Effects of a central dark matter core on time-delay cosmography with galaxy clusters

The central dark matter distribution probes the nature of dark matter and baryon physics, yet its impact on time-delay cosmography is not fully explored. 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. We update previous analyses of the cluster-lensed quasar SDSS J1004+4112 and the supernova Refsdal in MACS J1149. We jointly infer $H_0$ and the dimensionless core size parameter $β=r_{\rm c}/r_{\rm s}$, while the Brightest Cluster Galaxy (BCG) is either tied to the scaling relation of.

For SDSS J1004+4112, constraints on both $β$ and $H_0$ strongly depend on the treatment of the BCG. A large core is implied when no prior is added to the BCG stellar mass, while a reasonable assumption on the BCG mass prefers a nearly cuspy halo with $β\sim 10^{-3}$.

For MACS J1149.5+2223, a large core of $β\sim 0.5$ is obtained with no prior, while a modest core of $β=0.0410^{+0.0865}_{-0.0276}$ is preferred when the Salpeter IMF is assumed. The constraint on $H_0$ from MACS J1149.5+2223 is stable against the BCG treatment, supporting its robustness.

For the Salpeter-prior case, we obtain $H_0=65.9^{+3.5}_{-3.4}\mathrm{~km~s^{-1}~Mpc^{-1}}$, and different BCG assumptions change the median $H_0$ by less than $1σ$. The core size constraints are translated into the self-interacting dark matter cross section, giving a 95% upper limit of $σ/m<0.

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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