Picture the universe as a loaf of bread rising in an oven — except no one switched the oven on, and the bread keeps rising faster and faster all by itself. That is roughly the puzzle cosmologists have wrestled with since 1998, when three astrophysicists discovered that the universe's expansion is not slowing down but accelerating. That discovery won Adam Riess, Brian Schmidt, and Saul Perlmutter the 2011 Nobel Prize in Physics. Then, in late 2025, a rival team shook the field by claiming the evidence for that acceleration — and for the mysterious force called dark energy behind it — was falling apart.

The challenge was serious. The rival researchers argued that the standard method of measuring cosmic expansion using Type Ia supernovae, the brilliant, predictable explosions that mark the deaths of certain stars, was fundamentally flawed. If they were right, nearly 30 years of cosmological research would have needed to be thrown out. Physicists on several continents held their breath.

Had one of the most celebrated discoveries in modern science been built on a mistake?