Tachyonic dark energy- Constraints from current observations

Abstract

Recent observations from the DESI survey have reignited the debate on the true nature of dark energy, challenging the standard model of cosmology. The results suggest a preference for dynamical dark energy rather than a 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 $\phi$ as a model for dark energy, assuming an exponential potential for the field and performed parameter estimation using MCMC techniques. Such a model offers solutions that have $w\sim-1$ and are decelerating without requiring a phantom like equation of state (EoS). The present day value of EoS 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 Pantheon+ and BAO measurements from DESI. The results show that both types of datasets consistently predict a turnaround in the EoS, regardless of whether $w_{\phi0}$ is treated as a free parameter or fixed to $-1$. The corresponding deceleration parameter also exhibits a future turnaround for both datasets when $w_{\phi0}$ is free. However, in the reference model with $w_{\phi0}=-1$, the deceleration parameter instead approaches $-1$ asymptotically. A model comparison shows that the Pantheon+ dataset favors free $w_{\phi0}$, while BAO observations prefer $w_{\phi0} = -1$. This indicates a disagreement in the future evolution predicted by the two datasets within the tachyon dark energy model.