Preprint: FRB–Planck analysis suggests about half of the missing baryons sit in warm-hot intergalactic gas

A new arXiv preprint reports evidence that roughly half of the universe’s long-sought missing ordinary matter may be sitting in the diffuse warm-hot gas between galaxies, based on a statistical link between fast radio bursts detected by CHIME and a map of hot gas from the Planck satellite.

If the result holds up, it would mark an important step in a decades-old cosmic census problem. But the finding is not yet peer reviewed, and the headline estimate depends on an assumed gas temperature, a key caveat in a field where the details matter.

The paper, “Locating the missing baryons in the warm-hot intergalactic medium with fast radio bursts and the Sunyaev-Zel’dovich effect,” was posted to arXiv on Aug. 10 UTC as arXiv:2608.09014v1 and says it has been submitted to AAS journals. Its authors are Dao-Hong Zhai, F.Y. Wang, Zi-Gao Dai and Renyue Cen.

The researchers used 2,656 extragalactic fast radio bursts, or FRBs, from the second CHIME/FRB catalog and cross-correlated their dispersion measures with a Planck PR4 Compton-y map. FRB dispersion measure is a readout of how many free electrons lie along the burst’s path to Earth. The Compton-y map traces the thermal Sunyaev-Zel’dovich effect, which is produced when hot electrons leave an imprint on the cosmic microwave background and therefore traces electron pressure.

After masking known galaxy clusters to try to isolate diffuse gas in the cosmic web, the authors report a positive spatial cross-correlation between the two signals. Under their main masking setup, they give the detection a significance of 3.05 sigma, corresponding to a p-value of 0.0023, or 99.77% confidence. From that, they infer that the fraction of cosmic baryons in the warm-hot intergalactic medium, or WHIM, is 0.48, with a 68% confidence interval of 0.27 to 0.61.

That does not mean the missing-baryons problem is solved. The paper’s baryon-fraction estimate is anchored to an assumed mean WHIM electron temperature of 2.4 million kelvin. The authors explicitly describe the inferred fraction as degenerate with temperature, meaning a different temperature assumption would change the answer. They also report that more aggressive masking of clusters lowers the detection significance to 2.06 sigma, though they say the inferred baryon fraction remains broadly similar, which they interpret as a sign that the signal is not mainly coming from gas in galaxy clusters.

The missing-baryons problem refers to a gap between the amount of ordinary matter predicted by cosmology and the smaller amount directly accounted for in stars, galaxies and known gas at late cosmic times. For years, simulations have suggested that much of that ordinary matter should reside in the warm-hot intergalactic medium, a thin web of gas between galaxies heated to roughly 100,000 to 10 million kelvin and therefore difficult to detect directly.

That is why FRBs and the Sunyaev-Zel’dovich effect are a useful pairing. FRBs trace the total number of electrons along a line of sight, while the thermal Sunyaev-Zel’dovich signal traces how much pressure those electrons exert. Using both together can help researchers separate out diffuse ionized gas and estimate how much of the baryon budget it contains.

The study also fits into a fast-moving line of research rather than standing alone. A 2020 Nature paper led by astronomer Jean-Pierre Macquart used localized FRBs to show that the intergalactic medium carries the expected baryon budget on average. The new preprint is narrower: It aims to identify the warm-hot component specifically. And while Zhai and colleagues describe their result as a first detection, earlier related preprints by Takahashi and collaborators in 2025 and Sharma and collaborators in 2026 also reported FRB cross-correlation results. The main distinction here is the much larger CHIME sample and a masking strategy intended to isolate diffuse WHIM rather than virialized cluster gas.

For now, the paper’s claim is best seen as new evidence, not a final verdict. It is under review, and independent confirmation will matter before astronomers can treat this as a firm accounting of the universe’s missing ordinary matter.