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https://www.quantamagazine.org/whisper-from-the-first-stars-sets-off-loud-dark-matter-debate-20180329/ March 29, 2018

Whisper From the First Stars Sets Off Loud DarkMatter DebateA surprise discovery announced a month ago suggested that the early universe looked very differentthan previously believed. Initial theories that the discrepancy was due to dark matter have comeunder fire.

By Liz Kruesi

Victor Mosquera for Quanta Magazine

Evidence pointing to the discovery of the earliest stars inspired excitement among cosmologists, but alsoskepticism.

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, JuddBowman. Bowman leads a small group of five astronomers who built and deployed a radio telescopein remote western Australia. Its goal: to find the whisper of the first stars. Bowman and his team hadpicked up a signal that didn’t quite make sense. He asked Barkana to help him think through whatcould possibly be going on.

For years, as radio telescopes scanned the sky, astronomers have hoped to glimpse signs of the first

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stars in the universe. Those objects are too faint and, at over 13 billion light-years away, too distantto be picked up by ordinary telescopes. Instead, astronomers search for the stars’ effects on thesurrounding gas. Bowman’s instrument, like the others involved in the search, attempts to pick out aparticular dip in radio waves coming from the distant universe.

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

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 andrealized that it could be a consequence of the presence of dark matter — the mysterious substancethat pervades the universe yet escapes every attempt to understand what it is or how it works. Hefound that the EDGES result could be interpreted as a completely new way that ordinary materialmight be interacting with dark matter.

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

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

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

The Source of a WhisperThe day after Bowman contacted Barkana to tell him about the surprising EDGES signal, Barkanadrove 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 universeduring the first few hundred million years after the Big Bang. This gas tended to absorb ambientlight, leading to what cosmologists poetically call the universe’s “dark ages.” Although the cosmoswas filled with a diffuse ambient light from the cosmic microwave background (CMB) — the so-

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called afterglow of the Big Bang — this neutral gas absorbed it at specific wavelengths. EDGESsearched for this absorption pattern.

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

The absorption signal as measured by EDGES contains an immense amount of information. As theabsorption pattern traveled across the expanding universe, the signal stretched. Astronomers canuse that stretch to infer how long the signal has been traveling, and thus, when the first stars flickedon. In addition, the width of the detected signal corresponds to the amount of time that the gas wasabsorbing the CMB light. And the intensity of the signal — how much light was absorbed — relatesto 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 absorptionthan 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 earliestgalaxies.

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Lucy Reading-Ikkanda/Quanta Magazine; Source: arXiv:1609.02312v3 Figure 1 (expected);doi:10.1038/nature25792 Figure 2 (observed)

The most obvious explanation for such a strong signal is that the neutral gas was colder thanpredicted, which would have allowed it to absorb even more background radiation. But how could

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the universe have unexpectedly cooled? “We’re talking about a period of time when stars arebeginning to form,” Barkana said — the darkness before the dawn. “So everything is as cold as it canbe. 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? Afterall, dark matter doesn’t seem to interact with normal matter via the electromagnetic force — itdoesn’t emit or absorb heat. So dark matter could have started out colder or been cooling muchlonger 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 interactwith ordinary matter, but only very weakly. Intergalactic gas might then have cooled by “basicallydumping 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.

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Courtesy of Rennan Barkana

Rennan Barkana, a cosmologist at Tel Aviv University, contributed the idea that a form of dark matter might explainwhy the early universe looked so cool in the EDGES observations. But he has also stayed skeptical about thefindings.

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 tocompare 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 atparticle accelerators like the Large Hadron Collider, and astronomers’ understanding of how the BigBang produced hydrogen, helium and lithium during the universe’s first few minutes. If millichargeddark 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 makeup a small fraction of the total dark matter in the universe — too small a fraction to create theobserved dip in the EDGES data. “You cannot have 100 percent of dark matter interacting,” saidAnastasia Fialkov, an astrophysicist at Harvard University and the first author of a paper submittedto Physical Review Letters. Another paper that Barkana and colleagues posted on the preprint sitearxiv.org concludes that this dark matter has an even smaller presence: It couldn’t account for morethan 1 to 2 percent of the millicharged dark matter content. Independent groups have reachedsimilar conclusions.

If it’s not millicharged dark matter, then what might explain EDGES’ stronger-than-expectedabsorption 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 wouldappear stronger even though the gas itself is unchanged,” Furlanetto said. Perhaps the CMB wasn’tthe 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 ARCADE2 reported a background radio signal that was stronger than would have been expected from theCMB 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 atblack holes as a possible explanation, since black holes are the brightest extragalactic radio sourcesin the sky. Yet black holes also produce other forms of radiation, like X-rays, that haven’t been seenin 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 incrediblydifficult, after all. Yet by all accounts the EDGES team took exceptional care to cross-check all theirdata — Price called the experiment “exquisite” — which means that if there is a flaw in the data, itwill be exceptionally hard to find.

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LoCo Lab

This antenna for EDGES was deployed in 2015 at a remote location in western Australia where it would experiencelittle radio interference.

The EDGES team deployed their radio antenna in September 2015. By December, they were seeinga signal, said Raul Monsalve, an experimental cosmologist at the University of Colorado, Boulder,and a member of the EDGES team. “We became suspicious immediately, because it was strongerthan expected.”

And so they began what became a marathon of due diligence. They built a similar antenna andinstalled it about 150 meters away from the first one. They rotated the antennas to rule outenvironmental and instrumental effects. They used separate calibration and analysis techniques.“We made many, many kinds of cuts and comparisons and cross-checks to try to rule out the signalas coming from the environment or from some other source,” Monsalve said. “We didn’t believeourselves at the beginning. We thought it was very suspicious for the signal to be this strong, andthat’s why we took so long to publish.” They are convinced that they’re seeing a signal, and that thesignal is unexpectedly strong.

“I do believe the result,” Price said, but he emphasized that testing for systematic errors in the datais still needed. He mentioned one area where the experiment could have overlooked a potentialerror: Any antenna’s sensitivity varies depending on the frequency it’s observing and the directionfrom which a signal is coming. Astronomers can account for these imperfections by either measuringthem or modeling them. Bowman and colleagues chose to model them. Price suggests that theEDGES team members instead find a way to measure them and then reanalyze their signal with that

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measured effect taken into account.

The next step is for a second radio detector to see this signal, which would imply it’s from the skyand not from the EDGES antenna or model. Scientists with the Large-Aperture Experiment to Detectthe Dark Ages (LEDA) project, located in California’s Owens Valley, are currently analyzing thatinstrument’s data. Then researchers will need to confirm that the signal is actually cosmological andnot produced by our own Milky Way. This is not a simple problem. Our galaxy’s radio emission canbe thousands of times stronger than cosmological signals.

On the whole, researchers regard both the EDGES measurement itself and its interpretation with ahealthy skepticism, as Barkana and many others have put it. Scientists should be skeptical of a first-of-its-kind measurement — that’s how they ensure that the observation is sound, the analysis wascompleted accurately, and the experiment wasn’t in error. This is, ultimately, how science issupposed to work. “We ask the questions, we investigate, we exclude every wrong possibility,” saidTomer Volansky, a particle physicist at Tel Aviv University who collaborated with Barkana on one ofhis follow-up analyses. “We’re after the truth. If the truth is that it’s not dark matter, then it’s notdark matter.”