This era, called cosmic reionization, occurred about 400 million years after the Big Bang. Early stars and galaxies emitted ultraviolet light, ionizing hydrogen gas. This made the universe transparent to radiation and allowed light to travel freely. Observing this period helps astronomers understand early galaxy formation. The timing of reionization offers clues about the first generation of stars, known as Population III. It marks the transition from darkness to the illuminated universe we see today.
It matters because reionization explains how the universe became transparent to light.
It also informs models of early star and galaxy formation, impacting cosmic history studies.
The first stars ended the universe’s long, dark infancy.
[European Space Agency, esa.int]