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
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, t