What is reionization in cosmology?
Quick Answer
Reionization was the era when the first stars and galaxies ionized the hydrogen gas in the Universe again. It ended the cosmic dark ages and began the transparent Universe we see today.
The Full Story
After the Big Bang, the Universe cooled enough for neutral hydrogen to form, leaving it opaque to some kinds of light. When the first stars, galaxies, and perhaps black holes formed, their intense radiation ionized this gas, making the Universe transparent again. This era is called reionization and likely occurred between roughly 400 million and 1 billion years after the Big Bang. Studying it helps astronomers understand the birth of the first luminous structures.
Key Facts
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Suggested video title for this topic:
"The moment the first stars switched the Universe’s lights back on"