The nature of the radio waves Bowman and his colleagues detected mostly matches theoretical predictions, but not everything lines up. When they tuned their instrument to listen to the frequency for hydrogen gas that models predicted, they didn’t hear anything. When they decided to search in a lower range, they got it. But the signal they found was stronger than expected. That meant that the hydrogen gas in the early universe was much much colder—perhaps nearly twice as cold—than previously estimated.
This is where things get even more interesting. Only two things could explain why the temperature of the gas was lower than predicted. There may have been more background radiation present after the Big Bang than astronomers previously thought, but they say that’s unlikely. The more plausible explanation involves an interaction between hydrogen gas and another cosmic mystery: dark matter, the invisible stuff that scientists say makes up most of the universe. “The idea is that the dark matter is colder than the gas,” Frebel says. “By interacting, the gas transfers some heat to the dark matter to warm it up a little. Hence the gas we observe has been cooled.”
This possibility is explored in a second paper, also published in Nature on Wednesday, by Rennan Barkana, an astrophysicist at Tel Aviv University who first proposed this potential interaction. If Barkana’s theory bears out, scientists may be able to figure out some of the properties of these dark-matter particles.
This would be huge for the field. The existence of dark matter has so far only been inferred indirectly from observational data. Astronomers can see the effects of dark matter by studying certain gravitational effects on the light of massive clusters of galaxies. But these effects only allow astronomers to map big chunks of dark matter, not individual particles, says Priyamvada Natarajan, a theoretical astrophysicist at Yale, who was not involved in the research.
“It’s like looking at a huge sand dune,” Natarajan says. “You know how the sand dune is assembled, you can show how it dissipates in the wind, how it’s washed out by water. But you don’t know what a grain of sand is made of.”
Natarajan says the study of the early signal detected by Bowman and his team could help theoretical astrophysicists uncover new, fundamental physics beyond our current understanding. In order for the observed cooling to make sense, the dark-matter particle would need to be much lighter than previously thought.
“Theorists are going to be busy,” Natarajan says.
Bowman says other teams around the world have been working to build and design instruments to detect this signal from the early universe, and he expects they should be able to confirm the results in the coming months.
“Only confirmation by other groups and experiments will truly usher in the new era of detecting hydrogen from the very first observable phase in the universe,” Frebel says. “As exciting and informative it is, we need to be cautious until it has been confirmed by others before entering it into the textbooks.”