Why is the universe transparent?

If we look out into space, what we will see are bright spots amidst the darkness of the universe, which could represent stars or even entire distant galaxies. However, what is transparent is not really the universe, but interstellar gas, and the James Webb Space Telescope has been able to explain why.

Immediately after the Big Bang, the gas in the universe was opaque, hot, and dense, and it had cooled over hundreds of millions of years. About a billion years after the beginning of the universe, a period known as the era of reionization, the gas became hot and ionized again and became transparent, possibly due to the appearance of the first stars in the primordial galaxies.

Until then it was not known what caused these constant changes, but now James Webb has shown that these transparent regions appear around relatively small galaxies like bubbles. Over millions of years, they grew up and joined together until they became gigantic and made the entire universe transparent.

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The era of reionization

To do this, a team of astronomers led by Simon Lilly located quasars in space just before the era of reionization, when there were clear and opaque regions in the universe. Because it is so luminous, the object acts like a flashlight, highlighting the gas between the quasar and telescopes.

During the light’s path to us, it passed through regions where it was absorbed by the gas and through other regions where it was moving freely. By combining data from Webb, telescopes at the W.M. Keck Observatory in Hawaii, the Very Large Telescope at the European Southern Observatory, and the Magellan Telescope at the Las Campanas Observatory in Chile, the researchers were able to determine the composition and state of the gas.

Subsequently, James Webb looked for primordial galaxies close to this path, showing that they are usually surrounded by transparent regions with a radius of up to two million years, hence the reason for reionization.

117 galaxies from that period have been found and will now be studied in five additional fields, all of which are fixed to the central quasar used in this study.

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