The significance of the achievement lies in what hydrogen can tell us about the largest questions in physics. Hydrogen is the most common element in the universe and the raw material from which stars form, and its atoms emit a characteristic radio signal at a wavelength of 21 centimetres. As that light travels across billions of light years, the expansion of the universe stretches it to longer wavelengths, carrying within it a record of how fast space was expanding when it was emitted. “Its faint radio emission acts like a cosmic tracer, revealing how matter is distributed across space,” said co-author Dr. Arnab Chakraborty, a postdoctoral fellow at the University of Toronto who first proposed the finding. By mapping where this glow is stronger or weaker across the sky, astronomers can reconstruct the large-scale structure of matter at epochs that are extraordinarily difficult to reach by any other means.

Dark energy sits at the heart of why this matters. In the late 1990s, observations of distant supernovae revealed that the universe is not merely expanding but expanding at an ever-increasing rate, a discovery that earned the Nobel Prize in Physics and upended the expectation that gravity would gradually slow cosmic growth. Whatever is responsible for that acceleration is called dark energy, and it currently accounts for roughly seventy per cent of the total energy content of the cosmos. Yet despite decades of effort, physicists still do not know what it is. Competing theories describe it in fundamentally different ways, from a constant energy density inherent to empty space to dynamical fields whose strength changes over cosmic time. Distinguishing between these possibilities requires measuring how the expansion rate of the universe has evolved across billions of years, which is precisely the kind of measurement CHIME was built to make.

CHIME is a radio telescope hosted by the National Research Council of Canada at the Dominion Radio Astrophysical Observatory near Penticton. Its distinctive design, four cylindrical reflectors arranged in a half-pipe configuration, gives it an enormous field of view and allows it to map the entire northern sky every day as Earth rotates beneath it. The project is a pan-Canadian collaboration built and operated by scientists at the University of British Columbia, McGill University, the University of Toronto and the Dominion Radio Astrophysical Observatory, together with North American partners including Arizona State University. From the outset, the instrument was conceived with one overriding scientific goal: to chart the distribution of hydrogen gas across the early universe, allowing astronomers to calculate how the cosmos has expanded and thereby investigate dark energy directly.

What makes the new result a genuine milestone is that CHIME achieved this detection autonomously. Previously, the collaboration had to cross-correlate its observations with galaxy survey data from other telescopes, using those optical catalogues as a reference to confirm that the telescope was genuinely seeing the cosmic hydrogen signal. Galaxy surveys investigate the same underlying questions about cosmic expansion, but they work in visible light and focus in fine detail on individual galaxies. That approach is extraordinarily expensive, costing millions of dollars more, and it carries a fundamental limitation: galaxies only exist in the parts of the universe that are hot and dense enough to have formed stars. By mapping the combined radio glow that hydrogen emits on its own, CHIME can explore the same questions at a greater scale, reach further back in cosmic time, operate at a fraction of the cost, and do so without relying on anyone else’s results.

The technique CHIME employs is known as hydrogen intensity mapping, and it is as elegant as it is technically punishing. Rather than resolving individual galaxies, the telescope measures the aggregate radio emission from all the hydrogen within large volumes of space, treating the gas as a continuous glowing background whose brightness fluctuations trace the clustering of matter. Because hydrogen pervades regions of the universe that never lit up with stars, intensity mapping can in principle survey cosmic epochs that optical surveys simply cannot reach. The trade-off is that the cosmological signal is astonishingly faint, buried beneath foregrounds that can be millions of times brighter, including radio emission from our own galaxy, from distant galaxies, and from human technology, as well as noise generated by the instrument itself. Extracting the signal demands extraordinary care in calibration, data processing and statistical analysis.

In an accompanying paper, the researchers examined what the observed signal actually reveals about the distribution of hydrogen in the universe. “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 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.” That figure carries real scientific weight, because the fraction of hydrogen remaining in neutral atomic form constrains models of how gas cools, collapses and ignites into stars across cosmic history. The consistency with independent measurements suggests the telescope is measuring something real about the intergalactic medium when the universe was about five billion years old.

