How fast is the Universe expanding? That's a billion-dollar question — literally — because different methods give different answers, and nobody knows who's right. This is called the "Hubble tension."
There's a way to settle the dispute — and it comes from an unexpected direction. When black holes or neutron stars merge, the gravitational wave carries distance information baked right in. No intermediate "rungs" needed — distance is read directly from the signal. These sources are called "standard sirens."
But there's a catch. If the merger produced a flash of light — great, you can find the host galaxy and measure its recession speed. That's a "bright siren." It worked once — with event GW170817 in 2017. The rest are "dark sirens": we hear the wave, but see no flash.
So what do you do? Statistically match gravitational wave event positions against galaxy catalogs. The cosmic web — the large-scale structure of the Universe — connects both.
A new study tested the robustness of this method: how you count, how you bin the data, how you handle incomplete catalogs. The conclusion is optimistic: with enough events, the cross-correlation method can deliver a precise Hubble constant — completely independent of all other measurements.