Astronomers have detected a signal from the first stars as they appeared and illuminated the universe, in observations that have been hailed as “revolutionary”.
The faint radio signals suggest the universe was lifted out of total darkness 180m years after the big bang in a momentous transition known as the cosmic dawn.
The faint imprint left by the glow of the earliest stars also appears to contain new and unexpected evidence about the existence and nature of dark matter which, if confirmed by independent observatories, would mark a second major breakthrough.
“Finding this minuscule signal has opened a new window on the early universe,” said Judd Bowman of Arizona State University, whose team set out to make the detection more than a decade ago. “It’s unlikely we’ll be able to see any earlier into the history of stars in our lifetime.”
Following the big bang, the universe initially existed as a cold, starless expanse of hydrogen gas awash with radiation, known as the Cosmic Microwave Background. This radiation still permeates all of space today and astronomers are beginning to scrutinise this cosmic backdrop for traces of events that occurred in the deep past.
During the next 100m years – a period known as the dark ages – gravity pulled slightly denser regions of gas into clumps and eventually some collapsed inwards to form the first stars, which were massive, blue and short-lived. As these stars lit up the surrounding gas, the hydrogen atoms were excited, causing them to start absorbing radiation from the Cosmic Microwave Background at a characteristic wavelength.
This led scientists to predict that the cosmic dawn must have left an imprint in the Cosmic Microwave Background radiation in the form of a dip in brightness at a specific point in the spectrum that ought, in theory, to still be perceptible today.
In practice, detecting this signal has proved hugely challenging, however, and has eluded astronomers for more than a decade. The dip is swamped by other, more local, sources of radio waves. And the expansion of the universe means the signal is “red-shifted” away from its original characteristic wavelength by an amount that depends on precisely when the first stars switched on. So scientists were also not sure exactly where in the spectrum they should be looking –and some predicted the task would prove impossible.
“The team have to pick up radio waves and then search for a signal that’s around 0.01% of the contaminating radio noise coming from our own galaxy,” said Andrew Pontzen, a cosmologist at University College London. “It’s needle-in-a-haystack territory.”
Remarkably, Bowman and colleagues appear to have overcome these odds using a small, crude-looking instrument the size of a small table. The Edges (Experiment to Detect Global EoR Signature) antenna sits in a remote region of Western Australia where there are few human sources of radio waves to interfere with incoming signals from the distant universe. The wavelength of the dip suggest that the cosmic dawn occurred about 180m years after the big bang, 13.6bn years ago and nine billion years before the birth of the sun.
The signal also indicated a second milestone at 250m years after the big bang, when the early stars died and black holes, supernovae and other objects they left behind heated up the the remaining free hydrogen with x-rays.
In a paper published in the journal Nature, Bowman and colleagues detail the elaborate experimental steps they took to prove the signal was real – several years of replications, changing the angle of the antenna, altering the setup.
“Telescopes cannot see far enough to directly image such ancient stars, but we’ve seen when they turned on in radio waves arriving from space,” said Bowman.
Emma Chapman, Royal Astronomical Society research fellow at Imperial College London, described the result as “an incredible achievement, constituting the first ever detection of the era of the first stars”. The huge significance of the result, she added, meant it needed to be replicated by an independent experiment.
The detection also contained a major surprise. The size of the dip was twice as big as predicted. This suggests the primordial hydrogen gas was absorbing more background radiation than predicted and would suggest the universe was twice as cold (about -270C) as thought.
One potentially groundbreaking explanation for this is that the hydrogen gas was losing heat to dark matter. Until now, the existence of dark matter – the elusive substance that is thought to make up 85% of the matter in the universe – has only been inferred indirectly from its gravitational effects. If confirmed, these results would suggest a new form of interaction between normal matter and dark matter, mediated by a fundamental force that until now has been entirely unknown.
The theory would also suggest that dark matter particles, the properties of which remain completely mysterious, must be light rather than heavy, which would rule out one of the leading hypothetical candidates for dark matter, known as weakly interacting massive particles – or wimps.
Lincoln Greenhill, a senior astronomer at Harvard University, said that if confirmed the dark matter observations could be revolutionary. “We know so little about it that there are many theories as to what dark matter is,” he said. “Many may shortly be eliminated from the running.”