Late-time acceleration with a scalar field source: Observational
  constraints and statefinder diagnostics
  This article discusses a dark energy cosmological model in the standard
theory of gravity - general relativity with a broad scalar field as a source.
Exact solutions of Einstein's field equations are derived by considering a
particular form of deceleration parameter $q$, which shows a smooth transition
from decelerated to accelerated phase in the evolution of the universe. The
external datasets such as Hubble ($H(z)$) datasets, Supernovae (SN) datasets,
and Baryonic Acoustic Oscillation (BAO) datasets are used for constraining the
model par parameters appearing in the functional form of $q$. The transition
redshift is obtained at $% z_{t}=0.67_{-0.36}^{+0.26}$ for the combined data
set ($H(z)+SN+BAO$), where the model shows signature-flipping and is consistent
with recent observations. Moreover, the present value of the deceleration
parameter comes out to be $q_{0}=-0.50_{-0.11}^{+0.12}$ and the jerk parameter
$% j_{0}=-0.98_{-0.02}^{+0.06}$ (close to 1) for the combined datasets, which
is compatible as per Planck2018 results. The analysis also constrains the omega
value i.e., $\Omega _{m_{0}}\leq 0.269$ for the smooth evolution of the scalar
field EoS parameter. It is seen that energy density is higher for the effective
energy density of the matter field than energy density in the presence of a
scalar field. The evolution of the physical and geometrical parameters is
discussed in some details with the model parameters' numerical constrained
values. Moreover, we have performed the state-finder analysis to investigate
the nature of dark energy.
