Cosmic records of the earliest days of the universe, 13.8 billion years ago, are starting to accumulate. Soon, we may have all the clues we need to piece together the events that transformed hydrogen and cosmic energy to the blooming universe astronomers study today.
Today (Feb. 28), scientists announced the discovery of some of the earliest traces of radiation left over from the Big Bang ever recorded. Using highly sensitive instruments, they found hydrogen atoms that had captured radiation from the formation of the universe’s very first stars. These stars formed about 180 million years after the massive event that transformed the universe from a mass of hydrogen and energy into organized galaxies.
Atoms can have different energy levels. At a baseline level, the atom’s nucleus (composed of protons and neutrons) holds onto its electrons at a particular distance. When atoms are exposed to more energy, through higher temperatures or radiation, their electrons may be pulled farther away from the nucleus. This state is called an excited state. Usually, atoms in an excited state will intermittently fluctuate back down to their ground state.
When the first stars formed, they gave off ultraviolet energy at precisely the right amount to bring some hydrogen atoms in the universe at the time into an excited state. In this excited state, these atoms absorbed some of the background radiation left over from the Big Bang.
For the billions of years since then, these hydrogen atoms have been floating around space, appearing nondescript among all the newer gases and galaxies. Scientists have long thought they could find these ancient hydrogen atoms if they listened for a precise radio frequency in the universe. This exact frequency is the energy the atoms give off when their electrons jump between their ground state and a specific excited state.
But space is a noisy place. Cosmic radiation from other stars and galaxies makes picking out a particular signal from a particular atom similar to hearing a single heartbeat at a music festival. So lead researcher Judd Bowman at Arizona State University and his team took a highly sensitive antenna-and-receiver out to a remote Australian desert to find the quietest place on Earth to see if they could identify the ancient-hydrogen frequency.
Their findings, which were published in the journal Nature, were the result of 12 years of work. “We now know there were stars about 180 million years [after the Big Bang],” says Bowman. But, he adds, they also discovered “something mysterious happening” was happening in the universe at that time. Although the hydrogen atoms’ signal turned up about where theoretical models had predicted it would, it was twice as loud as expected.
There are two working explanations for this surprise discovery: Either hydrogen at this point after the Big Bang was much colder than hypothetical calculations have led scientists to believe, or the background radiation temperature was hotter.
In a separate paper published simultaneously in Nature, Rennan Barkana, an astrophysicist at Tel Aviv University in Israel, argues that since current known astrophysical phenomena are not likely to account for either of these temperature discrepancies, the best explanation is that dark matter interacted with the atoms to cool them down.
At the moment, scientists don’t know a lot about dark matter. “It has mass—that’s how we infer that it’s there,” says Bowman. “But it doesn’t seem to interact with light,” like the rest of all matter that’s been studied on Earth. This research could provide another means to study this elusive material, in addition to the way that the universe itself formed.
This discovery is the kind of major progress in astronomy and physics that many scientists will want to see replicated before it’s accepted as true. “Although the signal in an absolute sense is not particularly small, there is contamination from other astronomical sources that is roughly 10,000 times brighter than the signal,” says Steven Furlanetto, an astrophysicist at the University of California-Los Angeles who is unaffiliated with either study. ‘There’s no doubt that confirmations from some of the other telescopes seeking this signal are necessary.”
Bowman and other astronomers have plans to try to replicate these results using larger radio telescopes in South Africa, hopefully getting even clearer signals from these ancient hydrogen atoms.