Tachyonic dark energy- Constraints from current observations

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.