Signal from our universe’s first stars deepens dark matter mystery
A tiny signal, dating back to the birth of the first stars in our universe, has been detected by astronomers for the first time.
- Astronomers detected a miniscule radio signal that indirectly indicates the presence of the earliest stars
- The discovery was made using of a small antenna in a pristine, radio quiet area in Western Australia
- While the frequency of the signal was predicted, scientists were surprised by the strength of the signal
- The discovery has thrown up new mysteries for physicists around the properties of dark matter
They have picked up a radio signature produced just 180 million years after the Big Bang using a simple antenna in the West Australian outback.
The ground breaking discovery, reported today in the journal Nature, sheds light on a period of time known as the "cosmic dawn", when radiation from the first stars started to alter the primordial gas soup surrounding them.
It could also completely revolutionise our understanding about dark matter, the invisible structure that makes up the bulk of our universe today.
"The signal confirms our expectations for when stars show up in the universe," said the study's lead author Judd Bowman of Arizona State University.
"But it's also telling us that there's something mysterious happening at this time beyond our previous expectations", he said.
It is thought the first stars were massive, blue stars that lived fast and died young.
Even though they emitted a lot of ultraviolet light they are too faint for current telescopes such as the Hubble to directly observe.
But astronomers proposed the stars could be indirectly detected by dips in cosmic background radiation — the afterglow of the Big Bang 13.8 billion years ago.
These dips, which produce a distinct radio signature, are caused by hydrogen gas absorbing the background radiation.
Hunting for signals in the remote outback
Professor Bowman and colleagues have been hunting for a signal from the early universe for more than a decade through the EDGES project — short for Experiment to Detect the Global Epoch of Reionisation Signature.
"It's challenging because the total amount of radio waves we receive on Earth from outer space is dominated by all the noise our … galaxy makes."
The signal they've been looking for is a miniscule fraction — between 0.1 and 0.01 per cent — of the radio noise from the sky.
"It's like trying to hear a whisper from the other side of a roaring football stadium," Professor Bowman said.
The signal is also within the lower range of FM radio, so finding a place on Earth that is free of human radio interference was essential.
That's why Professor Bowman and colleagues decided to base their experiment at CSIRO's Murchison Radio-astronomy Observatory, 300 kilometres north-east of Geraldton.
"Going to Western Australia and working at the Murchison Radio-astronomy Observatory was an absolutely critical first step for us," he said.
There they built a small table-sized radio spectrometer with a radio receiver attached to two metal panels that act as an antenna. Akin to a set-up from the 60s or 70s, the EDGES instrument is much simpler in design than bigger array telescopes around the world.
EDGES ground-based radio spectrometer Murchison Radio-astronomy Observatory
Supplied: CSIRO Australia
After years of calibrating the detector, Professor Bowman and colleagues finally found what they were looking for.
They detected a signal with a frequency of 78 megahertz, which was in the range predicted for a star formation by 180 million years.
But, to their surprise, the signal was twice as strong as it should be. This indicated the hydrogen gas in the early universe was around -270 degrees C — much colder than expected.
Professor Bowman said there are two possible explanations for this.
"Either there were other unknown objects that were making more radio waves than predicted, or there are interactions between dark matter and atoms or some other yet unknown type of physics that's making its mark in that era," he said, adding that the dark matter hypothesis is the mostly likely scenario.
Teasing out the mysteries of dark matter
Separate research based on the signal, also published in Nature, supports the dark matter hypothesis.
Rennan Barkana of Tel Aviv University came to the conclusion that the extra cooling seen in the signal could only have been caused by the interaction of normal matter with something even colder.
"Back then [at the cosmic dawn] there were no stars or they were only starting to form so the gas [in the early universe] was very, very cold," Professor Barkana explained.
"The only candidate that we know of that can be even colder than this cold, early gas is dark matter."
We know that dark matter exists by its pull on galaxies, but we don't know anything about its properties or what it's made of.
A timeline of the universe
Supplied: N R Fuller/National Science Foundation
Professor Barkana said if he is correct, his model points to new ways of detecting dark matter and exploring its mysterious properties.
"This would be the first clue of some interaction of the dark matter that is not just gravity," he said.
In order for dark matter to collide with normal matter and take away some of the heat, it must also be a lighter particle than current models predict, he added.
"This is a hint we need to look for dark matter in different parameter spaces to what people anticipated," he said.
"We observed something in a new range of physics … in the history of the universe.
"Hopefully in a few years this will be really clearly tested. There will be a lot more observational information because this is just the first measurement of its kind."
Cosmic dawn signal key to studying the later universe
Astrophysicist Cathryn Trott of Curtin University said the research provided evidence we didn't expect.
"This signal itself is telling us something very fundamental about our universe and the physical properties within that first 200 million years," she said.
Dr Trott leads separate, but related, research that uses the Murchison Widefield Array (MWA) to explore the next period of the history of the universe, 500 million to 1 billion years after the Big Bang.
During this period, known as the Epoch of Reionisation, it's thought increasing radiation from growing numbers of stars and galaxies changed the hydrogen gas into a plasma, enabling astronomers to directly detect ultraviolet light.
Cosmic microwave background radiation is the fingerprint of the Big Bang
Supplied: NASA/WMAP Science Team
Dr Trott said her team will be looking closely at the signal.
"For the MWA here in Australia, this [discovery] is extremely exciting because this provides us with a snapshot very early in the universe from which we can predict what we should see at those later times," she said.
"We can shape our signal to see if this is something with the hypothesis that Rennan Barkana is putting forward."
She said the experiment conducted by the EDGES team was extremely challenging.
"The analysis they've done is very, very thorough, however as with all these scientific experiments there are other experiments that are underway and they will need to confirm this signal," Dr Trott said.
She said the remote environment of Murchison Radio-astronomy Observatory was essential for the discovery made by EDGES — and for follow-up work by her team with the MWA and the future Australian Square Kilometre Array Pathfinder telescope.
"That part of Western Australia is designated as a radio quiet zone by the Australian Government," Dr Trott said.
"And that's the reason EDGES needs to be there, because it needs to be absolutely pristine with no signals around it at all."
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