When did the universe "wake up"?

 According to foreign media reports, the awakening of the first stars is undoubtedly an important moment for our universe. But this moment has always seemed extremely mysterious to scientists.

　　 But in a new study, a team of astronomers successfully discovered some of the oldest galaxies ever seen. Scientists point out that these celestial bodies were fully formed when the universe was only 680 million years old. They also found some evidence that these galaxies have emitted a large amount of extremely strong ultraviolet radiation to the surroundings.

　　 This turbulent wave of ultraviolet radiation has formed many huge bubbles in the universe, in which neutral gas is excited and ionized, presenting astronomers the first intuitive image of this period of major change in the universe.

  Before Dawn

　　 A long, long time ago, there was no single star shining in the universe. In the early days of the formation of the universe, all matter was very uniform, with similar densities everywhere, which was quite boring.

　　 The universe at that time was also very "neutral", which is quite different from when the universe was just formed. In the hundreds of thousands of years after the Big Bang, the universe was extremely hot and dense, filled with plasma everywhere. Atoms are also separated into electrons and nuclei under the huge pull.

　　 But when the universe was 380,000 years old, this chaos finally came to an end. The universe expanded enough and the temperature dropped low enough that electrons finally combined with the nucleus to form the first hydrogen and helium atoms. In the course of this series of events, a huge amount of radiation was also released, which is what we now call the "cosmic microwave background".

　　 For millions of years after this, the universe has always maintained this quiet and neutral state. But as the universe further expanded and the temperature further decreased, some small "seeds" began to form gradually, that is, some accidentally formed gas clumps with a density slightly greater than the surrounding ones. This allows them to have a greater gravitational force than their surroundings, which can attract surrounding matter. As their masses gradually increase, their gravitational pull becomes larger and larger, attracting more and more substances. As time passed, the quiet, dark, and neutral universe gave birth to the first stars and galaxies.

　　 dawn comes

　　 We do not know exactly when the first stars formed, but the scene when they formed must be very grand and spectacular. Because the universe at this time is no longer neutral, but ionized.

　　 Most of the substances we come into contact with are made up of complete atoms, and the nucleus is properly surrounded by outer electrons. These electrons fly back and forth in the outer layer of the atom and combine with other electrons to produce what we call a chemical reaction.

　　 But this situation is actually very unique. So far, most of the matter in the universe is plasma, just as long, long ago, electrons and atomic nuclei were "separated". The sun is made of plasma, other stars are made of plasma, nebulae are also made of plasma, and the matter between stars and nebulae is also plasma.

　　 When the universe was formed 380,000 years, the plasma in it was once transformed into neutral gas. But today, 13 billion years later, most of the matter in the universe has become plasma again. During this period, something special must have happened, tearing apart the atoms in the universe. Since there are all plasmas in our observable universe, this has been the case until the first universes and galaxies that appeared in the universe, indicating that the event that caused this "reionization" must have occurred a long time ago.

　　Astronomers believe that perhaps the large amount of ultraviolet radiation emitted by the first stars (and supernova explosions when they died) turned the entire universe back into a plasma state. But we do not know the exact time of the incident. Even the most powerful telescopes and deep space exploration missions are temporarily unable to see the situation so long ago. We can clearly see the cosmic microwave background, as well as the universe today, but what happens between these two is still an unsolved mystery. In other words, we neither know when the first stars appeared (the so-called "cosmic dawn" by astronomers), nor when the "reionization period" after this began.

  "blowing bubbles"

　　 But this situation has changed. In order to understand this critical "adolescence" in the growth and evolution of the universe, we are looking for more and more ancient galaxies, and we are also investigating the gas around them. Just recently, an international team of researchers has just discovered three galaxies with extremely weak light, small bodies, and extremely far away from us.

　　 These galaxies were fully formed when the universe was only 680 million years old. This is nothing surprising, after all, we have discovered such an ancient galaxy before. But in this study, the researchers analyzed the radiation in the surrounding environment of these three galaxies, and found that these galaxies have begun to blow ionized plasma "bubbles" into the surrounding universe. In other words, the radiation emitted by these galaxies has begun to transform the surrounding universe. This is the first time we have found a clear sign that the universe is in a period of reionization. Although astronomers had previously speculated that reionization would end before the universe reached one billion years old, no one thought it would have started so early.

　　 These galaxies will be excellent observation objects for the upcoming James Webb telescope, because the telescope is designed to study this period in the history of the universe. If the observations are in line with scientists' guesses and more examples of reionization can be found, we may finally be able to understand this ancient and turbulent past of the universe.

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