
Light
13.80 billion years ago
The Universe Becomes Transparent
Imagine 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.
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Imagine 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.






























