The path to the detection was anything but a sudden flash of insight. The team applied new data analysis and processing techniques to tease the faint signal out of the overwhelming noise of the background universe, human technology and even the telescope itself, and then spent more than a year testing the result to prove it was correct. The measurement is based on 94 nights of observation data collected in 2019. “We worked very hard to convince ourselves that this wasn’t a false alarm,” said Dr. Chakraborty. “After all the tests, the signal remained. That gave us confidence we were seeing real hydrogen from the distant universe.” That deliberate, sceptical pace reflects the hard lessons of the field, where earlier claims of hydrogen intensity mapping detections have been contested, and where the difference between a cosmological signal and a stubborn instrumental artefact can be agonizingly subtle.

For the collaboration, the moment carries both scientific and national significance. “This is a completely new technique for probing the cosmos, delivered by an instrument that was conceived, built and funded by Canadians,” said co-author Dr. Mark Halpern, 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.” Co-author Dr. Simon Foreman, an assistant professor at Arizona State University, framed the broader implications: “For a long time, astrophysicists have believed there is great potential in this hydrogen mapping technique with this kind of telescope. 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.”

Perhaps the most striking aspect of the announcement is how much untapped data remains. The current measurement uses only a small fraction of what CHIME has collected since operations began, and researchers now have nearly seven years of observations available for analysis. The team is working to expand the analysis to earlier periods in cosmic history, targeting epochs when the universe was only three billion years old, when dark energy’s influence on cosmic expansion was far weaker and its underlying nature may be easier to disentangle from ordinary matter and gravity. If the technique continues to perform at scale, hydrogen intensity mapping could become one of the most cost-effective tools in cosmology, complementing billion-dollar galaxy surveys and offering an independent check on the theories that attempt to explain why the universe’s expansion is speeding up. The project is funded by the Canada Foundation for Innovation, the National Research Council of Canada, the Natural Sciences and Engineering Research Council and the provinces of British Columbia, Ontario and Quebec, with support from the Digital Research Alliance of Canada. What began as a Canadian experiment in the hills of British Columbia has now proven it can hear the universe’s oldest storyteller, and the story it tells is only beginning.

Subject of Research: Hydrogen intensity mapping of the early universe with the CHIME radio telescope to probe dark energy

Article Title: Canadian telescope directly maps earliest glow of hydrogen, opening a new window on the universe

Article References: Canadian telescope directly maps earliest glow of hydrogen, opening a new window on the universe. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: CHIME, hydrogen intensity mapping, dark energy, cosmology, 21-cm radio emission, cosmic expansion, radio telescope, large-scale structure, galaxy formation, neutral hydrogen, The Astrophysical Journal, University of British Columbia

That simple fact has driven years of careful prototyping, and the latest chapter in that effort has now been published in the Journal of Astronomical Telescopes, Instruments, and Systems. A team led by Dr. Shin’ichiro Asayama of the SKA Observatory built and tested a prototype station known as AAVS3, short for Aperture Array Verification System 3, and used real observations of the sky to evaluate a novel antenna layout borrowed from an unexpected source of inspiration: the sunflower. The work matters because even small, subtle losses in sensitivity, if baked into the design of hundreds of stations, would diminish the scientific reach of the entire multitelescope array.

The predecessor prototype, AAVS2, arranged its antennas in a scattered, pseudo-random pattern. Randomization of this kind is a classic technique in antenna engineering, deployed to suppress unwanted artifacts in the station’s response to the sky. But pseudo-random layouts carry their own liability. When antennas are placed too close to one another, they interact electromagnetically, a phenomenon known as mutual coupling. The current induced in one antenna by its neighbor distorts the carefully calibrated response of the whole station. Critically, this interference grows stronger at lower frequencies, becoming particularly troublesome below about 150 megahertz, a region squarely inside SKA-Low’s operating range.

To address the problem, researchers within the Observatory proposed a layout modeled on the spiral phyllotaxis pattern found in the seed heads of sunflowers, an arrangement known in mathematics as the Vogel layout. The sunflower pattern distributes points across a disk with a pleasing, near-uniform density while avoiding the rigid rows and columns that cause coherent artifacts. In the antenna context, the spacing inherent to the Vogel layout reduces mutual coupling between neighboring elements while preserving the station’s collecting area. Encouraged by simulations, the SKA Observatory constructed AAVS3 using 256 dual-polarized antennas in this sunflower-inspired configuration, creating a full-scale test bed for evaluating the design under genuine sky conditions rather than relying solely on computer models.

