Tachyonic dark energy- Constraints from current observations

TL;DR

This paper investigates tachyonic dark energy with an exponential potential as an alternative to the cosmological constant in light of DESI and Pantheon+ observations. The authors formulate a flat FRW tachyon model with $V(\phi)=V_0 e^{-\phi/\phi_a}$, derive the dynamical equations for the field and the expansion history, and constrain the model using MCMC fits to Pantheon+ SN data, BAO measurements (BAO1), and DESI DR2 BAO. They find that allowing the present-day EoS parameter $w_{\phi0}$ to vary yields a turnaround in $w_\phi$ and in the deceleration parameter $q$, while fixing $w_{\phi0}=-1$ leads to $q\to -1$ asymptotically; however, AIC/BIC show dataset-dependent preferences, with SNe favoring $w_{\phi0}\neq -1$ and BAO/DESI favoring $w_{\phi0}=-1$. The results highlight a partial agreement across datasets and a divergence in future evolution predictions between SNe and BAO within this tachyon framework, underscoring the need for further joint analyses and high-precision data.

Abstract

Recent observations from the Dark Energy Spectroscopic Instrument (DESI) survey have reignited the debate on the true nature of dark energy, challenging the standard cosmological constant model of cosmology. The results suggest a preference for dark energy to be dynamical rather than a cosmological constant. Several recent analyses of DESI data indicate that the universe's expansion may not be accelerating in the way suggested by supernova based cosmology. Motivated by these studies, we investigated a tachyon type scalar field $φ$ as a model for dark energy, assuming an exponential potential for the field and performed parameter estimation using Markov Chain Monte Carlo (MCMC) techniques. Such a model offers solutions that have $w \sim -1$ and are decelerating without requiring a phantom like equation of state. The present day value of the equation of state parameter is treated as a free parameter; however, for the reference model, we fix its present value to $-1$. The analysis is carried out using the latest Supernovae dataset (Pantheon+) and BAO measurements from DESI. The results show that both types of datasets consistently predict a turnaround in the equation of state, regardless of whether $w_{φ0}$ is treated as a free parameter or fixed to $-1$. The corresponding deceleration parameter also exhibits a future turnaround for both datasets when $w_{φ0}$ is free. However, in the reference model with $w_{φ0} = -1$, the deceleration parameter instead approaches $-1$ asymptotically. A model comparison using the Akaike and Bayesian Information Criteria shows that the Pantheon+ dataset favors the free $w_{φ0}$ scenario, while BAO observations prefer the $w_{φ0} = -1$ case. This indicates a disagreement in the future evolution trends predicted by the two datasets within the tachyon type dark energy model.

Figures (5)

• Figure 1: Parameters at $2 \sigma$ confidence level for tachyonic scalar field model with exponential potential. Here $w_{\phi 0}$ is a free parameter, and the constraints are obtained via MCMC analysis with three different datasets i.e BAO (red), DESI (blue) and Pantheon+ (green). The uniform prior for MCMC is given in the table \ref{['Param_bound w']}. From the posterior plots, it is clear that the values of the parameters agree within the $\sim 1 \sigma$ range.
• Figure 2: Contours showing 2-$\sigma$ allowed region with $w_{\phi 0}=-1$. The uniform prior for MCMC is given in the table \ref{['Param_bound no w']}.
• Figure 3: This figure shows the equation of state parameter as a function of redshift. The dark curve represents the evolution of $w$ with redshift $z$ at the mean values of the parameters obtained from table \ref{['Param_bound w']}. The shaded region, referred to as the $1 \sigma$ region around this curve, represents the envelope formed by the outlier curves. The dotted curves correspond to scenarios where one parameter is varied by $1\sigma$, while the others are held fixed at their central values.
• Figure 4: Plots of deceleration parameter as a function of redshift. The dark curve represents the evolution of $q$ w.r.t. redshift $z$ at the mean values of the parameters obtained from table \ref{['Param_bound w']}. This suggests that for all the datasets used, this tachyon model predicts the slowing down of acceleration in the future.
• Figure 5: 1-$\sigma$ plots of equation of state parameter (first column) and deceleration parameter (second column). The black, solid curve represents the evolution of $w$ (first column) and $q$ (second column) as a function of redshift $z$ at the mean values of the parameters. The shaded region corresponds to the 1-$\sigma$ ranges of the parameters.