A new parametrization for dark energy density and future deceleration

In this work, we have proposed a general dark energy density parametrization to study the evolution of the universe. We have also constrained the model parameters using the combination of Type Ia supernova (SNIa), baryonic acoustic oscillations (BAO), cosmic microwave background radiation (CMB) and observational $H(z)$ datasets. For the $H(z)$ dataset, we have used the direct observations of the Hubble rate, from the radial BAO size and the cosmic chronometer methods. Our result indicates that the SNIa+$H(z)$+BAO/CMB dataset does not favour the $\Lambda$CDM model at more than $2\sigma$ confidence level. Furthermore, we have also measured the percentage deviation in the evolution of the normalized Hubble parameter for the present model compared to a $\Lambda$CDM model, and the corresponding deviation is found to be $4-5\%$ at low redshifts ($z\sim 0.5$). Finally, we have also investigated whether the deceleration parameter $q$ may have more than one transition during the evolution of the universe. The present model shows a transient accelerating phase, in which the universe was decelerated in the past and is presently accelerating, but will return to a decelerating phase in the near future. This result is in great contrast to the $\Lambda$CDM scenario, which predicts that the cosmic acceleration must remain forever.