In the last two decades, a set of interesting ideas based upon unexpected connections between the quarks and the cosmos and the emergence of a new generation of observations and experiments have transformed cosmology into a full-fledged, precision science. The ten-microKelvin fluctuations in the cosmic microwave background radiation are constraining cosmological parameters and shedding light on the earliest moments of the universe. Maps of the distribution of galaxies and clusters in volumes approaching billions of megaparsecs on a side are testing the cold dark matter paradigm. The current expansion rate of the universe has finally been pinned down to 10% precision, and measurements of the past rate have revealed we are now in a period of cosmic acceleration. The contribution of ordinary matter to the overall mass-energy budget has been shown to be small, with more than 95% of the universe existing in new and unidentified forms of matter and energy.
Many, creative theoretical ideas have emerged that provide a way to understand the expansion of the universe, its composition, and the origin of structure. Still, big questions remain. Why are there three different forms of matter/energy of comparable abundance, with the transition to accelerated expansion occurring very recently? How much of the truth does inflation hold about the early universe and what is the hypothetical inflaton field that drove inflation? What is the dark matter and the strange dark energy? Could the complicated recipe and accelerated expansion indicate that we don't yet fully understand gravity?
Astronomers and physicists are in the midst of carrying out ambitious new experiments, completing large surveys of the Universe, and commissioning powerful new telescopes with novel technology and advanced instrumentation. There is great promise of increasingly sharper tests of inflation, cold dark matter, and dark energy, and always the potential for further new surprises. There is no doubt that we are in the midst of a revolutionary period of discovery in cosmology.