Mapping large-scale structure without galaxy formation - combining fast radio bursts with CMB lensing

Fast radio bursts are an emerging precision probe of cosmological fluctuations. The amount and quality of observed FRB data will grow enormously over the next decade. The new analysis still awaits peer review, but it already lays out the central claim clearly.

That matters because cosmology operates at the edge of what current instruments can measure, where systematic errors and model assumptions are never trivial. Small discrepancies between independent measurements have historically pointed toward missing physics rather than simple calibration errors, and the ongoing tension in the Hubble constant is a live example of how a persistent disagreement between methods can reshape the theoretical landscape. Each new dataset that approaches this territory with independent systematics adds real information to a problem that has resisted easy resolution for more than a decade. Fast radio bursts (FRBs) are an emerging precision probe of cosmological fluctuations. The amount and quality of observed FRB data will grow enormously over the next decade.

FRB dispersion data directly probe the free electron distribution up to intergalactic scales. Precision models of the free electron distribution are necessary to exploit this exciting new probe of large-scale structure.

We show that the free electron distribution almost exactly follows the total matter distribution. We confirm this fact by estimating the electron field-level bias parameters for the first time from the FLAMINGO simulations.

With simulation-based knowledge of a handful of electron free parameters, the electron distribution largely acts as a second realization of the matter distribution. Working on large scales, we can leverage this fact to combine FRB correlations with those of Cosmic Microwave Background (CMB) weak lensing to constrain cosmological parameters.

The relevance goes beyond one dataset because even small shifts in measured parameters can matter when the field is testing the limits of the standard cosmological model. The Lambda-CDM framework describes the observable universe with remarkable economy, but its success rests on two components, dark matter and dark energy, whose physical nature remains entirely unknown. Any credible measurement that tightens or loosens the constraints on those components moves the entire theoretical enterprise forward, regardless of whether the immediate result looks dramatic on its own terms.

Assuming access to localized FRBs with redshifts, we perform cosmological forecasts using these two large-scale matter probes, which have no strong dependence on baryonic feedback. We show that the parameters $σ_8$ and $Ω_m$ are constrained at a level similar to or better than that obtained by combining FRB dispersion with weak lensing of galaxy shapes under.

Because this is still a preprint, the result should be read with genuine interest and proportionate caution. Peer review is not a guarantee of correctness, but it is a process that forces authors to respond to technical criticism from specialists who have no stake in a particular outcome. Preprints that survive that process, often with substantive revisions, emerge with a stronger evidential base than the version that first appeared. Until that stage is complete, the responsible reading keeps uncertainty explicitly visible rather than treating the claims as established findings.

The next step is to see whether the effect survives when independent surveys, different calibration strategies and tighter control of systematic uncertainties enter the picture. Programmes such as Euclid, DESI and the Rubin Observatory will deliver datasets over the next several years that cover the same parameter space with largely independent methods. If the current signal persists through those tests, its theoretical implications will become impossible to set aside. Until peer review and independent follow-up address those open questions, skepticism is not a failure of appreciation for the work; it is part of how science decides what to keep.

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