The verification campaign was deliberately thorough. The team combined simulations with on-sky observations to assess both sensitivity and calibration accuracy, the two quantities that ultimately define a radio telescope’s performance. AAVS3 was operated in two modes: as a beamformer, in which the antenna signals are phased and combined to point at a single direction on the sky, and as an imaging array capable of forming pictures of extended radio emission. To exercise both modes, the researchers pointed the station at a diverse set of celestial targets, including the Sun, bright cosmic radio sources, the extended glow of the Galactic Plane, and several pulsars, each of which stresses the instrument in different ways.

Calibration, the process of correcting raw data so that it faithfully reflects the sky, received equal attention. The team developed two independent methods. The simpler approach used the Sun as a known reference source, exploiting its precisely predictable position and substantial brightness to solve for the complex gains of every antenna and signal path. The second, more rigorous technique combined solar observations with a model of the sky’s overall brightness and detailed profiles of how each antenna responds to radiation from different directions on the sky. The added complexity paid off in one decisive way: unlike the Sun-only method, the fuller approach worked at night, when the Sun is below the horizon and unavailable as a reference.

It was during the construction phase that simulations delivered an unwelcome surprise. Modeling predicted a localized loss of sensitivity directly overhead at around 125 megahertz. The culprit was traced to repeated antenna spacings of roughly 2.4 meters that occur throughout the sunflower pattern. Because the Vogel construction repeats characteristic distances across the array, the signals from many antenna pairs arrive with systematic phase relationships, creating interference patterns that conspire to reduce the station’s response in particular directions and at particular frequencies. The older prototype, AAVS2, with its scattered pseudo-random layout, showed no such deficit, highlighting a genuine trade-off between the two design philosophies.

The decisive test came from the sky itself. When the Galactic Plane, the bright band of emission tracing the disk of the Milky Way, drifted directly overhead of AAVS3, the resulting images clearly displayed the same dip in sensitivity at 125 megahertz that the simulations had forecast. The agreement between prediction and observation confirmed that the sensitivity loss was not a fault in the electronics, the data processing, or the calibration method. It was an intrinsic consequence of the antenna geometry, a subtle signature of the sunflower’s mathematics written into the telescope’s response to the sky.

The team separately verified the station’s sensitivity at 230 megahertz, well above the problematic frequency. At night, the actual measurements matched predictions closely when the more comprehensive calibration method was applied, a strong endorsement of both the hardware and the analysis pipeline. Daytime measurements, however, showed larger discrepancies from the modeled values. The researchers attribute these gaps most likely to unusually high solar activity during the observation period, which made the Sun’s brightness harder to predict accurately and degraded the performance of a calibration that depends on treating the Sun as a stable reference source.

The practical consequence of the 125 megahertz finding was swift and consequential. Engineers ruled out the original sunflower layout for the finished telescope. Further simulations pointed them toward a modified arrangement, dubbed Perturbed Vogel, which retains the neighbor spacing that suppresses mutual coupling while disrupting the repeating distance pattern responsible for the sensitivity dip. On-sky testing of the perturbed layout lies beyond the scope of the current study, but the AAVS3 results have already fed directly into the construction phase. Together with lessons learned from AAVS2, this work has shaped the observatory’s verification procedures and supported a key construction milestone known as AA0.5, marking the first SKA-Low stations to come online, bringing the sunflower’s lesson from a field in Western Australia to the front line of twenty-first-century radio astronomy.

Subject of Research: On-sky verification of the Vogel sunflower-inspired antenna layout for the SKA-Low prototype station AAVS3

Article Title: Testing an antenna layout for the SKA-Low telescope

Article References: Testing an antenna layout for the SKA-Low telescope. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: SKA-Low, Square Kilometre Array, radio telescope, AAVS3, Vogel layout, mutual coupling, antenna calibration, radio astronomy, Western Australia, sensitivity verification, Galactic Plane, low-frequency astronomy