CHIME picks out a 9-billion-year-old hydrogen glow using only its own radio data

The galaxy surveys CHIME used to check itself against are sharper instruments. They observe light from galaxies in much greater detail, but according to the University of British Columbia's announcement they mainly see regions hot and dense enough for stars to form ^[16][7]. CHIME measures something broader: the combined radio emission of the hydrogen itself ^[8]. Measuring that combined signal, the release says, lets the telescope examine much larger regions of the cosmos and reach farther back in time ^[8].

Hydrogen is the target because of how much of it there is. "Hydrogen is the most common element in the universe and the raw material from which stars form," said Arnab Chakraborty, a University of Toronto postdoctoral fellow and co-author. "Its faint radio emission acts like a cosmic tracer, revealing how matter is distributed across space." ^[11]

Mark Halpern, a UBC physics professor and CHIME's principal investigator, put the result in national terms. "This is a completely new technique for probing the cosmos, delivered by an instrument that was conceived, built and funded by Canadians," he said ^[12].

The first physical number from the data is a hydrogen census. "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 Shabbir Shaikh, an Arizona State University postdoctoral fellow and co-author of an accompanying paper ^[10]. That leaves about 98 percent in some other state at the time ^[17]. In my view "broadly consistent" is the most useful phrase in the announcement. A new instrument's first number tests the instrument as much as the universe. Agreement with independent measurements is the closest thing this result has to a control.

The dark-energy case is a longer chain. As the release lays it out, astronomers chart how hydrogen was spread through the earlier universe, use that large-scale structure to reconstruct how the universe expanded, and then use the expansion history to investigate dark energy ^[13]. This paper completes the first link with CHIME's observations alone ^[1]. For the rest, the release uses the conditional: the result "could" give scientists a faster way to investigate dark energy ^[6].

The thing the announcement doesn't tell you is how strong the detection is. Shaikh's two percent comes without an error bar in the release, and the university's summary of the detection does not state a significance level ^[10].