Hannah Devlin “Cosmic dawn: astronomers detect signals from first stars in the universe” https://www.theguardian.com/science/2018/feb/28/cosmic-dawn-astronomers-detect-signals-from-first-stars-in-the-universe
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*3, 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 second Nature paper*5, Rennan Barkana*6, a professor of astrophysics at Tel Aviv University, proposes a potentially groundbreaking explanation: 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.”
*2:Judd D. Bowman, Alan E. E. Rogers, Raul A. Monsalve, Thomas J. Mozdzen & Nivedita Mahesh “An absorption profile centred at 78 megahertz in the sky-averaged spectrum” https://www.nature.com/articles/nature25792
*5:Rennan Barkana “Possible interaction between baryons and dark-matter particles revealed by the first stars” https://www.nature.com/articles/nature25791 See also “Dark matter’s nature is illuminated by the earliest stars” https://www.nature.com/articles/d41586-018-02536-7