Fast Radio Bursts: Angular Power Spectrum as a Cosmological Probe
The first measurement of the angular auto-power spectrum of dispersion measures (DMs) of fast radio bursts (FRBs) is presented, based on a sample of 3455 presumably single events from the CHIME/FRB Catalog 2. An angular correlation signal with a significance level >3σ is detected, linked to large-scale fluctuations in free-electron density. Fitting the measured spectrum to theoretical models enabled constraints on two parameter combinations: Ω_b h²–H₀ (baryonic matter density and the Universe’s expansion rate) and Ω_b h²–f_d (the fraction of baryons in large-scale structure). Using simulated catalogs, the method’s robustness was tested against systematic uncertainties from the redshift distribution of FRBs and DM contributions from host galaxies, the Milky Way halo, and the Galactic interstellar medium. The angular power spectrum proved nearly insensitive to uncorrelated DM components, like the host galaxy contribution, effectively reducing the impact of poorly constrained systematics. Unlike traditional DM–z analysis, this method does not require individual redshifts — only the overall distribution — and partially breaks parameter degeneracies in the Ω_b h²–H₀ and Ω_b h²–f_d planes. These results establish the DM angular power spectrum as a robust cosmological probe and a powerful indicator of baryon distribution.
Context
Where did half of the ordinary matter in the Universe go? This question has plagued astrophysicists for decades. According to the cosmic microwave background, the Big Bang produced a strictly defined amount of baryons, yet in the modern Universe we see only a small fraction of them — in galaxies, stars, and gas. The rest is thought to be hidden in the diffuse ionized medium of intergalactic space and galaxy halos, dominated by hydrogen and helium. The missing baryon problem dates back to the work of Fritz Zwicky, who in the 1930s already noticed a lack of luminous matter in galaxy clusters. Fast radio bursts (FRBs) — millisecond radio pulses, presumably associated with neutron stars — carry in their dispersion measure (DM) the integrated amount of free electrons along the line of sight, making them ideal probes of this invisible plasma. So far, however, their use has been limited by the need for precise redshifts and interfering systematics. A new approach based on the angular power spectrum of DM fluctuations promises to overcome these barriers.
Methods
The study used 3,455 non-repeating FRBs from the second catalog of the CHIME telescope. For each burst, the contribution of the Milky Way's interstellar medium (according to the NE2025 model) was subtracted from the observed DM, after which a map of residual DM values on the celestial hemisphere was constructed. Then, using an algorithm implemented in the NaMaster package, the angular autocorrelation power spectrum was computed in six logarithmically spaced multipole bins (ℓ from 10 to 1000). This spectrum essentially shows how electron density fluctuations correlate at different angular scales. To assess significance, DM values were randomized among FRB positions, which allowed determination of how much the real signal exceeds random noise. The theoretical model spectrum was built within the framework of standard ΛCDM cosmology, accounting for nonlinear matter evolution and baryonic feedback effects. Building on the cosmological model pioneered by Edwin Hubble, the scientists modeled the expected spectrum and compared it with the data.
Results
The measured spectrum displays a statistically significant signal at more than 3σ relative to the null hypothesis. Physically, this means that the residual DMs are not random but reflect real large-scale clumping and rarefaction of electron density in the cosmic web. Comparison with the model allowed simultaneous constraints on two parameter sets. In the first case, with the fraction of baryons in the large-scale structure fixed at f_d = 0.83, values of the baryon density Ω_b h^2 = 0.035+0.010–0.021 and the Hubble constant H0 = 74+20–30 km/s/Mpc (68% confidence level) were obtained. In the second case, with H0 fixed, Ω_b h^2 = 0.047+0.022–0.033 and f_d = 0.56+0.44–0.13 were found. These numbers show that, although the errors are still large, the method can "untangle" parameters that are completely degenerate in classical analysis (based on the DM–redshift relation). The diagrams show that the main limitation comes from the sample size, not from systematics.
Implications
The work proves for the first time that the angular DM power spectrum is indeed extractable from real data and carries cosmological information. Crucially, this method does not rely on individual FRB redshifts, requiring only the distribution in z — meaning it can use tens of thousands of unlocalized bursts from future surveys. Moreover, the spectral approach naturally separates the cosmological signal from uncorrelated noise associated with host galaxies and the Milky Way halo. This reduces dependence on poorly known DM_host models and opens the way to a "clean" measurement of cosmic parameters, free from many systematic errors that plague traditional analysis. As the galaxy rotation measurements of Vera Rubin showed, dark matter dominates, but ordinary baryons are also unevenly distributed, and the new method captures this structure.
Future development
With the commissioning of next-generation instruments like SKA and DSA-2000, a deluge of detected FRBs is expected — up to hundreds of thousands of events with far better statistics. This will allow measurement of the power spectrum with high signal-to-noise, refinement of the redshift dependence of the baryon fraction f_d, and possibly detection of subtle effects such as the influence of feedback from active galactic nuclei on the electron distribution. The next logical step will be cross-correlation of the DM field with maps of the cosmic microwave background, X-ray background, and dark matter via gravitational lensing, which will greatly multiply the cosmological return. In the future, FRBs may become a primary tool for mapping "dark" baryons and testing theories of dark energy.
Impact
The results will impact observational cosmology, intergalactic medium astrophysics, and the theory of large-scale structure formation. The method also stimulates the development of statistical approaches in radio astronomy and can be adapted for other types of transients.
Next steps
In the near future, the method will be applied to growing FRB samples, and joint analysis with cosmic microwave background and X-ray survey data will be conducted to calibrate systematics and refine models of the Milky Way halo.
Key open problems
The work directly addresses the "missing baryons" problem — a long-standing mystery about the whereabouts of up to 50% of the ordinary matter in the Universe. Moreover, it offers a new way to independently measure the Hubble constant, which could help resolve the cosmological tension crisis between local and early estimates of H0. This method continues the legacy of Edwin Hubble, who first measured the expansion of the Universe nearly a century ago. At a deep level, understanding the distribution of baryons in the cosmic web is inextricably linked to the nature of dark matter and the feedback mechanisms that shape the visible structure of the Universe.
🎯 Fast radio bursts are so energetic that a single pulse in milliseconds releases as much energy as the Sun does in several days. And they occur in all directions, serving as kind of "lighthouses" that illuminate the cosmic web.
🎬 The idea of using distant pulses to study hidden matter echoes the concept of 'interstellar radio beacons' from Carl Sagan's novel Contact, where an extraterrestrial signal carries information about the structure of the Universe.
Key numbers
- Number of FRBs used: 3455
- Detection significance level: >3σ
- Hubble constant (estimate): 74+20-30 km/s/Mpc
- Baryon density Ω_b h^2 (estimate): 0.035+0.010-0.021
- Fraction of baryons in large-scale structure f_d (estimate): 0.56+0.44-0.13