Experiments and observations over the last two decades have provided strong support for a "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. I review the basic physics, astronomy, and history of ideas on which this model is based. I describe the 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. I conclude with a list of open cosmological questions.
Bharat Ratra is a theoretical cosmologist and astroparticle physicist, who is currently a Distinguished Professor of Physics at Kansas State University. He had completed his doctorate in physics at Stanford University under the supervision of Leonard Susskind and Michael Peskin. Along with Jim Peebles, Ratra had proposed the first dynamical dark energy scalar field, or quintessence, model. Ratra's early universe research includes the first consistent semi-classical computation of the spectrum of energy density perturbations from inflation. Ratra had also proposed the first inflation model that can generate, from quantum fluctuations, large-enough primordial cosmological magnetic field to be able to explain observed galactic magnetic fields.