Dark energy is the leading candidate for the mechanism that is
responsible
for causing the cosmological expansion to accelerate. Dr Ratra will
describe the astronomical data which persuade cosmologists that (as yet
undetected) dark energy and dark matter are by far the main components
of
the energy budget of the universe at the present time. He will review
how
these observations have led to the development of a quantitative
"standard" model of cosmology that describes the evolution of the
universe from an early epoch of inflation to the complex hierarchy of
structure seen today. He will also discuss the basic physics and the
history of ideas, on which this model is based.