The universe is expanding, but at what rate is a question that has been troubling astronomers for decades. Two methods of measurement give different answers, and this discrepancy is called the Hubble tension. Some look at the early universe through the cosmic microwave background, others at nearby supernovae and Cepheid variables. The difference is small but stubborn: about 9%. It's like having two thermometers in the same room showing 20 and 22 degrees — both seem right, but they disagree.
The new approach doesn't argue about the exact value of the Hubble constant; instead, it builds a test that doesn't depend on it at all. The idea is to take two types of distances: one measured by the delay of light in a gravitational lens (when a massive object bends the path of light, and we see multiple images of the same source arriving at different times), and the other by gravitational waves from neutron star mergers, which act as standard sirens: their true brightness is known from physics, so the distance can be computed from the observed brightness. The ratio of these two distances depends only on how the expansion rate changed over time, not on its absolute value today.
By comparing this ratio as predicted from early universe data with the measured value at late times, scientists get a clean test: if they match, the standard model of expansion is correct, and the Hubble tension is due to measurement errors. If not, then at some point after the Big Bang, the universe expanded differently than we thought. And if you add an independent measurement of the Hubble constant, you can even pinpoint the epoch when the deviation occurred.
For now, the method awaits data: we need to catch gravitational waves that have passed through a gravitational lens. No such events have been detected yet, but the LIGO and Virgo detectors are improving, and in the coming years they may deliver the first results. Then we'll have a new cosmic ruler — without markings, but with a very precise scale.
Gravitational waves from neutron star mergers are so weak that LIGO detectors pick up changes in the length of their four-kilometer arms by a thousandth of a proton's diameter: that's like measuring the distance to the nearest star with the precision of a human hair's width.