Explore the groundbreaking discovery of the universe's accelerating expansion in this 3-hour World Science Festival conversation featuring Nobel Laureate Adam Riess and physicist Brian Greene. Delve into the concept of dark energy, its implications for our understanding of the cosmos, and the challenges it poses to established theories. Learn about the historical context of this paradigm-shifting revelation, including Einstein's cosmological constant and the initial skepticism faced by researchers. Examine the role of supernovas in measuring cosmic distances and the ongoing debate surrounding the Hubble constant. Gain insights into inflationary cosmology, the Big Bang theory, and the philosophical implications of our evolving understanding of the universe. Part of the Big Ideas series, this in-depth discussion offers a fascinating look at cutting-edge astrophysics and cosmology.