When two dead stars collide in a distant galaxy, two messengers bring us the news: a shiver in space, a gravitational wave, and a flash of light. Both messengers race toward us across the entire Universe, but their paths can differ slightly. Light sometimes gets delayed by cosmic dust, while gravitational waves pass through everything almost without noticing obstacles.
The authors of the study found a way to cross-check the accounts of these two messengers. They derived simple mathematical rules that connect distances measured by light and by gravitational waves. The rules act like a lie detector: if both methods give consistent results, our picture of the world is correct. If a discrepancy appears, it means we are missing something about how the cosmos works.
What's especially interesting is that the rules do not depend on whether our Universe is flat or slightly curved, like the surface of a giant soap bubble. Astronomers usually assume the cosmos is flat for simplicity, but this assumption can introduce errors in calculations. The new formulas allow us to bypass this assumption and test whether the world is actually curved. It's as if a cartographer learned to draw an accurate map of the terrain without knowing in advance whether they are standing on a plain or a hillside.
as written by the authors arXiv:2605.09660
We show that within the framework of the Friedmann cosmological model it is possible to derive general multimessenger consistency conditions, between the gravitational wave (GW) and the electromagnetic wave (EMW) luminosity distances. We first study the effects of the flatness assumption often used in the estimation of cosmological parameters from GWs observations, deriving a general relation for the curvature parameter, uniquely in terms of the flatness biased GW and the EMW luminosity distances, and a multimessenger consistency relation for the Friedmann model, valid independently of the dark energy equation of state. We then derive a general multimessenger consistency relation for the cosmological constant, which is independent of any flatness assumption made in the analysis of observational data.
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