Reionization

Reionization is a key period in the universe's history when the first stars and galaxies formed, emitting energy that ionized the surrounding hydrogen gas. Initially, after the Big Bang, the universe was mostly filled with neutral (uncharged) hydrogen, making it opaque to certain light. As stars and galaxies formed, their radiation ionized this hydrogen, turning it into a charged state and making the universe transparent to light again. This process opened the "cosmic window," allowing light from distant objects to reach us, and marks the universe's transition from a dark, neutral state to the luminous one we observe today.