Space forensics: signs of the first stars in the universe

What were those first stars like and when did they form? How did they affect the rest of the universe? These are questions astronomers and astrophysicists have long pondered.

Now, after 12 years of experimental effort, a team of scientists, led by Arizona State University School of Earth and Space Exploration astronomer Judd Bowman, has detected indications of the earliest stars in the universe.

Using radio signals, the detection provides the first evidence for the oldest ancestors in our cosmic family tree, born a mere 180 million years after the universe began.

“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.”

To find these fingerprints, Bowman’s team used a ground-based instrument called a radio spectrometer, located at the Australian national science agency's (CSIRO) Murchison Radio-astronomy Observatory (MRO) in Western Australia.

Through their Experiment to Detect the Global EoR Signature (EDGES), the team measured the average radio spectrum of all the astronomical signals received across most of the Southern Hemisphere sky and looked for small changes in power as a function of wavelength (or frequency).

As radio waves enter the ground-based antenna, they are amplified by a receiver, and then digitized and recorded by computer, similar to how FM radio and TV receivers work. The difference is that the instrument is very precisely calibrated and designed to perform as uniformly as possible across many radio wavelengths.

The signals detected by the radio spectrometer in this study came from primordial hydrogen gas that filled the young universe and existed between all the stars and galaxies. These signals hold a wealth of information that opens a new window on how early stars — and later, black holes and galaxies — formed and evolved.

“It is unlikely that 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.”

This detection highlights the exceptional radio quietness of the MRO, particularly as the feature found by EDGES overlaps the frequency range used by FM radio stations.

Australian national legislation limits the use of radio transmitters within 161.5 miles (260 km) of the site, substantially reducing interference that could otherwise drown out sensitive astronomy observations.

This updated timeline of the universe reflects the recent discovery that the first stars emerged by 180 million years after the Big Bang.

The results of this study have been recently published in Nature by Bowman, with co-authors Alan Rogers of the Massachusetts Institute of Technology’s Haystack Observatory, Raul Monsalve of the University of Colorado, and Thomas Mozdzen and Nivedita Mahesh also of ASU’s School of Earth and Space Exploration.

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 is that some of the radiation from the very first stars is starting to allow hydrogen to be seen," said Rogers.

"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 team originally tuned their instrument to look later in cosmic time, but in 2015 decided to extend their search. “As soon as we switched our system to this lower range, we started seeing things that we felt might be a real signature,” Rogers 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.”

The study also revealed that gas in the universe was probably much colder than expected — less than half the expected temperature. This suggests that either astrophysicists’ theoretical efforts have overlooked something significant or that this 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, a concept that was originally proposed by Rennan Barkana of Tel Aviv University.

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Image credit: Arizona State University.