of non-phantom tracker models, the data show no evidence for dynamical dark energy. The $Λ$CDM model continues to provide a better fit to current observations in the non phantom regime. We emphasise, however, that our analysis does not rule out the possibility of phantom-crossing dark energy models, which have been found in other studies to provide a better fit to some datasets.

Figures (8)

• Figure 1: Top figures show the thawing and scaling behaviours for the potential \ref{['eq:pot_pl']}. Top left: Evolution of energy densities of matter (long dashed green), radiation (short dashed red) and scalar field (solid ($n=1$) and dashed ($n=20$) blue lines) normalised with the present value of critical density $\rho_{\rm c0}$ have been shown. Top right: Scalar field EoS for $n=1$ (solid blue) and $n=20$ (dashed red) has been shown. $V_0/\rho_{\rm c0}=0.5\; {\rm and}\; 1700$ for $n=1$ and $n=20$ respectively. Initial conditions are $\phi_i=0.1 M_{\textrm{Pl}}$ and $0.5$ fo $n=1$ and $n=20$ respectively with $\phi_i'=\mathrm{d}\phi_i/\mathrm{d} \ln(1+z)=10^{-5}M_{\textrm{Pl}}$ for both values of $n$. Bottom figures are similar to the top figures but represent the tracker dynamics for $n=6$. $V_0/\rho_{\rm c0}= 1700$, $\phi=0.1M_{\textrm{Pl}}$ and $\phi'=10^{-5}M_{\textrm{Pl}}$. For all figures we have considered $\Omega_{\mathrm{m}0}=0.3$.
• Figure 2: Left: Similar to the top left figure of Fig. \ref{['fig:rho_para_track_pl']}. Right: Scalar field EoS for different $n$ (top) and $f$ (bottom) along with the effective EoS (dashed red) have been shown. Initial conditions are $\phi_i=0.1 M_{\textrm{Pl}}$ and $\phi_i'=\mathrm{d}\phi_i/\mathrm{d} \ln(1+z)=10^{-5}M_{\textrm{Pl}}$. For all the figures we have considered $\Omega_{\mathrm{m}0}=0.3$.
• Figure 3: Similar to the figures of Fig. \ref{['fig:rho_para_track_pl']}. Top: Dashed blue line is for $n=2$ and solid blue line for $n=10$ while $\mu=1$. Bottom: Dashed blue line is for $\mu=0.2$ and solid blue line for $\mu=1$ while $n=10$. Initial conditions are $\phi_i=0.7 M_{\textrm{Pl}}$ and $\phi_i'=\mathrm{d}\phi_i/\mathrm{d} \ln(1+z)=10^{-5}M_{\textrm{Pl}}$. For all the figures we have considered $\Omega_{\mathrm{m}0}=0.3$.
• Figure 4: Evolution of the density parameters ($\Omega$) (left) and energy densities ($\rho$) (right) have been shown for the MSE potential \ref{['eq:potMSE']}. In both the figures long dashes green line, short dashed red line and solid blue line represent matter, radiation and scalar field respectively. Initial conditions are $\phi_i=0.45 M_{\textrm{Pl}}$ and $\phi_i'=\mathrm{d}\phi_i/\mathrm{d} \ln(1+z)=10^{-5}M_{\textrm{Pl}}$ with $\phi_0=0.5$. For all the figures we have considered $\Omega_{\mathrm{m}0}=0.3$.
• Figure 5: (Left:) Power spectrum is shown for the IAX potential \ref{['eq:pot_iax']} (top left) with $n=4$ and $f=0.3$ (solid green) and for the ISE potential \ref{['eq:pot_ise']} (top left) with $n=10$ and $\mu=1$ (solid green) at $z=0$ along with the power spectrum for the $\Lambda$CDM (dashed red) model. (Right:) Evolution of $f\sigma_8(z)$, for the similar cases as the left figure, has been shown. The brown dots are the observational data of $f\sigma_8$ along with their $1\sigma$ error bars Nesseris:2017vor. For both the figures $\Omega_{\mathrm{m}0}=0.3$.