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Evidence for neutral hydrogen gas in gamma-ray burst 1 billion years after Big Bang

Universe before “reionization” now within reach of observation


Graduate School of Science / Faculty of Science

The most abundant element in the universe is hydrogen. When the universe was born 14 billion years ago, ionized hydrogen atoms existed separately as nuclei and electrons. Around 400 thousand years after the Big Bang, neutral hydrogen gas was formed when electrons and nuclei combined as a result of cooling. However, it is known that hydrogen in the present universe is ionized again. It is believed that this “cosmic reionization” occurred around 1 billion years after the birth of the universe as a result of the formation of the first generation of stars and galaxies, but it is observationally still highly uncertain. Astronomical observations have been made to detect neutral hydrogen gas before reionization, but no strong evidence has been found yet.

© Tomonori Totani.
Optical spectrum of a gamma-ray burst that occurred about 1 billion years after the Big Bang. This time, the amount of neutral hydrogen around this gamma-ray burst was accurately measured by using the “damping wing feature” seen in 8400-8500 A (1 Angstrom = 0.1 nanometer).

In this latest study, Professor Tomonori Totani at the University of Tokyo’s Graduate School of Science and researchers at the National Astronomical Observatory of Japan, Tokyo Institute of Technology and other institutes analyzed a very high quality optical spectrum of a gamma-ray burst (a sudden, bright burst of gamma rays lasting from a few seconds to several tens of seconds) that occurred about 1 billion years after the Big Bang, and found for the first time a signature of optical light absorption by neutral intergalactic hydrogen gas around the host galaxy of the burst. This observation made with the Subaru telescope has the least uncertainty yet and was obtained by a straightforward methodology.

This research suggests that humankind’s observation of the distant universe has now reached the epoch before reionization.

Press release (Japanese)


Totani, Tomonori; Aoki, Kentaro; Hattori, Takashi; Kosugi, George; Niino, Yuu; Hashimoto, Tetsuya; Kawai, Nobuyuki; Ohta, Kouji; Sakamoto, Takanori; Yamada, Toru,
“Probing Intergalactic Neutral Hydrogen by the Lyman Alpha Red Damping Wing of Gamma-Ray Burst 130606A Afterglow Spectrum at z = 5.913”,
Publications of Astronomical Society of Japan,
Article link


Graduate School of Science

Department of Astronomy, Graduate School of Science

Dr. Tomonori Totani’s website , Department of Astronomy, Graduate School of Science

Subaru Telescope

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