Light
Light emerged ~380,000 years after the Big Bang, when the universe cooled enough for electrons to bind with protons, making space transparent. For the first time, photons could travel freely. This 'recombination epoch' marks the birth of light itself—the moment the universe became visible. The Cosmic Microwave Background is the fossil record of this first light, still traveling to us from the edge of the observable universe.
Light
13.8 Ga — Everywhere
For 380,000 years, the universe was opaque. Then the fog lifted — and the first light is still travelling.
The Universe Becomes Transparent
magine a fog so thick that light cannot travel farther than a few centimetres. Now imagine that fog filled the entire universe. For the first 380,000 years after the Big Bang, that was reality: a plasma so hot and dense that particles of light — photons — were trapped, scattering off free electrons before they could travel anywhere.
Then the temperature dropped below 3,000 Kelvin. Electrons bound to protons for the first time, forming neutral hydrogen. The fog cleared. Light — which had existed since the first fraction of a second — was finally free to travel. In roughly 100,000 years, the universe went from opaque to transparent.
The Physics of Recombination
The name is a misnomer. "Recombination" implies that electrons and protons were reuniting — but they had never been together before. This was their first meeting. At temperatures above 3,000 K, collisions were too energetic for atoms to form: electrons were stripped away the moment they attached. Below that threshold, hydrogen atoms assembled across the entire universe simultaneously.
The transition took roughly 100,000 years — fast enough that the Cosmic Microwave Background is a snapshot, not a blur. The light released in that window has been travelling ever since, stretched by cosmic expansion from visible glow to microwave hum, but still carrying the imprint of the moment the universe first became see-through.
The First Light
Every photon we detect from the CMB is a messenger from the moment light was born. It has been travelling for 13.8 billion years. When it left, there were no stars, no galaxies, no planets, no eyes to see it. The universe it departed was a featureless glow. The universe it arrives in contains observers who built an antenna to catch it — and who mistook it, at first, for pigeon droppings on the equipment.
