Preprint: CHIME Fast Radio Bursts Trace Diffuse Baryons Across the Cosmic Web

A new arXiv preprint argues that fast radio bursts, or FRBs, can be used in aggregate to trace ordinary matter threaded through the cosmic web. Using dispersion measurements from 3,455 sources detected by the Canadian Hydrogen Intensity Mapping Experiment, or CHIME, the authors report statistically significant links between the bursts and multiple maps of galaxies, hot gas and matter distribution. In their interpretation, the same dataset may also help distinguish between models of how galaxies heat and redistribute surrounding gas.

The study, titled “Backlighting the Cosmic Web with Fast Radio Bursts: An Anthology of Dispersion Measure Cross-Correlations with Large-Scale Structure and Baryon Tracers,” was posted to arXiv on April 23. It is a preprint led by Kritti Sharma with co-authors including Elisabeth Krause, Vikram Ravi, Liam Connor, W.L. Kimmy Wu and Simone Ferraro, and it has not yet been peer reviewed. The analysis uses 3,455 unique CHIME/FRB sources, a large subset of CHIME detections, with typical localizations of about 15 arcminutes — coarse on the sky, but numerous enough for statistical studies.

According to the abstract, the team measured significant cross-correlations between FRB dispersion measures and 10 probes of large-scale structure and baryonic matter at redshifts up to about 1.5. Reported detection significances range from 2.6 sigma to 5 sigma. Among the strongest were X-ray emission tracing galaxy clusters at 5.0 sigma, the soft X-ray background at 4.1 sigma, the cosmic infrared background at 4.0 sigma and the thermal Sunyaev-Zel’dovich, or tSZ, effect at 3.8 sigma. The authors also report signals with CMB lensing at 3.3 sigma, superclusters at 3.3 sigma, radio continuum emission at 3.2 sigma, galaxies at 2.8 sigma and weak gravitational lensing at 2.6 sigma.

In plain terms, an FRB’s dispersion measure records how much ionized material its radio pulse passed through on the way to Earth. Radio waves at different frequencies arrive at slightly different times, and that delay reveals the total column of free electrons along the line of sight. Because much of the universe’s ordinary matter exists as thin, diffuse ionized gas between and around galaxies, FRBs have become a promising probe of baryons that are otherwise hard to detect directly.

That is why the new paper matters. A 2020 Nature paper led by Jean-Pierre Macquart showed that precisely localized FRBs could help account for much of the universe’s previously “missing” low-redshift baryons. The new work takes a complementary route. Instead of relying on a smaller sample of tightly pinned-down bursts, it uses a much larger CHIME sample and asks a statistical question: Do burst sightlines through denser environments pick up more electrons? The authors’ answer is yes. As the abstract puts it, “These measurements reveal a consistent picture in which FRB sightlines intersecting overdense environments carry systematically larger DMs.”

The paper also pushes beyond galaxy counts to hot-gas tracers, which the authors say provide leverage on feedback — the processes that heat gas and move it around galaxies and dark matter halos. Here the conclusions are more model-dependent and should be read that way. In the authors’ interpretation, the measured tSZ-DM and soft X-ray background-DM amplitudes are consistent with a moderate-feedback model at about 0.5 sigma, while a weaker-feedback scenario is ruled out at about 3.5 sigma by the soft X-ray background-DM correlation.

Those claims will need scrutiny as the preprint moves through review. But if they hold up, the work would strengthen the case for FRBs as a statistical tool for studying diffuse baryons, even when the bursts are not localized with high precision. The authors frame the result as groundwork for future surveys with BURSTT, CHORD, DSA and SKA. As they write in the abstract, the measurements could help support “a new era” in which FRBs and other probes are used together to map baryons across the cosmic web.