Whisper From the First Stars Sets Off Loud Dark Matter Debate

The news about the first stars in the universe always seemed a little off. Last July, Rennan Barkana, a cosmologist at Tel Aviv University, received an email from one of his longtime collaborators, Judd Bowman. Bowman leads a small group of five astronomers who built and deployed a radio telescope in remote western Australia. Its goal: to find the whisper of the first stars. Bowman and his team had picked up a signal that didn’t quite make sense. He asked Barkana to help him think through what could possibly be going on.

For years, as radio telescopes scanned the sky, astronomers have hoped to glimpse signs of the first stars in the universe. Those objects are too faint and, at over 13 billion light-years away, too distant to be picked up by ordinary telescopes. Instead, astronomers search for the stars’ effects on the surrounding gas. Bowman’s instrument, like the others involved in the search, attempts to pick out a particular dip in radio waves coming from the distant universe.

The measurement is exceedingly difficult to make, since the potential signal can get swamped not only by the myriad radio sources of modern society — one reason the experiment is deep in the Australian outback — but by nearby cosmic sources such as our own Milky Way galaxy. Still, after years of methodical work, Bowman and his colleagues with the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) concluded not only that they had found the first stars, but that they had found evidence that the young cosmos was significantly colder than anyone had thought.

Barkana was skeptical, however. “On the one hand, it looks like a very solid measurement,” he said. “On the other hand, it is something very surprising.”

What could make the early universe appear cold? Barkana thought through the possibilities and realized that it could be a consequence of the presence of dark matter — the mysterious substance that pervades the universe yet escapes every attempt to understand what it is or how it works. He found that the EDGES result could be interpreted as a completely new way that ordinary material might be interacting with dark matter.

The EDGES group announced the details of this signal and the detection of the first stars in the March 1 issue of Nature. Accompanying their article was Barkana’s paper describing his novel dark matter idea. News outlets worldwide carried news of the discovery. “Astronomers Glimpse Cosmic Dawn, When the Stars Switched On,” the Associated Press reported, adding that “they may have detected mysterious dark matter at work, too.”

Yet in the weeks since the announcement, cosmologists around the world have expressed a mix of excitement and skepticism. Researchers who saw the EDGES result for the first time when it appeared in Nature have done their own analysis, showing that even if some kind of dark matter is responsible, as Barkana suggested, no more than a small fraction of it could be involved in producing the effect. (Barkana himself has been involved in some of these studies.) And experimental astronomers have said that while they respect the EDGES team and the careful work that they’ve done, such a measurement is too difficult to trust entirely. “If this weren’t a groundbreaking discovery, it would be a lot easier for people to just believe the results,” said Daniel Price, an astronomer at Swinburne University of Technology in Australia who works on similar experiments. “Great claims require great evidence.”

This message has echoed through the cosmology community since those Nature papers appeared.

The Source of a Whisper

The day after Bowman contacted Barkana to tell him about the surprising EDGES signal, Barkana drove with his family to his in-laws’ house. During the drive, he said, he contemplated this signal, telling his wife about the interesting puzzle Bowman had handed him.

Bowman and the EDGES team had been probing the neutral hydrogen gas that filled the universe during the first few hundred million years after the Big Bang. This gas tended to absorb ambient light, leading to what cosmologists poetically call the universe’s “dark ages.” Although the cosmos was filled with a diffuse ambient light from the cosmic microwave background (CMB) — the so-called afterglow of the Big Bang — this neutral gas absorbed it at specific wavelengths. EDGES searched for this absorption pattern.

As stars began to turn on in the universe, their energy would have heated the gas. Eventually the gas reached a high enough temperature that it no longer absorbed CMB radiation. The absorption signal disappeared, and the dark ages ended.

The absorption signal as measured by EDGES contains an immense amount of information. As the absorption pattern traveled across the expanding universe, the signal stretched. Astronomers can use that stretch to infer how long the signal has been traveling, and thus, when the first stars flicked on. In addition, the width of the detected signal corresponds to the amount of time that the gas was absorbing the CMB light. And the intensity of the signal — how much light was absorbed — relates to the temperature of the gas and the amount of light that was floating around at the time.

Many researchers find this final characteristic the most intriguing. “It’s a much stronger absorption than we had thought possible,” said Steven Furlanetto, a cosmologist at the University of California, Los Angeles, who has examined what the EDGES data would mean for the formation of the earliest galaxies.

The most obvious explanation for such a strong signal is that the neutral gas was colder than predicted, which would have allowed it to absorb even more background radiation. But how could the universe have unexpectedly cooled? “We’re talking about a period of time when stars are beginning to form,” Barkana said — the darkness before the dawn. “So everything is as cold as it can be. The question is: What could be even colder?”

As he parked at his in-laws’ house that July day, an idea came to him: Could it be dark matter? After all, dark matter doesn’t seem to interact with normal matter via the electromagnetic force — it doesn’t emit or absorb heat. So dark matter could have started out colder or been cooling much longer than normal matter at the beginning of the universe, and then continued to cool.

Over the next week, he worked on a theory of how a hypothetical form of dark matter called “millicharged” dark matter could have been responsible. Millicharged dark matter could interact with ordinary matter, but only very weakly. Intergalactic gas might then have cooled by “basically dumping heat into the dark matter sector where you can’t see it anymore,” Furlanetto explained. Barkana wrote the idea up and sent it off to Nature.

Then he began to work through the idea in more detail with several colleagues. Others did as well. As soon as the Nature papers appeared, several groups of theoretical cosmologists started to compare the behavior of this unexpected type of dark matter to what we know about the universe — the decades’ worth of CMB observations, data from supernova explosions, the results of collisions at particle accelerators like the Large Hadron Collider, and astronomers’ understanding of how the Big Bang produced hydrogen, helium and lithium during the universe’s first few minutes. If millicharged dark matter was out there, did all these other observations make sense?

They did not. More precisely, these researchers found that millicharged dark matter can only make up a small fraction of the total dark matter in the universe — too small a fraction to create the observed dip in the EDGES data. “You cannot have 100 percent of dark matter interacting,” said Anastasia Fialkov, an astrophysicist at Harvard University and the first author of a paper submitted to Physical Review Letters. Another paper that Barkana and colleagues posted on the preprint site arxiv.org concludes that this dark matter has an even smaller presence: It couldn’t account for more than 1 to 2 percent of the millicharged dark matter content. Independent groups have reached similar conclusions.

If it’s not millicharged dark matter, then what might explain EDGES’ stronger-than-expected absorption signal? Another possibility is that extra background light existed during the cosmic dawn. If there were more radio waves than expected in the early universe, then “the absorption would appear stronger even though the gas itself is unchanged,” Furlanetto said. Perhaps the CMB wasn’t the only ambient light during the toddler years of our universe.

This idea doesn’t come entirely out of left field. In 2011, a balloon-lofted experiment called ARCADE 2 reported a background radio signal that was stronger than would have been expected from the CMB alone. Scientists haven’t yet been able to explain this result.

After the EDGES detection, a few groups of astronomers revisited these data. One group looked at black holes as a possible explanation, since black holes are the brightest extragalactic radio sources in the sky. Yet black holes also produce other forms of radiation, like X-rays, that haven’t been seen in the early universe. Because of this, astronomers remain skeptical that black holes are the answer.

Is It Real?

Perhaps the simplest explanation is that the data are just wrong. The measurement is incredibly difficult, after all. Yet by all accounts the EDGES team took exceptional care to cross-check all their data — Price called the experiment “exquisite” — which means that if there is a flaw in the data, it will be exceptionally hard to find.