Astronomers have uncovered evidence suggesting that the Universe was warm before the emergence of the first stars and galaxies. A team from the Curtin University and the International Centre of Radio Astronomy Research (ICRAR) conducted this groundbreaking research using the Murchison Widefield Array (MWA) telescope located in Western Australia. Their work sheds new light on the elusive ‘Epoch of Reionisation,’ a critical phase in the Universe’s history.
The research was conducted in two phases. During the initial phase, the team, led by Dr. Ridhima Nunhokee, gathered their first evidence of gas heating up between galaxies nearly 800 million years after the Big Bang. This period, known as the Epoch of Reionisation, signifies the end of the Cosmic Dark Ages, approximately a billion years after the Big Bang. It marks the transition of gas between galaxies from opaque to transparent, enabling light from the first stars and galaxies to travel across the Universe.
To investigate this early period, astronomers faced the challenge of isolating faint signals from the Epoch of Reionisation amidst a backdrop of radio emissions. “We must identify and remove every other source of radio waves in the Universe,” Dr. Nunhokee explained. This includes emissions from nearby stars and galaxies, interference from the Earth’s atmosphere, and even noise generated by the telescope itself. The team developed innovative methods to manage this foreground contamination, allowing for a clearer signal.
The integration of nearly ten years of MWA data significantly enhanced the team’s ability to observe the sky. This extensive dataset proved vital in their discovery, as a cold Universe would have produced detectable signals that were not observed. The absence of such signals indicates that the Universe must have been ‘pre-heated’ prior to reionisation.
In the second phase of the research, Professor Cathryn Trott, who leads the Epoch of Reionisation project at ICRAR, contributed further insights. “As the Universe evolved, the gas between galaxies expands and cools. We would expect it to be very cold,” she said. “Our measurements show that it is at least heated by a certain amount. Not by a lot, but it tells us that very cold reionisation is ruled out. That’s really interesting.” The findings suggest that this heating could be driven by energy from early X-ray sources originating from black holes and stellar remnants.
The implications of their findings extend beyond this research. The techniques developed for data processing will assist in the ongoing search for the Epoch of Reionisation using the upcoming Square Kilometre Array (SKA) telescopes, currently under construction in Western Australia and South Africa. “All these existing techniques will help us find what’s missing,” Dr. Nunhokee stated. “The signal is definitely buried in there. It’s just improving on our data, and getting more data, cleaner data, to reach it.”
The initial research paper, titled “Limits on the 21cm power spectrum from MWA observations,” was published in The Astrophysical Journal on August 8, 2025. The second phase paper, “Improved limits with the MWA using Gaussian information,” was published shortly thereafter, marking a significant milestone in the understanding of the Universe’s early history.
