Since the late 1990s, humanity has built its understanding of the cosmos on a single unsettling truth: the universe is not merely expanding, but accelerating outward, driven by an invisible force called dark energy. Now, new instruments are whispering that this force may itself be changing — weakening with time — and the mathematical scaffolding that has held cosmology together for two decades may need to be dismantled. NASA's Nancy Grace Roman Space Telescope, launched in August 2026, enters this moment of uncertainty as an independent witness, tasked with either confirming one of the most co