First light: the signature of the earliest stars detected

A dozen years of searching pays off in a find that could profoundly alter our understanding of the universe. Andrew Masterson reports.

180 million years ago, the first stars ignited.
180 million years ago, the first stars ignited.

The traces of the first daybreak in the universe have been detected by a small radio-telescope in Western Australia.

Astronomers know that shortly after the Big Bang, 13.7 billion years ago, everything was dark. After its intensely energetic burst of expansion, the universe cooled. Gradually, in the ensuing eons, gravity pulled bits of matter together into clumps, then into larger bodies, until finally, around 180 million years after the process began, a critical mass was reached and the first stars ignited.

As the light interacted with hydrogen gas, a signal was created. That signal has now been finally detected by a collaboration between Australian research organisation, the CSIRO, and Arizona State University in the US.

The accepted models for predicting events in the very early universe suggest that when ultraviolet light produced by the first stars interacted with hydrogen gas, it would change how the gas absorbs radiation. This change was expected to show up as a dip in radiowaves coming from space at frequencies lower than 200 megahertz.

In a paper published in the journal Nature, scientists led by Judd Bowman of Arizona State report the detection of such a dip around 78 megahertz, with a profile “largely consistent with expectations” for signals produced by early stars.

There are, however, some significant differences between it and the predictions, Mainly, Bowman and colleagues reveal the amplitude of the signal is twice as strong as suggested in even the uppermost estimates.

This, they write, may be because the primordial hydrogen was colder than expected, or the radiation temperature was hotter. The scientists suggest the best-fit explanation is cooling caused by interactions between dark matter and subatomic particles.

Bowman has been searching for the elusive trace of star ignition for 12 years. His success, in the end, came down only partly to improvements in technology. Mostly, it was about geography.

It was always assumed that the signal, known as the “global epoch of reionisation signature”, would be very, very faint. After all, it was generated an extremely long way away, an extremely long time ago. This was a problem, however, compounded by the fact that predictions showed that it would be contained in a region of the radio frequency spectrum reserved for FM radio broadcasts.

The faint evidence of the first morning breaking, therefore, might be forever buried under the loud blips and whoops of Redfoo singing.

Acknowledging this difficulty, many years ago the CSIRO began building the Murchison Radio-astronomy Observatory (MRO) in the centre of Western Australia. The region is very sparsely populated and falls in a naturally “radio-quiet” zone. This quietness is further enforced through legislation, with a law passed restricting the use of radios in an area extending for 240 kilometres around the site.

Even in such a quiet environment, picking out the extremely faint target signal from cosmic background was very, very difficult.

“This is one of the most technically challenging radio astronomy experiments ever attempted,” says MRO manager Antony Schinckel.

“The lead authors include two of the best radio astronomy experimentalists in the world and they have gone to great lengths to design and calibrate their equipment in order to have convincing evidence for a real signal.”

Schinckel describes the result as “an absolute triumph”.

Other scientists have been equally effusive.

“The apparent detection of the signature of the first stars in the universe will be a revolutionary discovery if it stands the tests of time,” says astrophysicist and Nobel laureate Brian Schmidt.

“While the detection appears robust, it is an incredibly challenging measurement, and needs to be confirmed. The fact that the detection is much stronger than expected, and that can be easily explained, is particularly exciting.”

Lister Staveley-Smith, science director of the International Centre for Radio Astronomy Research in Western Australia, says it is “a truly amazing result”.

“If the signal is confirmed by other experiments (which may happen soon),” he adds, “the implications for our understanding of the evolution of the universe and the nature of cosmic dark matter will be profound.”

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Andrew Masterson is news editor of Cosmos.
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