Cosmic Reionization

Cosmic reionization is a period in the universe's history, roughly 400 million to 1 billion years after the Big Bang, when the first stars and galaxies formed. Their intense light ionized the surrounding neutral hydrogen gas, turning it from opaque to transparent and allowing light to travel freely through space. This process transformed the universe from a dark, opaque state into the clear, star-filled cosmos we observe today. Reionization marks a major phase change, helping shape the universe's evolution and making it possible for galaxies and stars to shine across cosmic history.