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About 300,000 years after the Big Bang, the Dark Ages began. In those long days, time was almost frozen, there were no stars, no galaxies, and the entire universe was filled with neutral hydrogen.

Where did the first light that once again illuminate the universe come from? When did the universe start to get brighter?

Professor Wang Junxian of the University of Science and Technology of China, Zheng Zhenya, a researcher at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, and their collaborators recently observed a sample of a galaxy from the early universe (about 800 million years after the Big Bang, about 6% of the current age of the universe), and found that the ionization of hydrogen in the interstellar medium in the universe at that time was about 50%. Just like the darkness before dawn, at the end of this dark age, the first generation of stars and galaxies in the universe began to form, and the ultraviolet radiation they emitted ionized the neutral hydrogen around them, making the whole universe begin to brighten up little by little.

Recently, this breakthrough was published in the world's leading astrophysics journal Astrophysics Letters. On July 11, local time, the National Optical Astronomy Observatory of the United States wrote an article on this research breakthrough with the title of "Distant Galaxies Unveiling the End of the Dark Age of the Universe".

An extremely challenging question

Let's take a look at the past and present lives of the universe.

About 13.8 billion years ago, our universe was formed in a Big Bang when temperatures reached over 1 billion degrees Celsius. Hydrogen and helium were produced at this time. The Big Bang determined the respective proportions of hydrogen and helium, with hydrogen accounting for about 90% of the total number (75% by mass).

Imagine that the whole universe was a hot porridge at that time, hydrogen was in an ionized state, and the universe was bright at that time. As the temperature drops, the universe gradually cools down, and the hydrogen element that was once in an ionized state becomes neutral hydrogen, which can absorb ultraviolet light in the universe, thus binding these photons and preventing them from reaching distant places freely. About 300,000 years after the Big Bang, the entire universe was plunged into darkness.

However, in this dark period of the universe, the structure of the universe began to gradually take shape under the influence of gravity: hydrogen formed the first generation of stars and galaxies, and the mass of the first generation of stars could be very large, equivalent to hundreds of suns. When these stars fuse, they produce a large number of ultraviolet photons, producing many ionized bubbles that resemble bubbles. As the ionization accelerates, at a certain special stage, the interstellar medium of the entire universe becomes an ionization environment again, thus ending the dark ages of the universe.

This process is called "reionization". While astronomers know that it occurred between 300 million and 1 billion years after the Big Bang and that the first galaxies of the universe played a significant role in it, determining the detailed process of reionization and when the first galaxies formed has been a challenging issue at the frontier of astrophysics.

After 800 million years in the universe, the "fog" began to dissipate

Suppose that a photon at the time of the Big Bang has been running non-stop for years, and scientists can get a glimpse of what the early universe looked like if they can decipher the information it carries. The farthest signal we can now see in the universe is the cosmic microwave background radiation. In the early 60s of the 20 th century, two scientists in the United States established a high-sensitivity horn-type receiving antenna system in order to improve satellite communications. To reduce the noise, they even removed the bird droppings from the antenna, but there was still background noise that could not be eliminated. This is caused by the cosmic microwave background radiation, a discovery that earned them the 1978 Nobel Prize in Physics.

Although cosmic microwave background radiation is an important method for studying the reionization period of the universe, this method has limitations, and it is generally combined with the study of galaxies in the early universe, such as the study of quasars, gamma bursts, and star-forming galaxies at that particular time, to obtain the evolutionary history of reionization. However, the number of quasars in the early universe is very small, and the early cosmic gamma explosion is difficult to capture, so the star-forming galaxies of the early universe are now a hot spot for studying cosmic reionization. The Lyman alpha photons emitted by these early celestial bodies have been a key means for scientists to detect cosmic reionization, because these emitted line photons are scattered by neutral hydrogen atoms scattered between the universe. If the neutral hydrogen environment of the universe as a whole is like a fog, these Lyman Alpha galaxies in the early universe are like car lights in a fog, obscured a little. Once the surroundings begin to ionize, the fog will gradually weaken, and when the hydrogen is completely ionized, the fog will disappear.

"Lyman Alpha Galaxy in the Cosmic Reionization Period" (LAGER) is an international research project initiated and organized by Professor Wang Junxian of the University of Science and Technology of China, with the participation of astronomers from China, the United States and Chile.

Due to the challenges of observation, similar international searches for this universe age and the more distant Lyman Alpha galaxy have detected 23 Lyman Alpha emission line galaxies at the age of 800 million years in the first target region. This batch is also the largest sample of galaxies ever obtained at that cosmic age. The analysis found that the number of Lyman Alpha galaxies at the age of 1 billion years is about four times that of the universe at 800 million years. "This suggests that the process of reionization of the universe began much earlier, was still incomplete at 800 million years of the age of the universe, was about half ionized and half electrically neutral, and was inhomogeneous." Researcher Zheng Zhenya, the first author of the paper, said.

This result means that the universe is less than 6% of its current age, and this "fog" has begun to dissipate (50% ionized); A large proportion of the first galaxies of the early universe were formed 800 million years ago.