I used to wonder: what if there was a light switch that controlled the light of the Universe? One flick, and all the stars would go dim and space would be pitch-black? As I later came to know, in fact, that is how our Universe was at 380,000 years, before the creation of the first stars. The real question is what caused them to ignite.
Back to the beginning
To understand the reionization of the Universe, we have to go back to moments after the Big Bang. In a fraction of a second, protons and electrons were in a dense soup pervading the cosmos, preventing photons from moving very far before they bumped into a particle. At this time, the Universe was opaque. After three minutes, hydrogen, began to form as the Universe cooled and expanded rapidly. Over the next 380,000 years, the Universe continued to cool and hydrogen and helium ions attracted electrons, forming neutral atoms in an event known as Recombination. Now, photons could move freely and the Universe became transparent. This was the start of the Dark Ages as there were no light sources except for the Cosmic Microwave Background Radiation from the aftermath of the Big Bang.
About 100-200 million years later, stars made of only hydrogen and helium gas began to appear. They came into being from clouds of hydrogen and helium gas collapsing under gravity, the trigger for star formation. Gravity and dark matter drew more matter to the centre of the cloud and the outer matter heated up to 50,000 degrees Celsius. The lump held together by gravity then collapsed further to create a massive, bright star.
These newly created stars are not our everyday stars; they were massive gas-guzzlers that shone not visible light, but ultraviolet light. When the ultraviolet radiation from the first stars interacted with the surrounding neutral hydrogen and helium gas, it created a bubble of ionized gas. The radiation created an abundance of free electrons and ionized the hydrogen gas in the bubble. Over time, each star created its own bubble, the bubbles merged, and they eventually cleared away the fog pervading the universe, ending the Dark Ages. Photons from stars ripped the hydrogen atoms apart into protons and electron so photons could travel freely, and the first visible light shone. It took roughly a billion years for Reionization to complete.
Understanding how the first stars worked and what triggered them is vital to our understanding of how the universe was reionized. Scientists can find out if a star or galaxy was born during this era by looking for glowing gas. Its appearance signifies that it had once helped to reionize the neutral hydrogen gas. Without the first stars, the universe would still be pitch-black. However, many state that the work of the first stars’ ultraviolet radiation is not enough to reionize the whole Universe. Scientists are now looking to galaxies and other alternatives to this theory. With ongoing research, scientists may be able to find out what truly triggered the reionization of the universe.