Our Universe was ‘pre-heated’ in its early moments, according to a new study from the International Centre of Radio Astronomy Research (ICRAR), challenging assumptions it emerged from an ultracold state.
The discovery was made as astronomers hunted for an elusive signal from the Epoch of Reionization – the time when the first suns began to fire up about a billion years after the Big Bang.
Finding that the cosmos was at least a little warm around that time rules out models that suggest the ionization process occurred under extremely low temperature conditions.
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“As the Universe evolved, the gas between galaxies expands and cools, so we would expect it to be very, very cold,” says Cathryn Trott, radio astronomer and lead of the Epoch of Reionization project at ICRAR.
“Our measurements show that it is at least heated by a certain amount. Not by a lot, but it tells us that very cold reionization is ruled out. The research suggests this heating is likely driven by the energy from early sources of X-rays from early black holes and stellar remnants spreading through the Universe.”
Humanity’s so-called ‘Dark Ages’ were positively fluorescent compared to what the Universe went through in its youth. Before stars and galaxies formed, there were no major sources of light at all – just clouds of neutral hydrogen molecules hanging around, slowly coalescing over hundreds of thousands of years.
Eventually, pockets of gas collapsed into the first generation of stars, which burned with a greater intensity than those around today. This release of energy ionized the cosmic hydrogen, which is why it’s known as the Epoch of Reionization.
Freed of its electrons, the hydrogen making up the vast cosmic clouds became transparent to light, allowing us to now see far through space and time. On the other hand, it also means it’s hard to see what the Universe was like during or before this epoch.
Astronomers are forced to rely on a radio signal called the hydrogen line; a 21-centimeter-long wave of electromagnetic radiation predicted to pass right through clouds of material that scatter shorter waves of visible light, carrying with it vital information about the cosmic dark ages.
The ICRAR team has been using the Murchison Widefield Array (MWA) radio telescope in Western Australia to hunt for this signal. The problem is that the Universe is absolutely lousy with sources of radio signal.
“These include emissions from nearby stars and galaxies, interference from the Earth’s atmosphere, and even noise generated by the telescope itself. Only after carefully subtracting these ‘foreground signals’ will the remaining data reveal signals from the Epoch of Reionization,” says Ridhima Nunhokee, a radio astronomer at ICRAR.
“From this research, we have developed methods to deal with the foreground contamination, and subtract the signals we don’t want, but also better understand our telescope and come up with a clean signal. We’ve also been able to integrate about 10 years of MWA data together, to observe the sky for longer than we ever have before.”
While the current work still hasn’t revealed the long-sought hydrogen line, the amount of high-quality data not only brings the team closer than ever but also reveals some new tidbits of information. Case in point: the revelation that the early cosmos had been ‘pre-heated’ before the reionization took place.
As more radio telescopes join the search in the near future, it should only be a matter of time before the hydrogen line turns up.
“The signal is definitely buried in there. It’s just improving on our data, and getting more data, cleaner data, to reach it,” says Nunhokee.
The research was published in two papers in The Astrophysical Journal.