Dark Matter May Have Kept the Early Universe Cool

Astronomers have detected hydrogen formed just 180 million years after the Big Bang. But the gas is much colder than theoretical predictions.

Artist's rendering of how the first stars in the universe may have looked | N.R.Fuller, National Science Foundation
Artist's rendering of how the first stars in the universe may have looked | N.R.Fuller, National Science Foundation

Astronomers have used a small radio telescope to find faint radio waves emitted by clouds of primordial hydrogen gas formed just 180 million years after the Big Bang. These are the earliest signals of hydrogen ever observed. They also found evidence that the first stars were already shining at this point in the early universe. While their observations match most theoretical predictions for the origins of our universe, there also is a bit of a mystery: The hydrogen gas was colder than expected.

“Our observations are consistent with the big bang,” Judd Bowman from Arizona State University told Seeker. He is the lead author of one of two papers on the research published in the journal Nature. “They are also consistent with our expectations about when the first stars form in the universe, too. But they suggest that we are missing at least one piece of the puzzle — either something else is present at this time to make more radio waves, or some mechanism cooled the gas more than expected.”

Bowman said that prior to these observations, astronomers expected that they knew the temperature of the primordial gas quite well based on physical modeling and previous observations of the cosmic microwave background — the remnant, electromagnetic radiation from the earliest stages of the universe.

“We expect that gas cools slowly from the time it is formed 380,000 years after the big bang until stars started to appear,” he said. “However, the strength of the signal we detect requires either the intensity of background radio waves in the early universe to be larger than predicted or the gas to be colder than expected.”

A timeline of the universe, updated to show when the first stars emerged. This updated timeline of the universe reflects the recent discovery that the first stars emerged by 180 million years after the Big Bang. The research behind this timeline was conducted by Judd Bowman of Arizona State University and his colleagues, with funding from the National Science Foundation. | N.R.Fuller, National Science Foundation

While astronomers typically rely on light to make their observations, the most useful for observing the early universe are radio waves — specifically microwaves. By studying this part of the electromagnetic spectrum, they can measure the cosmic microwave background, which is the oldest light still in existence. Before this was emitted, the universe was effectively opaque: Light couldn't travel freely because the universe was so hot and dense.

The prevailing theory is that when the first stars turned on, they provided ultraviolet radiation that caused changes to the distribution of hydrogen atom. The transition is what astronomers call the 21-cm hyperfine line. This means that the hydrogen gas would absorb photons from the cosmic microwave background, imprinting a signature in the radio spectrum that should be observable today at radio frequencies below 200 megahertz. The intensity of waves from this early era would provide indications of the temperature of the gas.

Bowman and his colleagues used a small ground-based radio antenna located in western Australia called EDGES, short for Experiment to Detect Global EoR [Epoch of Reionization] Signature. They detected the signature at 78 megahertz, which was within the range they expected, but the signal had a larger amplitude, indicating that the primordial gas was colder than expected.

EDGES ground-based radio spectrometer, CSIRO’s Murchison Radio-astronomy Observatory in Western Australia | CSIRO Australia

Bowman said they investigated possible explanations and reached out to close colleagues for assistance and ideas.

“It is difficult to find mechanisms that could increase the radio background at this age in the universe, whereas an explanation for how the gas might be cooler than expected was more assessable,” he said.

One colleague, Rennan Barkana from Tel Aviv University had an intriguing idea based on previous observations. The gas could have been cooled through the interaction of hydrogen with something quite cold: dark matter.

“Barkana realized that previous work exploring the possible effects of dark matter interacting with baryons — atoms, for example — could be applied to the era probed by our observations and would provide a mechanism for cooling the gas to the needed temperature,” Bowman said. “In essence, if the gas interacts — even weakly — with dark mater, it could lose energy to the dark matter and cool. This is the best explanation we have at the moment and it is very exciting if it holds.”

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Bowman added that additional ideas will likely emerge as more astrophysicists analyze their research. But, he said, it is hard to conjure up any other objects or processes that can have such an effect at these early ages.

But thanks to upgrades to the antenna and other systems of EDGES made by Bowman and fellow astronomers at ASU, MIT, and the University of Colorado, the scientists were especially intrigued to find signature of light from the first stars and that the profile of the radio waves match theoretical predictions of what would be produced if hydrogen were indeed influenced by the first stars.

"It is unlikely that we'll be able to see any earlier into the history of stars in our lifetimes," Bowman said in a statement. "This project shows that a promising new technique can work and has paved the way for decades of new astrophysical discoveries."