The gas between galaxies was already heated 800 million years after the Big Bang, according to astronomers who have spent a decade hunting signals from the Universe’s infancy. The finding rules out certain models of how the cosmos evolved and suggests early black holes and stellar remnants spread energy through space earlier than expected.
A team from the International Centre for Radio Astronomy Research used the Murchison Widefield Array telescope in Western Australia to search for evidence of the Epoch of Reionisation, when neutral hydrogen gas shifted from opaque to transparent. Rather than finding the signal itself, they discovered what it is not. A cold Universe would have produced a detectable signature in their data. Its absence means the intergalactic medium must have been pre-heated before reionisation occurred.
“Our research was conducted over two phases. During the initial research, we obtained our first evidence of heating of the Intergalactic Medium, the gas between galaxies, 800 million years after the Big Bang.”
Dr. Ridhima Nunhokee, who led the initial phase, explained that isolating this ancient signal requires removing every other radio source in the Universe from observations. That includes emissions from nearby stars and galaxies, interference from Earth’s atmosphere, and noise from the telescope itself. Only after subtracting these foreground signals will the remaining data reveal information from the Epoch of Reionisation.
Ten Years of Sky Cleaning
The researchers integrated about ten years of telescope data, observing the sky longer than ever before. They developed new methods to handle contamination and better understand their instrument, producing what they describe as the cleanest radio signal yet from the array. The quality and quantity of this dataset made the discovery possible.
Professor Cathryn Trott, who leads the project and authored the second research phase, noted the counterintuitive nature of the finding. As the Universe evolved, the gas between galaxies should expand and cool dramatically.
“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.”
Hunting Cosmic Dawn
The Epoch of Reionisation marks the end of the Cosmic Dark Ages, roughly a billion years after the Big Bang. During this period, light from the first stars and galaxies began traveling freely throughout space. Astronomers predict this transition based on theory, but radio telescopes have yet to directly detect it.
The team suspects X-rays from early black holes and stellar remnants drove the heating. These energetic sources would have spread through the Universe, warming the gas before the first galaxies ionized it. The research suggests reionisation began with the intergalactic medium already above its coldest possible temperature.
The techniques developed for processing these data will accelerate the search when the Square Kilometre Array telescopes come online. Those instruments, currently under construction in Western Australia and South Africa, will offer unprecedented sensitivity to signals from the early Universe.
The signal remains buried in the data, according to the researchers. Finding it requires cleaner observations and more integration time. After a decade of work removing contamination layer by layer, they have narrowed the possibilities and set the stage for future detection.
The Astrophysical Journal: 10.3847/1538-4357/adff80
The Astrophysical Journal: 10.3847/1538-4357/adda45
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