Today, a small team of researchers is announcing that its correspondingly small telescope picked up something that theoreticians had only suggested might exist: a signal produced by the very first stars in our Universe. Their radiotelescope, only two meters across, didn't image the stars directly. Instead, it picked up an imprint on the Cosmic Microwave Background left by the matter that these stars interacted with.

And, while the signal had been predicted by theoreticians, calculations had suggested that it would be substantially smaller than it actually is. If the results hold up, then it could be a sign that dark matter looks very different from what we had expected.

Ignition

The Cosmic Microwave Background was produced when the Universe cooled enough to allow electrons to settle down into the Universe's first atoms, releasing radiation as they did. It famously captures the state of the Universe when it was formed, telling us about the Big Bang that produced it, as well as the composition of the Universe's contents. But in many ways, it's the gift that keeps giving, as subtle details in the Background provide further details of the Universe's physics, and theorists regularly think up ways to extract more information from it.

Theoretical considerations helped motivate the EDGES project, or Experiment to Detect the Global Epoch of reionization Signature. In an era of massive telescopes, EDGES is refreshingly simple: its largest component is just a 30-meter square of metal plates on the ground. At the center sit two radio antennae sensitive to a specific region of the spectrum that overlaps with part of the Cosmic Microwave Background.

That area of the spectrum has been predicted to capture a rather indirect interaction of the Cosmic Microwave Background and the first stars. At the time the Cosmic Microwave Background was formed, the Universe was still extremely hot and dense, with the first atoms at roughly the same temperature as the radiation they were producing. This allowed them to interact with the Microwave Background radiation for a time, but that time came to an end as the Universe continued to expand and the atoms cooled down.

The ignition of the first stars produced lots of high-energy radiation that heated up the surrounding gas, temporarily pushing it back up to the threshold where it could interact again with the Cosmic Microwave Background. These interactions would reduce the amount of Background radiation in a specific area of the spectrum (currently in the radio wavelengths), with the precise details of the reduction depending on the state of the gas at the time. The EDGES observatory was designed for a single purpose: examine that area of the spectrum.

Lots of noise

The problem is that the Cosmic Microwave Background is not the only signal at these wavelengths. Our own galaxy produces lots of radiation there; so does the Earth's ionosphere, a high-altitude region of the atmosphere. So a lot of the team's work involves identifying these other signals and removing them. For example, the galaxy's signal tends to be a cluster of individual absorbance and emissions lines, rather than a smooth spectrum. And the ionosphere's signal fluctuates both with the seasons and based on what's going on with space weather.

So the researchers took measurements for hundreds of hours over a couple of years. Part way through, they rotated one of their two receivers in order to ensure that the signal wasn't the product of local hardware. And, to make sure any signals were real, they ran their data through two independently written pieces of analysis software.

When they were done, there was a clear signal. The most distant, oldest galaxy we know about is distant enough that its light is dramatically shifted to the red; a measure of this termed z produces a z of 11. The signal detected by EDGES starts at a z of 15 and gets progressively older. z = 15 corresponds to the Universe being just 270 million years old.

This isn't conclusive evidence that the signal of the first stars has been found. Physics typically wants a confidence level of five sigma before it announces discovery, and this is at 3.8 sigma. But it's pretty compelling and about as good as this instrument is likely to do without a lot more observing time. Fortunately, there are a number of similar projects in the works, so we may have independent verification relatively soon.

Cold and maybe dark

The detection itself is an impressive feat and will help verify our theoretical models of the conditions of the Universe when the first stars were formed. But, as mentioned above, the signal also contains information about those conditions. And that information is pretty weird.

The amount of energy absorbed is going to be related to the temperature of the Universe's gas at the time the first stars ignited. And here, the data provides a bit of a surprise: the absorption was twice the amount predicted by even the more extreme models of the early Universe. That means that the gas present in the Universe cooled much faster than we had expected.

Why is that weird? Because almost everything we know about in the Universe was hot at the time. There is no obvious mechanism to cool the gas down.

The EDGES team discusses this issue extensively, but an accompanying theoretical paper by Tel Aviv University's Rennan Barkana lays out the issue and its solution pretty succinctly. "The extra cooling indicated by the data is possible only through the interaction of the [gas] with something even colder," Barkana writes. "The only known cosmic constituent that can be colder than the early cosmic gas is dark matter." In fact, nearly everything other than particles that have been proposed to account for dark matter's gravitational effects, such as primordial black holes, should be heating the gas up.

The problem is that cooling the gas requires dark matter to interact with it, and dark matter as currently understood doesn't do much interacting with anything. Plus calculations based on the cooling suggest that the dark matter particles should be far lighter than we expect them to be—a small fraction of the mass of the Higgs boson.

It's a bit of a cliché in science reporting to say we need more data. But we really need more data here. It would be good to push the significance of this signal up to where we could be fully confident that we had a discovery, instead of it just looking extremely probable. And we'd definitely want some indication that the amount of energy absorbed is really double that predicted by theorists. Barkana also shows that the signal should be distributed unevenly in space, with hot and cold spots. To pick these up, we'll probably want to place an instrument in space—perhaps orbiting the Moon so that it is partly shielded from radio sources on Earth.

Until we have more data, though, the theoreticians are going to have a field day with these results.

Nature, 2018. DOI: 10.1038/nature25792, 10.1038/nature25791 (About DOIs).