The so-called green pea galaxies are believed to have reheated the early universe about a billion years after the Big Bang. According to a new study published in the journal Nature, the universe was so hot and dense after the Big Bang that matter was ionized. But approximately 380,000 years later, it began cooling as the universe expanded. Matter became neutral and first structures such as gas clouds of hydrogen and helium began to form.
Researchers solve the mystery of cosmic re-ionization
After some time, matter condensed even more to form the first stars and galaxies. About a billion years after the Big Bang, another major transformation occurred that had perplexed scientists until now. The universe suddenly reheated and its most abundant element hydrogen became ionized for a second time in an event that astronomers call “cosmic re-ionization.”
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Now scientists at the University of Geneva have found the answer to the mystery of reheating of the early universe. Using data from an ultraviolet spectrometer aboard the Hubble Space Telescope, astronomers discovered a nearby dwarf galaxy that was sending large amounts of ionizing photons into the intergalactic space.
These galaxies were first discovered in 2007. They are called green pea galaxies because they are round and compact, and appear green to sensors on the Sloan Digital Sky Survey. Since 2013, researchers have suspected that these galaxies host stellar explosions and winds that are powerful enough to eject ionizing photons into outer space. Ionizing a hydrogen atom is not very difficult. But there is so much dust and debris in most galaxies that their radiation doesn’t make its way into intergalactic space.
Green pea galaxies actively ejecting photons
To figure out how early galaxies emitted sufficient energy to reheat the entire universe, scientists examined data from the Sloan Digital Sky Survey and identified about 5,000 green pea galaxies. Then they selected only five galaxies for further observation. They found that one of the green pea galaxies was actively ejecting photons capable of ionizing hydrogen.
It means when the universe cooled and was about one billion years old, the violent star explosions and galactic winds in the green pea galaxies started ejecting massive amounts of energy into intergalactic space, reheating the whole universe.