Only a few hundred million years after the Big Bang, the infant Universe was brimming with opaque hydrogen gas that trapped light at some wavelengths from stars and galaxies. Over the first billion years, the gas became fully transparent — allowing the light to travel freely. Astrophysicists have long sought definitive evidence to explain this flip. The new data from the NASA/ESA/CSA James Webb Space Telescope recently pinpointed the answer using a set of galaxies that existed when the Universe was only 900 million years old. Stars in these galaxies emitted enough light to ionize and heat the gas around them, forming huge, transparent bubbles; eventually, those bubbles met and merged, leading to today’s clear and expansive views.
“Not only does Webb clearly show that these transparent regions are found around galaxies, we’ve also measured how large they are,” said Nagoya University astronomer Daichi Kashino.
“With Webb’s data, we are seeing galaxies reionize the gas around them.”
“These regions of transparent gas are gigantic compared to the galaxies — imagine a hot air balloon with a pea suspended inside.”
“The Webb data show that these relatively tiny galaxies drove reionization, clearing massive regions of space around them.”
“Over the next hundred million years, these transparent bubbles continued to grow larger and larger, eventually merging and causing the entire Universe to become transparent.”
Kashino and colleagues targeted a time just before the end of the Epoch of Reionization, when the Universe was not quite clear and not quite opaque — it contained a patchwork of gas in various states.
They focused Webb on the quasar SDSS J0100+2802, an extremely luminous active supermassive black hole that acts like an enormous flashlight, highlighting the gas between the quasar and the telescope.
As the quasar’s light traveled toward us through different patches of gas, it was either absorbed by gas that was opaque or moved…
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