This artist’s illustration shows the universe’s first, massive, blue stars in gaseous filaments, with the cosmic microwave background just visible at the edges. Credit: N.R.Fuller, National Science Foundation
Shortly after the Big Bang, the universe was dark, devoid of the stars and galaxies that define it today.
Over the next 100 million years, gravity pulled dense regions of the early universe’s neutral hydrogen gas together until the force was so strong, these balls of gas collapsed in on themselves to form stars. Unlocking the secrets of what these stars looked like and how they formed could give us unprecedented insight into our universe’s birth – and we’ve just taken a significant step in that direction.
As a result of 12 years of research, a team of scientists, led by ASU School of Earth and Space Exploration astronomer Judd Bowman, has detected the “fingerprints” of the earliest stars in the universe, which formed just 180 millions years after the universe began. Considering our universe is approaching 14 billion years old, this is a relative drop in the ocean of its age.
Peering into the early universe
A timeline of the universe, updated to show when the first stars emerged. Credit:N.R.Fuller, National Science Foundation
The clue to detecting these stellar fingerprints lay in radio astronomy. Using a radio spectrometer at the Australia’s national science agency (CSIRO) Murchison Radio-astronomy Observatory (MRO) in Western Australia, the team looked for small changes in wavelength across the radio spectrum.
As radio waves hit the antenna, they are amplified by a receiver before being digitised to store them as a computer file, like FM and TV receivers transform signals.
The team originally tuned its instrument to look later (so more recently) in cosmic time, but in 2015 extended its search. “As soon as we switched our system to a lower range, we started seeing things we felt might be a real signature,” study author Alan Rogers of the Massachusetts Institute of Technology’s Haystack Observatory said. “We see this dip most strongly at about 78 megahertz, and that frequency corresponds to roughly 180 million years after the Big Bang.
“In terms of a direct detection of a signal from the hydrogen gas itself, this has got to be the earliest.”
In particular, the signals detected by the radio spectrometer in this study came from so-called primordial hydrogen gas which would have been abundant at this point in the early universe, filling the gaps between the first stars.
“There was a great technical challenge to making this detection, as sources of noise can be a thousand times brighter than the signal – it’s like being in the middle of a hurricane and trying to hear the flap of a hummingbird’s wing,” said Peter Kurczynski, the National Science Foundation program officer who supported this study. “These researchers with a small radio antenna in the desert have seen farther than the most powerful space telescopes, opening a new window on the early universe.”
Deciphering the results
The results from this 12-year experiment are significant for a number of reasons. Firstly, the signals offer insight into how early stars formed, unlocking theories which, until now, were seemingly impossible to prove. Secondly, by unlocking the clues to how the first stars form, astronomers will be able unlock how galaxies, black holes and solar systems evolved. This study could even reveal the secrets of the elusive dark matter. It truly is unprecedented.
“It is unlikely we’ll be able to see any earlier into the history of stars in our lifetimes,” said Bowman. “This project shows that a promising new technique can work and has paved the way for decades of new astrophysical discoveries.”
The results confirm the general theoretical expectations of when the first stars formed and the most basic properties of early stars.
“What’s happening in this period,” said co-author Rogers of MIT’s Haystack Observatory, “is that some of the radiation from the very first stars is starting to allow hydrogen to be seen. It’s causing hydrogen to start absorbing the background radiation, so you start seeing it in silhouette, at particular radio frequencies. This is the first real signal that stars are starting to form, and starting to affect the medium around them.”
The study also revealed that gas in the universe was likely much colder than expected, as much as half the temperatures expected and previously hypothesised.
The antenna used to detect the early signals
This suggests either astrophysicists have overlooked something significant or, more excitingly, it may be the first evidence of non-standard physics: specifically, that baryons (normal matter) may have interacted with dark matter and slowly lost energy to dark matter in the early universe, an idea originally proposed by Rennan Barkana of Tel Aviv University.
“The only known cosmic constituent that can be colder than the early cosmic gas is dark matter,” explained Barkana in his paper Possible interaction between baryons and dark-matter particles revealed by the first stars. “The reason for this is that dark matter is assumed to interact with itself and with baryons mainly gravitationally, and so it is expected to decouple thermally in the very early universe and cool down thereafter.”
“If Barkana’s idea is confirmed,” continued Bowman, “then we’ve learned something new and fundamental about the mysterious dark matter that makes up 85% of the matter in the universe, providing the first glimpse of physics beyond the standard model.”
“Now that we know this signal exists,” says Bowman, “we need to rapidly bring online new radio telescopes that will be able to mine the signal much more deeply.”
The results have been published in Nature in two papers