Hydrogen may be the most common element in the cosmos, but scientists could use it to do something unique: uncover the nature of dark energy, the mysterious force driving the accelerating expansion of the universe.
This new line of dark energy investigation comes from the fact that researchers have determined that the CHIME (Canadian Hydrogen Intensity Mapping Experiment) project can detect the faint radio glow of hydrogen from the universe's distant past.
Not only does this represent a faster and much less expensive way to investigate dark energy, but it is a major milestone for CHIME. The instrument was specifically devised to investigate dark energy and its pervasive influence.
"This is a completely new technique for probing the cosmos, delivered by an instrument that was conceived and built by Canadians," CHIME principal investigator Mark Halpern of the University of British Columbia said in a statement. "It's a bold new step in the global cosmology program and a Canadian success story."
Why is dark energy so problematic?
Dark energy is the mysterious force that is accelerating the expansion of the universe. It is so problematic for scientists because it started acting on the fabric of space only after the expansion triggered by the Big Bang had halted.
That's akin to pushing a child on a swing, watching it stop, and then seeing the swing start up again without a push. And, not only that, but the swing starts oscillating faster and faster. Consider scientists akin to very worried parents, who would really like to know where this ghostly push came from and how it is going to proceed.
Just as, if the push on the swing continues, the child could end up in the bushes, or in orbit, the strengthening of dark energy could result in the cold death of the cosmos, or even a "Big Rip" scenario that sees space shredded. Alternatively, if it weakens and allows gravity to reassert dominance, then the universe could collapse back in on itself, resulting in a "Big Crunch" ending that sees the cosmos compressed back into a singularity, as it was at the beginning of time.
Dark energy is now the dominant element of the universe, accounting for 68% of the universe's matter-energy budget. It's little wonder that scientists are very keen to know what this mysterious force is.
So how can hydrogen help the investigation of dark energy?
"Hydrogen is the most common element in the universe and the raw material from which stars form," team member Arnab Chakraborty of the University of Toronto said. "Its faint radio emission acts like a cosmic tracer, revealing how matter is distributed across space."
Previously, CHIME could map the distribution of hydrogen in the early universe but only by cross-correlating its observations with galaxy survey data from other telescopes, operations that cost millions of dollars and can only focus on regions of the universe dense enough to form stars. CHIME has now shown it can map the radio glow of hydrogen all on its own.
Indeed, the team has already begun making steps in this direction using just a tiny fraction of existing CHIME data. Using 94 nights of observations from 2019, they detected a hydrogen signal from when the universe was just 5 billion years old.
"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," team member Shabbir Shaikh of Arizona State University said. "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."
CHIME has now collected seven years of data, and using this, the team is hoping to probe the distribution of hydrogen back to 3 billion years after the Big Bang.
The team's research was published across two papers in The Astrophysical Journal.
