\n The early universe was filled with a fog made up of hydrogen atoms until the first stars and galaxies burned it away. Credit: NASA\/JPL-Caltech, CC BY
\n <\/figcaption><\/figure>\n<\/p><\/div>\n<\/div>\nHowever, a fraction of a second after the Big Bang, the universe expanded extremely rapidly. This expansion eventually allowed the universe to cool enough for light and matter to separate out of their “soup” and\u2014some 380,000 years later\u2014form hydrogen atoms. The hydrogen atoms appeared as an intergalactic fog, and with no light from stars and galaxies, the universe was dark. This period is known as the cosmic dark ages.\n<\/p>\n
The arrival of the first generations of stars and galaxies several hundred million years after the Big Bang bathed the universe in extremely hot UV light, which burned\u2014or ionized\u2014the hydrogen fog. This process yielded the transparent, complex and beautiful universe we see today.\n<\/p>\n
Astronomers like me call the first billion years of the universe\u2014when this hydrogen fog was burning away\u2014the epoch of reionization. To fully understand this time period, we study when the first stars and galaxies formed, what their main properties were and whether they were able to produce enough UV light to burn through all the hydrogen.\n<\/p>\n
The search for faint galaxies in the early universe<\/b><\/p>\n