Event: Photon Epoch
Date: 10 seconds – 380,000 years after the Big Bang

“For hundreds of thousands of years, light could not travel far. Only when atoms formed did the universe finally become transparent—and photons could at last tell the tale of creation.”
-Lawrence Krauss

Dear Human,

There was a time when light could not move. Not because it didn’t exist—photons, the tiny messengers of light, were everywhere—but because the universe was too dense and hot to let them travel. This was the Photon Epoch, beginning just seconds after the Big Bang and lasting nearly 400,000 years.

Back then, temperatures were over a billion degrees Kelvin. The universe was a fog of protons, electrons, and photons in constant collision. Photons bounced endlessly off free electrons, unable to move freely through space. It was like trying to see through boiling water—too thick with motion to glimpse anything clearly.

During this time, the fundamental forces of nature had already separated, and neutrinos had escaped into the cosmos. But light remained trapped. The very force that would one day make vision, communication, and discovery possible was still caged by chaos.

As the universe expanded, it cooled. By around 380,000 years in, it had dropped to roughly 3,000 K. Protons and electrons finally combined to form neutral hydrogen atoms. This process—recombination—meant photons no longer had free charges to scatter off. They could move at last. The universe became transparent.

That light never stopped traveling. Today, we detect it as the Cosmic Microwave Background—an ancient glow stretched into microwaves by billions of years of expansion. It’s the earliest light we can see, and it carries a perfect imprint of the infant universe.

The Photon Epoch was the turning point. For the first time, the universe became visible. Light in motion allowed galaxies to take shape, eyes to evolve, and minds to question what they see. It gave the universe a way to communicate across time and space.

From chaos came order. In a cosmos once too wild to witness itself, light became a messenger of memory and meaning. That same light still surrounds you, whispering stories older than stars.

Pathfinder

Photon Epoch – Wikipedia