The Accelerating Expanding Universe: Dark Matter, Dark Energy, and Einstein’s Cosmological Constant

What is the universe made of, and why is its expansion accelerating? Professor Bharat Ratra, a distinguished cosmologist at Kansas State University, will describe the astronomical evidence indicating that dark matter and dark energy constitute most of the present-day universe. He will explain how these observations contributed to the development of the standard cosmological model, which traces the evolution of the universe from an early period of inflation to the galaxies and large-scale structures observed today. This accessible, nontechnical lecture will also examine the physics and history behind our changing picture of the cosmos. Professor Ratra will include a live weather-balloon demonstration illustrating the expansion of the universe.

Professor Ratra studies cosmology and astroparticle physics, with particular emphasis on the structure and evolution of the universe and on testing cosmological models against observational data. In 1988, he and Jim Peebles proposed the first dynamical dark-energy model. He is a Fellow of the American Physical Society, the American Association for the Advancement of Science, and the American Astronomical Society, and he received the American Physical Society’s 2025 Julius Edgar Lilienfeld Prize for his contributions to physics and scientific communication.