Hydrogen’s distant glow opens new way to investigate dark energy

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) has shown for the first time that it can detect the faint glow of hydrogen gas from deep in the universe’s past using only its own data.

The breakthrough paves the way for a faster, less expensive way to study dark energy—the mysterious force thought to be driving the universe to expand at an ever-increasing rate and one of the biggest open questions in physics. The achievement also marks a milestone for the telescope, which was built for this purpose. The findings are 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, a 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.”

There are conflicting theories about the nature of dark energy in the astrophysics community. Using its own data, CHIME can investigate these theories independently and help prove or disprove them.

“This is a completely new technique for probing the cosmos, delivered by an instrument that was conceived, built by Canadians,” said co-author Dr. Mark Halpern, a 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 hydrogen’s glow

CHIME is a radio telescope near Penticton, British Columbia, hosted by the National Research Council of Canada (NRC). It maps the entire northern sky every day.

It is a pan-Canadian research project built and operated by scientists at the University of British Columbia, McGill University, the University of Toronto and the Dominion Radio Astrophysical Observatory (NRC), as well as other North American collaborators, including Arizona State University.

CHIME was built to map the distribution of hydrogen gas in the early universe, allowing astronomers to calculate its expansion and investigate dark energy, the mysterious force thought to be driving the universe to expand faster over time.

Previously, CHIME had to cross-correlate its observations with galaxy survey data from other telescopes. Galaxy surveys investigate the same question using light to study galaxies in fine detail. They cost millions more dollars and focus only on the part of the universe hot and dense enough to form stars.

By mapping the combined radio glow that hydrogen emits on its own, CHIME can explore the same questions on a greater scale, further back in time, at a fraction of the cost and without relying on anyone else’s results.

Signal interpretation

In an accompanying paper, the researchers examined what the observed signal reveals about the distribution of hydrogen in the universe.

“Our data indicate that roughly 2% 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.”

Delayed celebration

The finding wasn’t a eureka moment. The researchers applied new data analysis and processing techniques to find the faint signal amid overwhelming noise from the background universe, human technology and even the instrument itself. They then spent more than a year testing the finding to confirm it was a signal from the universe when it was about 5 billion years old. The finding is based on data from 94 nights of observations collected in 2019.

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

“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.”

Seven years of data

The current measurement uses only a small fraction of the data CHIME has collected since operations began. Researchers now have nearly seven years of observations available and are working to expand the analysis to include earlier periods in cosmic history when the universe was only 3 billion years old.

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