The result could give scientists a faster and less costly way to investigate dark energy, the still mysterious phenomenon believed to be accelerating the expansion of the universe. Understanding dark energy remains one of the biggest unresolved problems in modern physics. The achievement is also an important milestone for CHIME, which was originally designed with this type of measurement in mind. The findings were published in The Astrophysical Journal.

"Hydrogen is the most common element in the universe and the raw material from which stars form," said co-author Dr. Arnab Chakraborty, postdoctoral fellow at the University of Toronto who first proposed the finding. "Its faint radio emission acts like a cosmic tracer, revealing how matter is distributed across space."

Scientists have proposed competing explanations for the nature of dark energy. Because CHIME can now make these measurements using its own data, researchers can test those ideas independently and gather evidence that could support or challenge existing theories.

"This is a completely new technique for probing the cosmos, delivered by an instrument that was conceived, built and funded by Canadians," said co-author Dr. Mark Halpern, professor in the UBC department of physics and astronomy and CHIME principal investigator. "It's a bold new step in the global cosmology program and a Canadian success story."

Mapping the Universe With Hydrogen

CHIME is a radio telescope located near Penticton, British Columbia, and hosted by the National Research Council of Canada (NRC). The instrument surveys the entire northern sky every day.

The project brings together researchers from across Canada, including scientists at the University of British Columbia, McGill University, the University of Toronto, and the Dominion Radio Astrophysical Observatory (NRC). Other North American collaborators, including Arizona State University, also participate in the effort.

One of CHIME's main goals is to chart how hydrogen gas was distributed in the earlier universe. By measuring that large-scale structure, astronomers can reconstruct how the universe expanded over time and use those measurements to investigate dark energy.

Until now, CHIME researchers needed to compare their radio observations with galaxy survey data collected by other telescopes. Those surveys address similar cosmological questions by observing light from galaxies in much greater detail. However, they can cost millions of dollars more and mainly probe regions of the universe that are hot and dense enough for stars to form.

CHIME instead measures the combined radio emission produced by hydrogen itself. That approach allows researchers to examine much larger regions of the cosmos, reach farther back in time and pursue similar questions at a fraction of the cost, without depending on data from other surveys.

What the Hydrogen Signal Reveals

In an accompanying paper, the researchers analyzed what the newly detected signal can tell scientists about how hydrogen was distributed throughout the universe.

"Our data indicate that roughly two per cent of the hydrogen in the universe was in neutral atomic form at this time, broadly consistent with other measurements," said co-author Dr. Shabbir Shaikh, postdoctoral fellow at Arizona State University. "By measuring how that hydrogen is distributed and clustered, CHIME gives us a new way to test our understanding of how galaxies form and evolve."

The measurement therefore offers more than a new way to study cosmic expansion. It can also help researchers test models of galaxy formation and better understand how matter became organized across the universe.

Finding a Signal Buried in Cosmic Noise

The discovery did not arrive as a sudden breakthrough. Researchers had to use new data processing and analysis methods to extract the extremely weak hydrogen signal from much stronger sources of interference.

That background included radio noise from the universe itself, signals produced by human technology, and even effects created by the telescope. After identifying the possible hydrogen signature, the team spent more than a year testing it to make sure the result was genuine.

Their analysis ultimately showed that the signal came from hydrogen in the distant universe when the universe was about five billion years old. The result was based on 94 nights of observations collected in 2019.

"We worked very hard to convince ourselves that this wasn't a false alarm," said Dr. Chakraborty. "After all the tests, the signal remained. That gave us confidence we were seeing real hydrogen from the distant universe."

Seven Years of CHIME Observations Remain

The new result draws on only a small portion of the information CHIME has gathered since it began operating.

Researchers now have nearly seven years of observations available for analysis. They are working to extend the technique farther into cosmic history, with the goal of studying an era when the universe was only three billion years old.

"For a long time, astrophysicists have believed there is great potential in this hydrogen mapping technique with this kind of telescope. By actually showing that the technique works in practice, we've opened up a whole new window on the universe. We can use it to test our current theories and learn new things about galaxies and other properties of the universe," said co-author Dr. Simon Foreman, assistant professor at Arizona State University.

This project is funded by the Canada Foundation for Innovation, the National Research Council of Canada, the Natural Sciences and Engineering Research Council and the provinces of British Columbia, Ontario and Quebec, and supported by the Digital Research Alliance of Canada.