Australian radio telescope narrows in on the elusive signal of the universe’s first stars

In the arid desert of Western Australia, near Perth, astronomers are on the hunt for evidence of the universe’s first-ever stars.

To do this, they are seeking out the signal of the neutral hydrogen that pervaded the early universe some 13 billion years ago, before the so-called ‘cosmic dawn’. 

Physicists expect this signal to decrease and eventually disappear as the first stars, galaxies and quasars formed and gradually ionised the hydrogen around them.

However, picking up this signal is like trying to zero in on a whispered conversation across a crowded, noisy room, meaning there is a lot of interference to isolate first.

Researchers from Perth’s Curtin University have now managed to reduce the noise from the most intense of these ‘foreground’ signals by a factor of three in models.

This is a big step in the hunt for the earliest stars — the detection of which would shine light on this key period when the cosmos evolved its large-scale structures.

To do this, they are seeking out the signal of the neutral hydrogen that pervaded the early universe some 12.8 billion years ago, before the so-called ‘ cosmic dawn’. Pictured: the evolution of the universe over time. The first stars formed around 400 million years after the big bang, bringing an end to the ‘cosmic dark’ ages

Physicists expect this signal to decrease and eventually disappear as the first stars, galaxies and quasars formed and gradually ionised the hydrogen around them. Pictured: a model of the formation of the first stars/galaxies (white) ionising the neutral hydrogen (red) around them for a cube-shaped part of the universe some 325 million light years across

The Universe began around 13.8 billion years ago. A millionth of a second later, the cosmos had cooled enough such that it became a soup of both protons (aka hydrogen ions) and electrons — a situation that lasted for some 380,000 years.

This period ended in what physicists bizarrely called ‘recombination’ — the tipping point at which the universe had expanded and cooled enough for protons and neutrons to combine for the first time, forming neutral hydrogen.

It would be several hundred million years, however, before denser pockets of this hydrogen finally collapsed in on themselves to ignite the first stars.

Astronomers refer to this phase in the universe’s history — during most of which there were no light sources other than the cosmic background radiation left over from the Big Bang — as the ‘Cosmic Dark Ages’.

The Dark Ages didn’t quite end, however, with the birth of the earliest stars. This is because, initially, most of the visible and infrared light they released was absorbed by the ‘fog’ of neutral hydrogen gas that still permeated the universe.

For this reason, light from these very first stars never managed to travel out into the cosmos — meaning that we can’t see them directly without our telescopes.

As the stars shone on, however, their emission of intense ultraviolet light acted to strip electrons from the ‘fog’, turning the hydrogen back into ions and allowing more light to spill out further into the universe — beginning the ‘Epoch of Reionization’.

Physicists believe that the process of reionization made the universe a bit like Swiss cheese — with spherical pockets of ionised gas forming in the neutral hydrogen and expanding out until there was no neutral gas left.

To study the birth of the first stars and learn how the universe changed during the Epoch of Reionization, astronomers have to come at the problem sideways — and instead try to look for the neutral hydrogen gas that once surrounded these bodies.

When they collide, neutral hydrogen atoms can, as a result of a weird quantum mechanics effect, sometimes radiate out energy.

This emission comes in the form of low-energy photos, with an initial wavelength of 21 centimetres (8.3 inches) — although it doesn’t remain this way.

‘The Epoch of Reionisation signal started life as a hydrogen atom radio wavelength of 21 centimetres,’ explained paper author and astronomer Christene Lynch of the Curtin University, in Perth, Australia. 

As a result of travelling across an expanding universe ‘over the intervening billions of years, it has been stretched and grown very, very faint,’ she added.

Nevertheless, detecting the signal — and establishing how it changed over time — would allow researchers to pinpoint when and how reionization took place and, by extension, where the earliest stars appeared.

As part of efforts to pick out the faint reionisation signal from all this background noise, Dr Lynch have been collecting data using the Murchison Widefield Array, a low-frequency radio telescope in the outback around 500 miles north of Perth. Pictured: 16 of the array’s antennas

‘Finding the weak signal of this first light will help us understand how the early stars and galaxies formed,’ said Dr Lynch.

