The Epoch of Reionization refers to the period in the early universe when the first stars and galaxies formed, and their radiation ionized the surrounding hydrogen gas. We are able to study this epoch through observations of distant astronomical objects. Light from these objects travels for billions of years before reaching us, essentially allowing us to look back in time.
The Cosmic Microwave Background (CMB) is the afterglow radiation from the Big Bang, arriving from when the universe was only approximately 380,000 years old. As light from the first stars and galaxies passed through the intergalactic medium, any neutral hydrogen gas would absorb it, leaving detectable imprints. This includes the 21-cm hydrogen line, which is a critical tool for studying this period. Additionally, by examining the redshift of distant objects, which indicates how much the universe has expanded since that light was emitted, we can infer details about the conditions during reionization.
We observe the Epoch of Reionization by looking at the universe when it was between about 400 million and 1 billion years old, corresponding to redshifts between roughly 6 and 20. Current studies and telescopes, like the James Webb Space Telescope and several radio arrays dedicated to detecting the 21-cm hydrogen line, provide direct and indirect measurements of this epoch. As technology advances, our understanding of the Epoch of Reionization will continue to improve, potentially offering deeper insights into the universe when it was only a few hundred million years old.