On Wednesday, February 11, 1931, Albert Einstein met for more than an hour with a small group of American scientists in the cozy library of the Mount Wilson Observatory, near Pasadena, California. The subject was cosmology, and Einstein was poised to make one of the more momentous statements in the history of science.

With his theories of relativity and gravity long confirmed and his Nobel Prize ten years old, he was by far the most famous scientist in the world. “Photographers lunged at me like hungry wolves,” he had written in his diary when his ship landed in New York…