Astronomers are gearing up on the search for the oldest light in the universe – the first stars that came to life and shine after the Big Bang.
These early stars formed amid thick clouds of hydrogen that absorb starlight and hide it from the prying eyes of today’s astronomers. But when these newborn stars bombarded the surrounding hydrogen clouds with stellar radiation, the clouds gave off their own radiation in response, like a cosmic backfire.
When the stars triggered them, the clouds emitted radio waves that were originally about 21 centimeters long. By searching for these telltale radio waves (now stretched to several meters in length due to the time and distance they travel to Earth), astronomers are hoping to find clues to the newborn stars that have been warming the baby universe.
So far, however, no one has had much luck.
“It’s a bit like trying to read a landscape by looking at the shadows in the fog.” Eloy de Lera Acedo told Vice versa.
De Lera Acedo and his colleagues want to change that soon – thanks to statistics.
What’s new – de Lera Acedo and his colleagues recently used computer simulations to test their method (and a new telescope design) for detecting these radio signals. They released the results on Thursday, July 21 in the diary natural astronomy.
“We want to separate the tiny cosmological signal we’re looking for from the much brighter signals coming from other places like our own galaxy,” says de Lera Acedo.
Finding radio waves emanating from primordial clouds of hydrogen is, perhaps unsurprisingly, challenging. Part of the problem is that there is a whole universe between us and these first star nurseries, and the universe is full of them Things that also emit radio waves. Also, until they reach a radio antenna here on Earth, the radio waves from these ancient hydrogen clouds are very weak. They are so weak that they can be disrupted by radio interference from the observatory itself.
To separate the cosmological chaff from the much louder, de Lera Acedo and his colleagues use Bayesian statistics, a method of number crunching that relies on machine learning, to calculate the probability that a datum fits into a given category.
“We want to quantify the probability that something in our data is introduced by our telescope or comes from the sky, and what the relationships are between the different components,” explains de Lera Acedo. And according to the simulations they ran recently, it works. Later this year they will test it with real observations.
Why it matters – If de Lera Acedo’s method works, it will allow astronomers to detect the faint radio waves from hydrogen clouds being heated by the first stars, even amid all the radio noise in the universe. The data will help fill in a missing chapter in the history of the universe: the era when the first stars formed and ignited, bathing the universe in radiation and heavier elements forming in their cores. Elements that eventually made up everything we know.
“We know pretty well what happened in the beginning with the Big Bang [thanks to] the cosmic microwave background radiation. We see a lot now [of data] about the latest evolution of galaxies and stars,” explains de Lera Acedo.
“What we haven’t seen yet is how we got from this empty, vast volume that was the post-Big Bang universe to the realm of celestial objects that we see with Hubble.”
The distance the signal travels to get to Earth shows how old it is and gives an indication of when the first stars lit up. Other properties, such as fluctuations in the amplitude of the signal, could reveal even more. A higher amplitude indicates the signal came from a hotter cloud than a lower amplitude signal.
“These fluctuations represent the history of temperatures in the universe: how hot were these hydrogen clouds?” asks de Lera Acedo.
“And if they get heated up, it’s because there was something inside them.”
Astrophysicists have several theoretical models of the early universe, although very little data is available. But de Lera Acedo and his colleagues can compare their temperature data with these existing models. Once they find a good fit, they can be fairly confident that the model’s predictions about other properties of the early Universe — like the mass of star-forming clouds and the rate at which new stars are being born — are also accurate.
And just like that, we will know a lot more about the infancy years of our universe than we do now.
“It will complete, or at least expand, our understanding of the universe, of how the universe evolved, and of our position within that universe – how everything came to be as it is,” says de Lera Acedo.
How it works – Since Bayesian statistics are based on machine learning, de Lera Acedo and his colleagues first had to teach their program what kind of signal — telescope interference, radio Signals from inside our galaxyand actual evidence of the first light of the universe – looks.
They “trained” their algorithm using measurements of radio interference from the team’s telescopes, along with data from radio observations of nearby objects in our galaxy. So far, no one has detected radio emissions from the universe’s first stellar nurseries, so de Lera Acedo and his colleagues simulated the signal based on theoretical models that predict what such radio waves should look like when they reach Earth.
The team used the data to create a computer simulation of the universe — or at least most of the radio signals it contains — and two radio antennas that are currently being built in South Africa (more on that later). This simulation also gave the algorithm the knowledge it needed to perform the actual Bayesian statistics calculations. For each radio signal the algorithm sees, it can calculate the probability that the signal is telescope noise, something in the Milky Way, or something much older, based on the signal’s properties.
What’s next – As we speak, de Lera Acedo and his colleagues at the REACH (Radio Experiment for the Analysis of Cosmic Hydrogen) project are building a pair of radio antennas in the Karoo Central Astronomy Advantage Area in South Africa. The Karoo is a 106,300 square kilometer strip of desert where the South African government has banned radio transmissions, providing a quiet zone for radio telescopes to focus on signals from the sky with a minimum of terrestrial airwave pollution.
By the end of 2022, REACH’s two new antennas will be searching for radio signals from the farthest corners of the universe. With these antennas and lots of statistical calculations, de Lera Acedo and his colleagues hope to find radio waves emitted by primordial clouds of hydrogen heated by the first stars.
And while de Lera Acedo says these observations could fill a huge gap in our understanding of the universe, he also concedes they will only be the beginning.
“In the future there will be other experiments and other telescopes that are more powerful or can give us a lot more information,” he says.
“So we’ll be able to tell a lot more about how those early stars were born and what happened there.”
One of those future telescopes is this square kilometer array, which will combine the power of radio receivers in South Africa and Australia to offer a higher definition view of the radio universe. When it comes online later this decade, the Square Kilometer Array could be able to directly image the earliest light in the Universe.
But for now, we can learn a lot from mapping shadows in fog.