Right now, as you sit here, roughly 400 million ancient photons are passing through every square centimeter of your body each second. This is not a metaphor, but the cosmic microwave background: the oldest light in existence, cooled and stretched over nearly fourteen billion years into a faint, invisible whisper that fills all of space. It is the afterglow of the moment the universe first became transparent, and it surrounds us completely, entirely undetectable to human senses.

This episode explores the journey of this ancient light, beginning with the unexplained radio static that puzzled Arno Penzias and Robert Wilson in 1965. We travel back to the first 380,000 years of the cosmos, when the universe was an opaque, seething fog of plasma, to understand how matter eventually cooled and combined, releasing a blinding flash of light that has ...