Observations from the James Webb Space Telescope are causing confusion about the timeline of the universe. The reionization period, which according to current theories ended about a billion years after the Big Bang, may have ended much earlier.
The reionization period was an important phase in the history of the universe. During this time, the first stars and galaxies began to radiate, causing a great change in their environment. The radiation changed practically all the hydrogen in the universe.
According to current theories, based on previous measurements, this change was completed about a billion years after the Big Bang. But new measurements from the James Webb Space Telescope tell a different story. These indicate that the reionization period ended at least 350 million years earlier than expected. These findings appeared in a professional journal Monthly Bulletins of the Royal Astronomical Society: Letters.
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Cold and neutral
For the first 380 thousand years after the Big Bang, the universe consisted of a hot plasma of positively charged nuclear particles: protons and negatively charged electrons. The universe expanded and the plasma cooled, causing protons and electrons to combine to form uncharged hydrogen atoms. About 100 million years after the Big Bang, the first stars and galaxies formed from this neutral hydrogen atomic gas.
The first stars emitted intense light and UV radiation. The UV radiation was so strong that it could split the neutral hydrogen atoms in the environment back into protons and electrons. This process is called ionization – and the phase in which this happened is called the reionization period.
This was happening everywhere, with stars all over the universe, and it had a huge impact. “About 75 percent of all matter in the universe is hydrogen,” says astronomer Julian Muñoz of the University of Texas at Austin in the United States. All that hydrogen gas went from cold and neutral to hot and electrically charged. It affected how fast and where new stars could form. “As a result, reioniasis played an important role in creating the cosmic web we now see around us.”
The end of reionization
When almost all the hydrogen gas in the universe has been ionized, the reionization period ends. But when exactly this happened, astronomers can only guess indirectly from observations.
For example, astronomers look at the cosmic microwave background radiation that was emitted right after the Big Bang. This radiation traveled through the universe during the reionization period, colliding along the way with free electrons that had been dislodged during reionization. These collisions affect the cosmic microwave background radiation, which can still be measured today.
“So the background radiation gives us a reasonable idea of how many free electrons there were in the universe at a given time compared to neutral hydrogen atoms,” says Muñoz. “If you put this data into our theoretical models, it shows that reionization ended about a billion years after the Big Bang.”
Observations of other radiation from the early universe, looking for fingerprints of neutral hydrogen or free electrons, also show that this period ended about a billion years after the Big Bang.
The Webb Telescope
The Webb telescope looks at the reionization period in a different way. Measurements from this telescope tell astronomers how much ionizing UV radiation was emitted by the first galaxies. This shows that there were more galaxies at that time, which collectively emitted more ionizing radiation than previously thought. There was so much radiation that reionization should have been complete 550-650 million years after the Big Bang.
But this is not consistent with, among other things, measurements of the cosmic microwave background radiation. So something strange happens. Maybe the measurements – for example with the Webb telescope – go wrong, or the models used by astronomers in their calculations are wrong. Or the measured radiation does not reach the hydrogen gas. “There’s a subtlety to it,” says Muñoz. “Not all of the radiation escapes the galaxy. Only a fraction does, and even the mighty Webb Telescope cannot determine how large that fraction is.
Astronomers must solve this puzzle before they can adjust the universe’s timeline. Muñoz: “The Web telescope sheds new light on reionization, and we should use it to better understand this exciting period in the cosmos.”