Scientists detect signal from ‘cosmic dawn,’ when stars first lit up the universe gasbuddy touch


For millions of years after it began, the universe was a chilly and lightless place. Its only contents were tenuous, invisible clouds of inert hydrogen gas. Its only energy was the radiation left over from the Big Bang — a persistent hum known as the cosmic microwave background.

No telescope today can detect the glow of these primitive stars. But theoretical models had suggested that their ultraviolet radiation would have pierced the hydrogen haze of the early universe, exciting the atoms within. This in turn would cause the atoms to absorb a small sliver of the cosmic microwave background radiation, imprinting it with a faint but indelible signature.

In a 1999 study, astrophysicists proposed that researchers could detect this signature by searching the entire sky for the muted wavelengths, which would appear as a dip in the radio end of the electromagnetic spectrum. Such a project would require only a small antenna but incredibly careful analysis — the dip overlaps with the most popular frequencies on the FM dial, as well as many natural radio signals. Peter Kurczynski, a program director for the National Science Foundation, has compared the effort to turning on every radio station but one simultaneously, then listening to figure out which station was missing.

A decade later, the NSF opted to fund the construction of an antenna in the Australian desert to listen for this signal. The instrument was deceptively simple, just two metal plates perched atop a dining room-table-type structure and situated at the center of a 30-meter-wide mesh carpet. But it sensed everything: the cacophony of human communication, the din of radio waves coming from the Milky Way and, beneath it all, the subtle buzz of the cosmic microwave background.

“The cosmological signal is … 0.1 to 0.01 percent of the total amount of radio power we’re received,” said Judd Bowman, an astrophysicist at Arizona State University and the lead author of the first Nature study. But by subtracting out the louder, closer signals, Bowman and his colleagues could pin down the signature of emerging stars emanating from the moment 180 million years after the Big Bang.

To ensure their signal was real, rather than a quirk of their instrument or a fluke in their data, Bowman and his colleagues spent two years considering and discarding a range of alternative explanations. They repositioned the antenna, tested it in a lab with simulated radio sky, even built a copy of the instrument to demonstrate that the experiment was reproducible.

The stars whose light led to the newly detected signal were very different from the ones that glitter in our night sky. Comprising just hydrogen and helium — the only elements that existed in the early universe — they burned blue, bright and fast. But when they died, they produced the explosions that gave rise to the heavier elements, including the carbon and oxygen atoms on which all life depends.

He and his colleagues were surprised to discover that the signal was twice as large as predicted, meaning the hydrogen fog must have absorbed more radiation than was thought possible. Bowman’s team approached Rennan Barkana, an astrophysicist at Israel’s Tel Aviv University, and asked for his take.

He noted that other astrophysicists may come up with differing explanations for the signal’s size once they’ve had time to analyze it. Meanwhile, two teams of researchers operating instruments similar to Bowman’s will attempt to confirm the cosmic dawn signal.

In an analysis for Nature, Harvard University astronomer Lincoln Greenhill imagined the possibilities raised by more and better detections: Scientists could use it to map the three-dimensional structure of the cosmos in this era. They could draw inferences about the “dark ages” before the first light sources emerged. They could probe the role of dark matter in creating the density fluctuations that allowed objects to form amid the dispersed hydrogen fog.