Schrödinger’s Alarming Phenomenon of Particle Creation in the Expanding Universe

In 1939, Erwin Schrödinger was “alarmed” to discover that particles may be created merely by the expansion of the universe.  For over forty years this result was regarded as a curiosity (to the degree it was regarded at all).  Today, it is appreciated that the phenomenon is the origin of primordial density perturbations and gravitational waves, and it may play an even wider role in cosmology.