Astronomers may not have found a sign of the universe’s first stars after all

 A new experiment finds no hint of the ‘cosmic dawn’ claimed by an earlier study

The radio waves picked up by an antenna on a lake in India (pictured) revealed no indication of the universe's initial stars, casting doubt on a previously reported measurement.

A new study casts a cloud on the first glimmers of starlight in the universe.

Researchers claimed in 2018 that a faint imprint in radio waves from early in the universe's history had revealed the epoch of the cosmic dawn, when the first stars turned on. However, scientists announce in Nature Astronomy on February 28 that the first experiment to test that study's conclusions discovered no evidence of those early stars.

The universe was a boiling stew of matter just after the Big Bang 13.8 billion years ago. Stars probably didn't start to shine until at least 100 million years later, during an age of the cosmos that is still poorly understood. Finding evidence of the earliest rays of starlight would help to fill in the gaps in the cosmic beginning tale. So the announcement in 2018 that the EDGES experiment in the Australian outback had pinpointed those first gleams sparked an astronomical frenzy.

"This amazing conclusion definitely completely enthralled our entire community," says radio astronomer Saurabh Singh of the Raman Research Institute in Bangalore, India.

The researchers discovered a decrease in radio waves at specific wavelengths, indicating that light from the early stars is interacting with surrounding hydrogen gas. However, because the dip was deeper than projected, the result instantly sparked doubt. More observations would be required to determine whether the glint of the first starlight was real.

With the Shaped Antenna Measurement of the Background Radio Spectrum 3, or SARAS 3, Singh and colleagues performed just that. The experiment, like EDGES, employs an antenna to collect up radio signals. However, SARAS 3 is not the same as EDGES, with a different antenna shape. SARAS 3 is meant to float on the surface of a lake. "We have a distinct advantage because of that," Singh says.

Radio waves arrive on Earth from a multitude of sources, all of which must be properly accounted for in order to detect the softer signal from the cosmic dawn. If those other sources of radio waves are misunderstood, an unaccounted-for experimental error could result in inaccurate conclusions.

Experiments on land, in particular, must struggle with radio waves released by the ground, which are challenging to quantify due to soil's layered complexity. It's easier to predict what sorts of radio waves come from the uniform water below when the antenna is atop a lake. There was no trace of the decline in data collected from two lakes in India.

According to physicist H. Cynthia Chiang of McGill University in Montreal, the new work "highlights just how challenging this measurement is." She admits that the differences between the two studies are unsettling, but adds that the differences "aren't quite enough to make any clear conclusions at this moment."

And, according to experimental cosmologist Judd Bowman of Arizona State University in Tempe, a member of the EDGES team, some of the same experimental difficulties that may influence EDGES could also harm SARAS 3. "We still have a lot of work to do to get to the ultimate result."

Later this year, an enhanced version of EDGES will be deployed, and the SARAS 3 team has more deployments planned. Similar measurements are being made in other investigations as well. These experiments could finally shed light on the universe's transformation from darkness to light.

References:

• S. Singh et al. On the detection of a cosmic dawn signal in the radio background. Nature Astronomy. Published online February 28, 2022. doi: 10.1038/s41550-022-01610-5.