Astronomers detect the first stars born after the Big Bang
Researchers discover an ancient signal that helps pinpoint the moment stars lit up the universe for the first time
Astronomers peering back in time have detected a faint radio signal from the very first stars, finally answering the question of when such celestial bodies burst into life. It would appear that the earliest stars began turning on their light some 180 million years after the Big Bang. If the findings regarding the timing of the so-called Cosmic Dawn are confirmed then it will have huge implications for our scientific understanding of the cosmos.
Scientists have long known that in the immediate aftermath of the Big Bang, the universe was cold, dark and featureless. It was filled with hydrogen and helium and there was much background radiation, known as Cosmic Microwave Background. But the question of how and when the universe transitioned from darkness to light has long troubled the best of minds. This is why a team led by Judd Bowman of Arizona State University sought to detect the earliest stars.
They based their work on the theory that gravity caused the densest regions of hydrogen gas to coalesce and form compact clouds in the wake of the universe's birth. Some of these eventually collapsed inwards, forming massive, blue, yet short-lived stars and, as they emitted their ultraviolet light into the dark areas that lay between them, the energy signature of the hydrogen atoms changed.
The atoms began to absorb radiation from the Cosmic Microwave Background at a frequency of 1.4 gigahertz, leaving an indelible mark. Understanding this led to a long-held idea that the absorption should be detectable – that it was possible to look for a dip in brightness of the background radiation. The problem is the radio waves have stretched because they have travelled for so long. Other signals also interfere.
Finding the right one was no mean feat, but Dr Bowman and his team made a breakthrough after 12 years of experimental effort. They used a table-sized ground-based radio spectrometer: Experiment to Detect the Global EoR Signature (EDGES). Based at the Murchison Radio-astronomy Observatory in Western Australia, where interference is low, the team says it was able to measure the average radio spectrum of all of the astronomical signals received across much of the Southern Hemisphere sky.
The eureka moment came after the team extended their search to lower frequencies in
2015. The instrument was then able to detect a tiny 0.1 per cent dip in the wavelength. “We see this dip most strongly at about 78 megahertz,” affirms Alan Rogers, co-author of the study. “And that frequency corresponds to roughly 180 million years after the Big Bang. In terms of a direct detection of a signal from the hydrogen gas itself, this has got to be the earliest.” If true – and the team spent two years checking that the finding was not caused by instrumental effect and noise – it means those early stars formed a staggering 13.6-billion-years ago.
Yet that is not the end of the team's findings. Since the size of the dip was twice as large as expected, the study also discovered that the universe prior to the formation of the first stars was far colder than astronomers had originally believed. It points to the universe at that stage being -270° Celsius (-454° Fahrenheit) – less than half the expected temperature. Rennan Barkana of Tel Aviv University says this points to the first evidence that dark matter, which he says is composed of low-mass particles, siphoned off energy from normal matter in the early universe. It means the hydrogen gas was losing heat to dark matter. “The first stars in the universe turned on the radio signal, while the dark matter collided with the ordinary matter and cooled it down,” Professor Barkana says.
This makes the discovery of the first stars even more important than initially imagined. “If Barkana’s idea is confirmed, then we've learned something new and fundamental about the mysterious dark matter that makes up 85 per cent of the matter in the universe, providing the first glimpse of physics beyond the standard model,” said Dr Bowman. Indeed, because it suggests that dark matter is interacting with hydrogen, it turns the theory that dark matter is made up of weakly interacting massive particles on its head.
As such, Dr Bowman is not about to stop there. “Now that we know this signal exists, we need to rapidly bring online new radio telescopes that will be able to mine the signal much more deeply,” he explains, referring to instruments such as the Hydrogen Epoch of Reionization Array (HERA) and the Owens Valley Long Wavelength Array (OVRO-LWA). The next step is to improve the performance of the instruments to learn more about those early stars. It is also crucial that the findings are independently confirmed.