‘Our challenge is that the Universe is very, very crowded. There are too many other radio sources that are much brighter than the Epoch of Reionisation signal lying between it and us.

‘It is like trying to hear someone whispering from across the room when between you and that person there are thousands of other people shouting as loudly as possible,’ she explained.

As part of efforts to pick out the faint reionisation signal from all this background noise, Dr Lynch have been collecting data using the Murchison Widefield Array, a low-frequency radio telescope in the outback around 500 miles north of Perth. 

The array — which is some three miles across — is comprised of thousands of spider-like antennas, each grouped together in regular grids known as ’tiles’.

Using a technique called interferometry, astronomers can combine the signals from multiple small antennas like these to simulate the signal that would be received by one large radio dish covering the same area — which would be impractical to build. 

In 2016, the number of individual tiles making up the array was doubled to 256, bringing the total number of antennas to 4,096 (although, at present, only half are ever used at one time), significantly increasing the power of the observatory.

In 2016, the number of individual tiles making up the Murchison Widefield Array (MWA) was doubled to 256, bringing the total number of antennas to 4,096 (although, at present, only half are ever used at one time), significantly increasing the power of the observatory. Pictured: a map of the central tiles in the array.

‘By using the new tiles and thus expanding the physical area over which the antenna work we were able to reduce a lot of that interference,’ said paper author Christene Lynch. Pictured: a map of the peripheral tiles in the array

The researchers’ experiment — the Long Baseline Epoch of Reionisation Survey (LoBES) — combined the use some of the original tiles with 56 of the new ones.

The team surveyed a whopping 80,824 sources of radio signals, taking 16 different spectral measurements for each one.

Entering their results into simulations, the team were able to show that they could reduce the intensity of the noisiest foreground radio signals by a factor of three. 

‘By using the new tiles and thus expanding the physical area over which the antenna work we were able to reduce a lot of that interference,’ said Dr Lynch. 

‘It’s clear that our new LoBES sky model will significantly improve efforts to properly locate [the reionisation signal]. 

‘As more and more of the tiles are added in, we’ll have a much better chance of finding the echo of that first light.’

‘Finding the weak signal of this first light will help us understand how the early stars and galaxies formed,’ said Dr Lynch, pictured here in the Murchison Widefield Array

‘This is our deepest and most detailed view to-date of the radio sky in these Epoch of Reionisation fields,’ said paper author and radio astronomer Cathryn Trott, also of Curtin university.

‘This new catalogue provides us with a cleaner path to locating the Epoch of Reionisation signal — a detection that will be a very major achievement for astronomy,’ she concluded.

The full findings of the study were published in the journal Publications of the Astronomical Society of Australia.

Pictured: the Murchison Widefield array is located in the outback, away from as many local sources of man-made radio interference as possible

THE BIG BANG THEORY DESCRIBES THE BEGINNING AND EVOLUTION OF THE UNIVERSE

The Big Bang Theory is a cosmological model, a theory used to describe the beginning and the evolution of our universe.

It says that the universe was in a very hot and dense state before it started to expand 13,7 billion years ago.

This theory is based on fundamental observations.

In 1920, Hubble observed that the distance between galaxies was increasing everywhere in the universe. 

The Big Bang Theory is a cosmological model, a theory used to describe the beginning and the evolution of our universe, based on observations – including the cosmic background radiation (pictured), which is a like a fossil of radiation emitted during the beginning of the universe, when it was hot and dense

This means that galaxies had to be closer to each other in the past.

In 1964, Wilson and Penzias discovered the cosmic background radiation, which is a like a fossil of radiation emitted during the beginning of the universe, when it was hot and dense. 

The cosmic background radiation is observable everywhere in the universe.

The composition of the universe – that is, the the number of atoms of different elements –  is consistent with the Big Bang Theory. 

So far, this theory is the only one that can explain why we observe an abundance of primordial elements in the universe.