Mapping Cosmic Expansion with CHIME's Hydrogen Signals

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) uses Hydrogen Intensity Mapping to observe faint radio signals from hydrogen gas, charting the universe's expansion history and investigating dark energy. This innovative technique leverages Baryon Acoustic Oscillations as a cosmic standard ruler, offering a faster, more cost-effective alternative to traditional galaxy surveys.

Marcus Cole

October 1, 2026 · 5 min read

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope is charting the universe's expansion history by observing faint radio signals emitted by hydrogen gas. This innovative approach, known as Hydrogen Intensity (HI) Mapping, allows astronomers to create large-scale maps of the cosmos and investigate the mysterious force of dark energy, which is believed to be accelerating the universe's expansion.

CHIME's methodology offers a faster and more cost-effective way to survey vast cosmic volumes compared to traditional methods. By focusing on the distribution of hydrogen and leveraging Baryon Acoustic Oscillations (BAO) as a cosmic "standard ruler," CHIME provides a unique window into the early universe and its evolution, offering independent results that can help resolve contradicting theories about dark energy.

Unveiling Cosmic Expansion with CHIME

The CHIME telescope, a collaborative project involving scientists from the University of British Columbia, McGill University, the University of Toronto, the Dominion Radio Astrophysical Observatory, and other North American institutions, was specifically designed to map the distribution of hydrogen gas in the early universe. This mapping allows astronomers to calculate the universe's expansion rate and investigate dark energy, the enigmatic force thought to be driving the universe to expand at an ever-increasing pace.

By observing hydrogen, the most common element in the universe and the raw material for star formation, CHIME aims to reconstruct the universe's expansion history. This endeavor is crucial for testing competing models of dark energy's behavior and addressing one of the biggest open questions in physics. The telescope's ability to directly map the earliest glow of hydrogen paves the way for a faster, less expensive method to study this cosmic mystery.

Hydrogen Intensity Mapping: CHIME's Observational Technique

CHIME employs a technique called Hydrogen Intensity (HI) Mapping to observe the universe. Instead of resolving individual galaxies, CHIME detects the combined radio glow emitted by neutral atomic hydrogen gas across vast cosmic distances. This faint emission, specifically the 21-centimeter (21cm) radio line, allows the telescope to create a three-dimensional map of the universe's large-scale structure.

This method enables CHIME to map the distribution of hydrogen between redshifts of 0.78 and 1.43, corresponding to a period when the universe was between approximately 2.5 and 7 billion years old. The CHIME Collaboration has reported the first 21cm intensity mapping measurements made with an interferometer, constraining the effective clustering amplitude of neutral hydrogen. The ability to isolate and map this hydrogen signal independently, without relying on other surveys to pinpoint locations, marks a fundamental achievement for the telescope, as reported by researchers examining the observed signal.

This technique is well-suited for measuring structure on large physical scales because it avoids reliance on the complicated physics of galaxy formation, as individual galaxies are not resolved and counted. This makes CHIME's measurements of large-scale structure more direct and less prone to astrophysical uncertainties inherent in galaxy surveys.

How CHIME Maps the Universe

CHIME's approach to mapping cosmic expansion integrates several key elements, forming a conceptual framework for understanding the universe's large-scale structure and expansion history:

Component	Description of Role	Significance for Cosmic Mapping
Hydrogen Intensity (HI) Mapping	CHIME observes the faint 21cm radio emission from neutral atomic hydrogen gas across vast cosmic distances. This technique creates a 3D map of the universe's large-scale structure by measuring the distribution of hydrogen.	This method is significantly faster and allows for a much larger survey volume compared to traditional galaxy surveys, enabling CHIME to map over 3% of the total observable universe volume during an epoch largely unobserved.
Baryon Acoustic Oscillations (BAO)	BAO are relic spherical shells of matter over-density, originating from pressure waves in the early universe. These shells represent regions where galaxies and gas are more likely to be found today.	The characteristic distance scale of BAO, established in the early universe, acts as a 'standard ruler' to measure the universe's expansion rate. This scale is detectable in the distant distribution of hydrogen gas and helps reconstruct expansion history.
Independence from Galaxy Physics	CHIME's method avoids reliance on the complicated physics of galaxy formation, as it does not resolve or count individual galaxies. This makes it well-suited for measuring structure on large physical scales.	This independence provides direct and independent results for large-scale structure measurements, reducing astrophysical uncertainties often present in galaxy surveys.
Dark Energy Investigation	By mapping hydrogen distribution and BAO, CHIME provides a new way to test understanding of cosmic expansion and investigate dark energy, the mysterious force driving accelerated expansion.	This capability contributes to resolving contradicting theories about the nature of dark energy by offering precise measurements of how the universe's expansion rate has changed over time.

Component

Description of Role

Significance for Cosmic Mapping

Hydrogen Intensity (HI) Mapping

CHIME observes the faint 21cm radio emission from neutral atomic hydrogen gas across vast cosmic distances. This technique creates a 3D map of the universe's large-scale structure by measuring the distribution of hydrogen.

This method is significantly faster and allows for a much larger survey volume compared to traditional galaxy surveys, enabling CHIME to map over 3% of the total observable universe volume during an epoch largely unobserved.

Baryon Acoustic Oscillations (BAO)

BAO are relic spherical shells of matter over-density, originating from pressure waves in the early universe. These shells represent regions where galaxies and gas are more likely to be found today.

The characteristic distance scale of BAO, established in the early universe, acts as a 'standard ruler' to measure the universe's expansion rate. This scale is detectable in the distant distribution of hydrogen gas and helps reconstruct expansion history.

Independence from Galaxy Physics

CHIME's method avoids reliance on the complicated physics of galaxy formation, as it does not resolve or count individual galaxies. This makes it well-suited for measuring structure on large physical scales.

This independence provides direct and independent results for large-scale structure measurements, reducing astrophysical uncertainties often present in galaxy surveys.

Dark Energy Investigation

By mapping hydrogen distribution and BAO, CHIME provides a new way to test understanding of cosmic expansion and investigate dark energy, the mysterious force driving accelerated expansion.

This capability contributes to resolving contradicting theories about the nature of dark energy by offering precise measurements of how the universe's expansion rate has changed over time.

Advantages Over Traditional Galaxy Surveys

CHIME's Hydrogen Intensity Mapping offers several significant advantages over traditional galaxy surveys. Galaxy surveys typically use visible light and focus on fine details, often costing millions more dollars and concentrating only on parts of the universe hot and dense enough to form stars. In contrast, CHIME's method is much faster and allows for a substantially larger survey volume, mapping over 3% of the total observable universe volume, an epoch largely unobserved by other large-scale structure surveys.

A crucial benefit of CHIME's approach is its independence from the complicated physics of galaxy formation. Since it does not resolve or count individual galaxies, it avoids the astrophysical uncertainties inherent in galaxy surveys, making its measurements of large-scale structure more direct. This allows CHIME to explore cosmic expansion at a greater scale, further back in time, at a fraction of the cost, and without relying on other telescopes' results for cross-correlation, as it had to do previously.

This operational independence, demonstrated by CHIME's standalone detection of hydrogen glow, means the telescope can isolate hydrogen signals without needing another survey to point out where to look. This capability is a fundamental hurdle cleared for a telescope designed to map the universe through hydrogen, enhancing its efficiency and the reliability of its independent findings.