  High precision cosmological observations in last decade suggest that about
70% of our universe's energy density is in so called "Dark Energy" (DE).
Observations show that DE has negative effective pressure and therefore unlike
conventional energy sources accelerates the cosmic expansion instead of
decelerating it. DE is highly uniform and has become a dominant component only
recently.
  The simplest candidate for DE is the time-independent cosmological constant
$\Lambda$. Although successful in fitting available data, the cosmological
constant model has a number of theoretical shortcomings and because of that
alternative models of DE are considered. In one such scenario a cosmological
scalar field that slowly rolls down its potential acts like a time-dependent
cosmological constant.
  I have used different independent cosmological data sets to constrain the
time dependence of DE's energy density in the framework of the slowly-rolling
cosmological scalar field model. Present data favors a time-independent
cosmological constant, but the time-dependent DE can not be ruled out at high
confidence level. Ongoing and planned cosmological probes and surveys will
provide more and better quality data over the next decade. When the new data
sets are available we will be able to either detect the time dependence of DE
or constrain it to a very small physically uninteresting value.
