Leveraging Lensing-driven Early Warnings: Joint Detectability of lensed Gravitational Waves and their LSST Optical Counterparts

Gravitational lensing of gravitational waves can be leveraged to provide early-warning times of $\mathcal{O}$ to $\mathcal{O}$ prior to the mergers of binary neutron stars and neutron star-black holes. 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. Gravitational lensing of gravitational waves (GWs) can be leveraged to provide early-warning times of $\mathcal{O}(\mathrm{hours})$ to $\mathcal{O}(\mathrm{days})$ prior to the. This capability could, in turn, enable electromagnetic (EM) observatories to capture transient emissions surrounding the time of the merger.

In this work, we demonstrate the viability of jointly detecting gravitationally lensed BNS and NSBH mergers via GWs during the O5 observing run, alongside their associated. We simulate a population of lensed BNS and NSBH mergers detectable during O5 and utilize current benchmark forward models to generate the corresponding lensed kilonova light.

We then assess the detectability of these light curves across LSST bands under various exposure limits. We find that among the detectable lensed mergers in O5, approximately 60\% of the BNS kilonova and 10\% of the NSBH kilonova will be observable within a single LSST exposure, with.

Furthermore, we define a detectability index ($\rm DI$), which determines the EM-detectability in optical of lensed GW-detectable BNS and NSBH mergers. We demonstrate the robustness and generalizability of the $\rm DI$ on our lensed BNS and NSBH population.

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.

Our work demonstrates that lensing-driven early warnings will be highly applicable in the near future, enabling the LSST to effectively follow up on BNS and NSBH triggers within. Both individuals and organizations that work with arXivLabs have embraced and accepted our values of openness, community, excellence, and user data privacy.

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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