But there’s more to this large-scale structure than meets the eye. Hydrogen atoms naturally emit radio waves with a characteristic wavelength of 21 centimeters, and since clouds of hydrogen gas tend to gravitationally cluster around galaxies, patterns of this radio emission reflect the underlying cosmic distribution of matter. In a recent preprint article, radio astronomers working on the Canadian Hydrogen Intensity Mapping Experiment (CHIME) report their first detection of these telltale patterns.
The result is an important first step towards a comprehensive map of the cosmic web using radio emissions from hydrogen, although CHIME’s measurements have yet to reach the precision of state-of-the-art infrared and optical surveys mapping the large-scale structure. “It’s not yet the ‘holy grail’ result, but it’s an important milestone for CHIME and also for the field,” says Tzu-Ching Chang, a researcher at NASA’s Jet Propulsion Laboratory, who has no not participate in the work.
In the ‘dark age’ of the universe, a few hundred million years after protons and electrons combined to form atoms after the big bang, no stars existed to illuminate all the hydrogen gas which then permeated the space. This gas became denser in some places and rarefied in others as gravity competed with cosmic expansion, and the denser regions eventually gave rise to bright stars, galaxies, and stars. galaxy clusters.
As early as the 1990s, cosmologists thought they understood the main lines of this story. They were therefore shocked to discover in 1998 that cosmic expansion had mysteriously begun to accelerate about five billion years ago, after more than eight billion years of contented navigation. Almost nothing is known about the “dark energy” responsible for this acceleration; an important open question is whether it is an immutable “cosmological constant” or rather a dynamic field whose strength changes with time.
Maps of the cosmic web might point to an answer. Light from more distant galaxies takes longer to reach us, and the expansion of the universe stretches the wavelength of this old light towards the red end of the visible spectrum: the further away the galaxy, the greater the redshift cosmic is important. Precise redshift measurements, based on the unique spectral fingerprints of atoms that are abundant in galaxies, thus allow astronomers to construct three-dimensional maps of the cosmic web. These maps encode a wealth of information about the history of cosmic expansion and the evolution of large-scale structure.
The most recent galaxy survey, called the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), cataloged the positions and redshifts of half a million galaxies and as many quasars – extremely bright regions in the heart of large galaxies fed by supermassive black holes. The eBOSS team then used this catalog to build a map covering around 15% of the sky and dating back more than 11 billion years. And even more ambitious follow-up surveys are underway.
A new hope
Yet despite their successes, galaxy surveys have their limits. Telescopes must first scan the sky to select galaxies to include in the survey, and subsequent measurements of the redshifts of individual galaxies tend to take time. Advanced surveys also require expensive spectrometers with thousands of moving parts.
Hydrogen intensity mapping, the strategy pursued by CHIME, could prove a cheaper and faster way to map the cosmos. Radio waves 21cm from distant gas clouds are redshifted, as is visible light. But radio telescopes can measure how the intensity of radio emission varies across the sky at several different wavelengths at once, allowing astronomers to construct three-dimensional maps without separate redshift measurements. Dedicated intensity mapping telescopes are also inexpensive, “an order of magnitude cheaper than comparable optical or infrared spectroscopic instruments,” says Kavilan Moodley, professor of astronomy at the University of KwaZulu-Natal in South Africa. Sud, which is not affiliated with CHIME.
Intensity mapping faces its own challenges. The main difficulty is that the cosmological signal is weak and the Milky Way itself is a strong radio transmitter. “You’re trying to look behind something that’s 1,000 or 10,000 times fainter,” says Moodley. Deciphering the footprint of the cosmic web requires precise telescope modeling and careful analysis.
CHIME is a row of four radio telescopes with no moving parts, each resembling a chicken wire snowboard half-pipe. As the Earth spins, telescopes scan a low-resolution map of the entire northern hemisphere. The resulting 3D map is made up of “voxels” rather than “pixels,” with each voxel being about 30 million light-years across, 10 million light-years deep, and typically filled with hundreds of galaxies. This coarse spatial resolution is a feature, not a bug: summing the radio emissions from all the hydrogen in each voxel allows astronomers to pick up faint signals they wouldn’t otherwise see. And because dark energy effects are more pronounced at very large range scales, the structure within individual voxels is not relevant to these studies.
In 2009 and 2010, Chang and other astronomers found the first traces of the cosmic web in hydrogen emission at 21cm using radio telescopes in Australia and West Virginia. But these telescopes are 100-meter dishes that collect light from a small region of the sky, so they couldn’t effectively map the large areas needed for a fuller view. These facilities are also in high demand and only a fraction of their observations can be devoted to 21 cm observations. The new results from CHIME, derived from data collected in 2019, are the first from a radio telescope specifically designed to map the cosmic web. This allowed the CHIME researchers to better control systematic errors, and they didn’t have to compete with other astronomers for telescope time. The project’s data goes back up to nine billion years, a billion years further in the past than previous radio measurements.
The first signal, but not the last
After processing their data to remove foreground emission from the Milky Way and terrestrial sources, the researchers used a technique called “stacking” to study correlations between the CHIME data and galaxy maps from the survey. eBOSS. They saw an unmistakable signal: regions of more intense radio emission overlapped with the positions of known galaxies and quasars. “When you have that first detection, it’s extremely motivating,” says Seth Siegel, a research scientist at McGill University and one of the CHIME team’s analysis leads. The result is an important milestone, he says, because it gives CHIME researchers a base from which to pursue further improvements.
The team is currently working on using more recent CHIME data to build a stand-alone map, without the help of the eBOSS catalog. He then plans to search for correlations in the distribution of hydrogen gas on longer distance scales, for which separating the signal from the foreground emission becomes particularly difficult. Such correlations are the remnants of sound waves – called “baryonic acoustic oscillations” by cosmologists – that propagated through the primordial fiery plasma that filled the early universe. The characteristic scale of these oscillations – around 500 million light-years in the current universe – has been accurately measured using other methods. Thus, the baryon’s acoustic oscillations can serve as a yardstick that the team can use to measure other distances in their maps looking for deviations from standard cosmology, such as changes in the strength of the dark energy.
Richard Shaw, a researcher at the University of British Columbia who co-led the analysis with Siegel, points out that this is just the beginning for CHIME. “We have bags of data in the box and more to come,” he says.