• They can cost millions of dollars more than CHIME's approach.
• They mainly probe regions that are hot and dense enough for stars to form.
• They cover smaller portions of the cosmos.

CHIME instead measures the combined radio emission produced by hydrogen itself. That lets researchers examine much larger regions of the universe, reach farther back in time, and pursue similar cosmological questions at a fraction of the cost — without depending on anyone else's data.

"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, a professor in the University of British Columbia's department of physics and astronomy and CHIME's principal investigator. "It's a bold new step in the global cosmology program and a Canadian success story."

What does the hydrogen signal reveal?

In an accompanying paper, the team analyzed what the newly detected signal says about how hydrogen was spread through 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, a 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 window on cosmic expansion. It can also help researchers test models of galaxy formation and better understand how matter became organized across the universe.

How hard was the signal to find?

The discovery did not arrive as a sudden breakthrough. The hydrogen signature is extraordinarily weak, buried under much stronger sources of interference: radio noise from the universe itself, signals produced by human technology, and even effects created by the telescope. The team had to develop new data processing and analysis methods to extract it.

After identifying the possible signature, the researchers spent more than a year testing it to make sure the result was genuine.

"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."

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

What comes next?

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, and they aim to push the technique farther back in cosmic history, targeting 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," said co-author Dr. Simon Foreman, an assistant professor at Arizona State University. "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."

The collaboration brings together researchers from UBC, McGill University, the University of Toronto and the Dominion Radio Astrophysical Observatory, with additional North American partners including Arizona State University. The project receives funding from 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, along with support from the Digital Research Alliance of Canada.