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Cosmic Microwave Background
13.80 billion years ago
A Fossil of the Early Universe
The CMB isn't just ancient light—it is the outer surface of our cosmic horizon. Because light takes time to travel, the farthest thing we can ever see is the glow released when the universe first became transparent. Anything beyond that horizon is invisible to us forever. The map of the CMB is therefore a fossil boundary: the limit of our vision and the beginning of our story.
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The isn't just ancient light—it is the outer surface of our cosmic horizon. Because light takes time to travel, the farthest thing we can ever see is the glow released when the universe first became transparent. Anything beyond that horizon is invisible to us forever. The map of the CMB is therefore a fossil boundary: the limit of our vision and the beginning of our story.
When we look at the CMB, we are seeing the universe as it was 13.8 billion years ago, when it was a hot, dense plasma of particles and radiation. At that moment, the universe cooled enough for electrons and protons to combine into neutral hydrogen atoms—an event called . This made the universe transparent, allowing light to travel freely for the first time. That light, stretched by cosmic expansion to microwave wavelengths, is what we detect today as the CMB.