Landmark Cosmic Observation Provides Tantalising Hints of Dark Matter

By Ryan F. Mandelbaum on at

Today, scientists announced that they’d seen evidence of a long-sought signal from the first stars. This slight change to some ambient radio waves could herald the first step in a new kind of astronomy. But maybe, just maybe, it’s also evidence of dark matter interacting with regular matter in the ancient universe.

The discovery consisted of the change in the radio signal coming from ancient hydrogen, signifying an absorption of ancient light. But the effect was twice as large as theorists had predicted. Some think this anomaly could be evidence of dark matter, and would mean that dark matter has less mass than we thought.

“This was a very big surprise,” Rennan Barkana, author of the new paper in Nature and head of the Department of Astrophysics School of Physics and Astronomy at Tel Aviv University, told Gizmodo. “My explanation is that the hydrogen gas is even colder than expected. That means that something has to cool it down.”

That something? Dark matter, perhaps. Dark matter is a theorised substance whose effects have been observed based on how light from distant galaxies seems to bend from excess gravity, and how galaxies seem to rotate too quickly at their edges. Calculations show that there should be five to six times more dark matter than regular matter in the universe. Some scientists hope that it interacts very weakly with regular matter, and are trying to detect it. Others worry that dark matter doesn’t interact with regular matter at all, and would thus seem to be undetectable.

A team of scientists at Arizona State University and MIT’s Experiment to Detect the Global Epoch of Reionisation Signature (EDGES) experiment measured the temperature of radiation coming from hydrogen 100 million years after the Big Bang. Ultraviolet radiation from the first stars excited this diffuse hydrogen gas, whose atoms then absorbed photons coming from universe’s most distant observable light, the cosmic microwave background. The signal showed the hydrogen was twice as cold as expected, and Barkana argues that the only thing available to do the extra cooling would be dark matter.

“It’s a very tantalising possibility,” Priyamvada Natarajan, a Yale astrophysicist who researches dark matter but was not involved in this study, told Gizmodo. She said the paper could have important implications for dark matter searches here on earth. Currently, scientists are hunting for dark matter that weighs in the neighborhood of a billion electron volts, just around the mass of the proton or heavier. “These papers are suggesting that we maybe need to move away from looking for GeV dark matter,” she said. “We would need a rethink of our current experimental setups for direct detection.”

But Natarajan noted that this theory is based on a single observation. “I think scientists are going to need a bit more time to see what else is possible,” she said. And Chanda Prescod-Weinstein, theoretical physicist at the University of Washington, told Gizmodo that the model Barkana used to analyse the potential dark matter-regular matter interaction in the universe “seems to be a relatively new idea, based on a paper from 2015 which still has yet to be widely cited.” Still, she said, “overall I think this is an exciting result and the evidence is compelling.”

Of course, this tantalising new evidence must be treated with caution until other scientists can reproduce it. But things are moving fast. “It’s not going to take so long to get more observations,” said Barkana. “There’s going to be more independent measurements of the same type just to verify the signal.”

And then maybe, in a few years, we’ll get an answer to this dark matter question, he said. “If the dark matter interpretation is correct, it predicts a specific kind of pattern due to the difference of how dark matter and ordinary matter behave in the early universe. There would be a clear signature.” [Nature]

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