A simple experiment has detected a signal from the first stars forming just 180 million years after the Big Bang. The observations have intriguing implications for the nature of dark matter.
The first stars began to shine just 180 million years after the Big Bang, according to new observations by a team of American radio astronomers. The evidence comes from observations of neutral hydrogen gas that pervaded the early universe. But surprisingly, the same observations show an unexpected chill in this gas — a result that could hint at non-gravitational interactions with dark matter.
“This is a really cool result,” says Michiel Brentjens (Netherlands Institute for Radio Astronomy), who was not involved in the study. “It’s an important first step in revealing how the very early universe behaved.”
For years, astronomers like Brentjens have been trying to detect the early universe’s neutral hydrogen via its telltale 21-centimeter radio emission. In particular, they want to see how the energetic radiation from the very first stars and galaxies heated and ionized surrounding gas. This occurred during the so-called Epoch of Reionization (EoR), when bubbles of ionized gas grew and spread across the universe, some 300 to 500 million years after the Big Bang.
However, success has eluded them so far. As the radio waves travel through expanding space for more than 13.7 billion years, they are stretched to wavelengths of a couple of meters, corresponding to difficult-to-detect frequencies below 200 megahertz. Moreover, the signal is swamped by foreground sources, such as radiation from electrons spiraling around magnetic field lines in our Milky Way Galaxy, man-made radio interference, and instrumental noise.
Detecting the First Stars
Now, Judd Bowman (Arizona State University), Alan Rogers (MIT Haystack Observatory), and their colleagues have tuned in to the infant universe at even lower frequencies, between 50 and 100 megahertz, corresponding to a longer look-back time. They used a relatively cheap detector called EDGES (Experiment to Detect the Global Epoch of reionization Signature), funded by the National Science Foundation and situated in the radio-quiet Australian Outback. About the size of a large desk, EDGES is extremely well calibrated to these lower frequencies. The simple setup also contains as few electronics as possible to prevent low-frequency interference.
At that very early stage in cosmic evolution, corresponding to redshifts between 15 and 20, the cosmic microwave background — the Big Bang’s fading afterglow — would have been hotter than the all-pervasive neutral hydrogen gas. As a result, the gas would show up not by emitting 21-centimeter radio waves, but by absorbing them.
As the team reports in the March 1st issue of Nature, EDGES successfully detected this absorption feature by averaging measurements across the sky. The subtle dip in the radio spectrum is centered at a frequency of 78 megahertz (a wavelength of 3.84 meters). If this is indeed a redshifted 21-centimer absorption signal, it corresponds to an epoch just 180 million years after the Big Bang. According to Rogers, this is the earliest direct detection of the hydrogen gas.
Brentjens is impressed by the team’s result. “They have succeeded in circumventing most of the instrumental noise that is so worrisome at these very low frequencies,” he says. “I hope it won’t be too long before we also detect the emission signal of the hydrogen gas from a somewhat later epoch.” Seeing the neutral gas by its emission rather than absorption would provide valuable additional information on precisely when and how reionization progressed across the universe.
Instruments on the lookout for this emission include the LOFAR telescope (Low-Frequency Array) in the Netherlands, the Murchison Wide-field Array in Western Australia, and HERA (Hydrogen Epoch of Reionization Array) in South Africa. As of yet, these facilities haven’t yet detected the emissions signal, though they’re getting close.
The discovery of the absorption signal by EDGES implies that the very first stars must already have formed when the universe was just 180 million years old, explains Bowman.
“After stars form, their ultraviolet light alters the energy states of the hydrogen atoms and knocks them out of equilibrium with the microwave background,” he says. “That causes the hydrogen to absorb some of the background radiation, creating the small dip that we’re able to observe. Without the stars, the hydrogen wouldn’t be able to produce this signal.”
Hints of Dark Matter
To the scientists’ surprise, the absorption they observed was stronger than expected. Since the temperature difference between the cosmic background radiation and the hydrogen gas determines the depth of the absorption feature, a deeper dip could mean either that the background radiation was hotter than expected or that the hydrogen gas was cooler than expected.
It’s hard to imagine how the cosmic microwave background could have been hotter than the expected 50K (50 degrees above absolute zero) at a cosmic age of 180 million years, as we can measure this background extremely precisely. Instead, the neutral hydrogen gas in the early universe must have been cooler than theory predicts: just over 3K instead of the expected 7K.
In a companion paper in the same issue of Nature, theoretical astrophysicist Rennan Barkana (Tel Aviv University, Israel) argues that the gas might have undergone additional cooling by interacting non-gravitationally with dark matter particles. Indeed, most dark matter theories do predict some very weak interactions between “normal” matter and dark particles, through collision and scattering. Based on the EDGES results, Barkana predicts a relatively low mass for dark matter particles — at most a few times the mass of a proton — and also relatively low velocities. “These results indicate that 21-centimeter cosmology can be used as a dark-matter probe,” he writes.
Rogers is open to the idea. “So far I think that dark matter interactions are the only proposed way of getting the temperature low enough to obtain the observed amount of absorption.” But he adds, “I think it’s still too early to draw firm conclusions.”
Given the incredible difficulty of detecting the absorption signal at all, his caution is understandable. After all, at 78 megahertz, the cosmic background radiation is some 10,000 times weaker than all the combined sources of foreground noise. As Peter Kurczynski (National Science Foundation) comments: “It’s like being in the middle of a hurricane and trying to hear the flap of a hummingbird’s wing.”
“We were one of the first groups to get started on this technique,” says Bowman. After 12 years of measurements and two years of tests to rule out instrumental errors, he next wants to see the same results repeated. “Several other groups around the world have set up similar experiments and are close to making the same measurement. The next step in the scientific process is for another group using a different instrument to confirm our detection.”