CHIME Detects 21 cm Hydrogen Signal to Probe Dark Energy

Estimated reading time: 5 minutes · Last updated: 10 October 2026

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) has reported a detection of the cosmological 21 cm hydrogen signal using only its own autocorrelation data, giving radio astronomers a new route to trace large-scale structure and probe dark energy. The measurement rests on 94 nights of observations taken in 2019 and is reported at roughly redshift z ∼ 1; the project team framed the finding as a 9‑billion‑year‑old signal while the analysis also discusses emission from an epoch when the universe was about five billion years old. As first reported by the University of British Columbia, the result appears in The Astrophysical Journal and marks a milestone for CHIME's original mapping goals.

Hydrogen is the most common element in the universe and the raw material from which stars form,

Dr. Arnab Chakraborty, co‑author

How CHIME pulled a 21 cm signal from its own data

CHIME is a fixed‑dish radio telescope near Penticton, British Columbia, designed to map the sky at frequencies sensitive to the hydrogen 21 cm transition. Rather than locating individual galaxies, CHIME measures the combined radio emission from neutral hydrogen across large volumes of space. The new result uses autocorrelation — a technique that analyses each antenna's self‑signal rather than cross‑correlating pairs — and the team reports a cosmological measurement at about redshift z ∼ 1 based on 94 nights of observations taken in 2019.

Autocorrelation was long considered challenging because an instrument's own response and terrestrial interference can mimic a faint cosmological signal. The CHIME collaboration applied new data‑processing steps and rigorous tests over more than a year to discriminate instrumental and human‑made radio interference from genuine sky emission. The finding is published in The Astrophysical Journal in two companion papers that present the detection and an interpretation of the autopower spectrum.

Why a 21 cm detection matters for dark energy

Measuring how matter is distributed at different epochs lets cosmologists infer how the Universe's expansion rate changed over time; that history carries information about dark energy. CHIME's 21 cm approach maps hydrogen directly across vast volumes instead of assembling galaxy catalogues one object at a time. That reach lets the telescope probe cosmic expansion at redshifts around z ∼ 1 while using a single instrument and, the team argues, at much lower cost than surveys that 'build' large samples of galaxies and typically require tens of millions of dollars.

Because the method samples larger scales, it yields complementary information to traditional optical surveys. The CHIME papers show the detected hydrogen clustering can test models of how structure grew and, by extension, provide an independent constraint on hypotheses for dark energy. The collaboration frames the discovery as opening a new window: it makes it possible to cross‑check dark energy inferences made from other tracers and to search for departures from standard cosmological models.

Verification, foregrounds and the work that remained

Extracting a cosmological 21 cm signal from radio data requires separating it from far stronger foregrounds: Galactic synchrotron emission, point sources, human transmissions and instrument systematics. The CHIME team reports they spent over a year subjecting the candidate detection to tests for these contaminants before concluding the signal persisted. The paper set out tests of spectral smoothness, instrument modelling and null tests to reduce the chance of a false alarm.

The collaboration emphasizes conservative analysis choices and crosschecks. Even so, autocorrelation measurements are sensitive to subtle calibration errors, and the authors lay out systematic uncertainties in the autopower spectrum interpretation. Independent confirmation, including comparison with other instruments or cross‑checks against galaxy surveys where overlap exists, will be central to turning this detection into precision cosmology.

Next steps: seven years of CHIME data and wider implications

CHIME now has nearly seven years of observations available for further analysis, and the collaboration intends to extend the autocorrelation technique to earlier cosmic times. The team states a goal of pushing the method toward epochs corresponding to when the universe was about three billion years old. Achieving that will expand the redshift baseline available to measure cosmic expansion and structure growth.

Beyond dark energy, the measurement constrains galaxy formation models because clustering of neutral hydrogen traces how matter organized into the structures that later hosted stars. The papers list specific follow‑ups: refine the autopower measurement across more nights, quantify remaining systematics, and compare results with independent surveys. If the method scales as the authors expect, radio intensity mapping could become a routine, cost‑effective probe used alongside optical surveys to sharpen constraints on cosmology.

How this finding could play out

The bottom line

CHIME's autocorrelation detection of the cosmological 21 cm hydrogen signal — based on 94 nights from 2019 and reported at roughly z ∼ 1 — promises a lower‑cost, wide‑volume way to map matter and test dark energy. The finding is a technical milestone but not the endpoint: the collaboration acknowledges significant systematic work remains and plans to exploit nearly seven years of data to push earlier in cosmic time. Independent confirmations and careful instrument modelling will determine whether intensity mapping becomes a routine, precision cosmological tool alongside optical surveys.

What to watch

• Watch for the team's expanded analysis of CHIME's nearly seven‑year dataset; no date has been set.
• Watch for follow‑up papers that push the autocorrelation method toward epochs when the universe was about three billion years old; no date has been set.