Discriminating Planck Reionisation Histories with the kSZ Effect

The epoch of reionisation is a key phase in cosmic history, but its detailed evolution remains poorly constrained by current cosmic microwave background observations. The new analysis still awaits peer review, but it already lays out the central claim clearly.

This 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. The epoch of reionisation is a key phase in cosmic history, but its detailed evolution remains poorly constrained by current cosmic microwave background (CMB) observations. We investigate whether the kinetic Sunyaev--Zel'dovich (kSZ) effect can discriminate among reionisation histories consistent with current large-scale CMB constraints.

Using histories derived from Planck data, we compute the corresponding kSZ angular power spectra within an analytical framework, separating late-time and patchy contributions and. The allowed histories fall into two broad classes, `short' and `long' duration reionisation, yielding distinct kSZ signatures.

Uncertainties from $x_e(z)$ and astrophysical parameters produce comparable amounts of dispersion, yet the two classes remain clearly separable, with variations within each class. Current kSZ measurements ($\sim$0--3 $μ$K$^2$) are not yet precise enough to distinguish between these scenarios, although they tend to favor a `short' reionisation.

The kSZ effect thus provides a promising probe of reionisation beyond optical depth constraints. In particular, a measurement of the kSZ power spectrum at $\ell \sim 2000$ with $\sim$0.4 $μ$K$^2$ sensitivity would discriminate between `short' and `long' reionisation scenarios.

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.

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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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