A multinational team of astronomers, led by a Mumbai-based scientist, has solved a long-standing cosmic puzzle: How many years after the Big Bang did the universe achieve conditions that determined and eventually led to the universe we see around us today? The seven-member team, including 35-year-old theoretical physicist Girish Kulkarni from the Tata Institute of Fundamental Research (TIFR), Mumbai, found that the universe finished heating up 12.7 billion years ago by a process called reionisation that occurred due to light from young stars formed in the first galaxies. That’s 1.1 billion years after Big Bang. The paper was published in the April issue of the Monthly Notices of the Royal Astronomical Society, London. “Reionisation led to the universe we see around us,” said Dr Kulkarni, the paper’s lead author. “Without reionisation, the universe would be dark and cold with temperatures close to absolute zero (–273.15 degrees Celsius). The universe would not be hot as is today. There would be no galaxies or solar systems, and humans would not exist.”
Previous studies had suggested that reionisation occurred much earlier, within one billion years of the Big Bang, the accepted theoretical birth of our universe 13.8 billion years ago.
The ‘reionisation epoch’ is the period of time when ultraviolet light from the first galaxies ionised the gas in deep space, transforming the universe from a neutral to an ionised state in which it remains today. Simply put, ionisation is when an atom or a molecule acquires a negative or positive charge by gaining or losing electrons. An atom thus formed is called an ion. This takes places in combination with other chemical changes. In a neutral state, an atom will have an equal number of protons (positively-charged sub-atomic particles) and negatively-charged electrons.
Researchers said understanding the thermal evolution of the universe since the Big Bang fills a gap in our understanding of the universe’s history. The study findings will also aid future experiments such as the 10-nation Square Kilometre Array (SKA) of which India is a member and which aims to detect neutral hydrogen from the early universe to uncover as-yet-unseen epochs of cosmic evolution.
“Late reionisation is also good news for future experiments that aim to detect the neutral hydrogen which is important to uncover cosmic evolution from the early universe,” Kulkarni said. “The later the reionisation, the easier it will be for experiments such as SKA to succeed.”
Knowing the accurate time of reionisation is important, said researchers, to better understand the formation and characteristics of the first galaxies.
“The findings are very significant,” said George Becker, professor at the department of physics and astronomy, University of California, Riverside, who was not involved in the study and whose research interests include cosmic reionisation. “The authors have shown, for the first time, that reionisation, one of the most significant events in the universe, may have lasted far longer than previously thought. When reionisation occurred carries implications for the properties of the first galaxies.”
Fifty million years after the Big Bang, the universe – mostly made of gas at temperatures a few degrees above absolute zero – was dark, and devoid of bright stars and galaxies. During reionisation, the universe transitioned out of these cosmic dark ages and is today filled with hot and ionised hydrogen gas at a temperature of around 10,000 degrees Celsius.
Hydrogen gas dims light from distant galaxies much like streetlights are dimmed by fog on a winter morning. By observing this dimming of a special type of bright galaxies called quasars, astronomers can study conditions in the early universe.
In the last few years, observations of this specific dimming pattern suggested that this fogginess of the universe varies significantly from one part of the universe to another, but the reason behind these variations was unknown.