Dark matter, the stuff that’s hypothesized to make up about a quarter of the universe yet doesn’t seem to interact with light at all, could have a tiny electric charge, according to a new study.
So far, dark matter has made its presence known only through gravity, by tugging on stars and galaxies. But now, astrophysicists Julian Muñoz and Abraham Loeb of Harvard University suggest that a small fraction of dark-matter particles could have a tiny electric charge — meaning dark matter could interact with normal matter through the electromagnetic force.
If true, this idea would not only represent a big step in understanding dark matter but would also explain a recent mystery that’s been confounding cosmologists.
In February, astronomers announced the first detection of an elusive signal from hydrogen gas from the cosmic dawn, the period about 180 million years after the Big Bang when the first stars began to shine. At this time, the hydrogen gas floating between the stars was cold — colder than the cosmic microwave background, the leftover radiation from the Big Bang that bathes the universe.
Because hydrogen is cooler than this afterglow, the gas absorbs the radiation — in particular, radiation with a wavelength of 21 centimeters (8.3 inches). By measuring the absorption of radiation by hydrogen, astronomers can better understand the cosmic dawn, a relatively unknown era of cosmic history. Using a radio antenna in Western Australia called the Experiment to Detect the Global Epoch of Reionization Signature (EDGES), a team of astronomers was able to detect this absorption for the first time.
“This is, in and of itself, an amazing scientific discovery,” Muñoz told Live Science. But more than that, he added, the astronomers found that twice as many photons were absorbed by the hydrogen than expected. For the gas to absorb so much radiation, it would have to be even cooler than scientists thought.
Muñoz and Loeb proposed that dark matter might be the culprit for the curious cooling. In a paper published May 30 in the journal Nature, they found that if less than 1 percent of the dark matter had about one-millionth of the electric charge of an electron, then this elusive matter could pull heat from the hydrogen — similar to how ice cubes cool your lemonade. “Ice, here, is the dark matter,” Muñoz said.
Their idea isn’t completely new. Decades ago, physicists proposed that dark-matter particles could have an electric charge.
And it’s not the only explanation for this cooling. In a March 1 paper in the journal Nature, Rennan Barkana, a cosmologist at Tel Aviv University in Israel, proposed that a more general form of dark matter, which doesn’t necessarily have a charge, could cool normal matter and explain the EDGES data.
Both dark-matter proposals make similar predictions, said Barkana, who was not involved in the current study.
“This is a time for cautious optimism and keeping an open mind, about both the radio observation and the interpretation,” Barkana told Live Science.
Dozens of ideas
Dark matter is just one of dozens of ideas proposed to explain the anomaly. For instance, instead of the gas being cooler, the background radiation might be hotter than expected, with some exotic process producing more radiation that has yet to be accounted for. Or, there simply could be errors in the analysis or measurement.
Indeed, the EDGES observation is the first of its kind, and although the team spent two years checking and double-checking the analysis, researchers will need more data to confirm the puzzling results.
“If EDGES is correct, I don’t think there’s any conventional explanation that’s compelling,” said Steven Furlanetto, an astrophysicist at the University of California, Los Angeles, who was not involved in the study. “You really need to go to one of these nonstandard physics scenarios, and in that case, I think it’s wide open.”
Muñoz, however, is partial to the dark-matter explanation. “If EDGES is indeed right, it seems very hard for this not to be the result of dark matter,” he said.
Several instruments around the world are already gearing up to make more detailed observations. Unlike EDGES, some experiments, such as a radio telescope in South Africa called the Hydrogen Epoch of Reionization Array, will be able to measure how the absorption varies across the sky. If a small fraction of dark matter is electrically charged as Muñoz and Loeb say, then it will create a distinct pattern in this variation — providing a key test for electrically charged dark matter.