Gravitational waves measure distance directly from the collision itself
For over a decade, cosmology has harbored a quiet crisis: two precise but disagreeing measurements of how fast the Universe expands have left scientists uncertain whether their foundational model of reality is complete. Now, a team from Swinburne University and CSIRO has offered a third voice in this debate — gravitational waves from a 2017 neutron star merger — and that voice leans toward the slower, early-Universe rate, suggesting the cosmos may be less mysterious than some feared. The measurement is not yet decisive, but it is the clearest independent signal yet that the standard model may hold, and it points toward a future where many such cosmic collisions will finally settle the question.
- A decade-long standoff between two irreconcilable measurements of cosmic expansion has pushed cosmologists to the edge of questioning their entire model of the Universe.
- A neutron star collision detected in 2017 — the first ever witnessed in both gravitational waves and light — has been reanalyzed with new radio telescope data, optical imaging, and advanced jet modeling to produce a fresh, independent measurement.
- The resulting Hubble constant of 65.5 km/s/Mpc lands strikingly close to early-Universe predictions and far from the nearby-supernova camp, tilting the scales in a debate that has resisted resolution for years.
- Because gravitational waves encode distance in the raw physics of a merger rather than in stacked astronomical assumptions, this method sidesteps the traditional distance ladder entirely — making its testimony uniquely credible.
- One event cannot close the case, but researchers estimate that roughly a dozen similar mergers could sharpen the measurement to 2% uncertainty — and new detectors coming online promise exactly that flood of data.
For more than a decade, cosmology has been divided against itself. Two extraordinarily precise methods for measuring the Universe's expansion rate — one reading the oldest light from the Big Bang, the other tracking nearby exploding stars — keep arriving at different answers. The gap, known as the Hubble tension, has grown serious enough that some researchers have begun wondering whether the standard model of the Universe needs to be rewritten.
A team led by Swinburne University of Technology and Australia's CSIRO has now entered the debate with a measurement that breaks clearly in one direction. Revisiting GW170817 — a neutron star merger first detected in 2017 and the only such event ever observed in both gravitational waves and light — they combined new gravitational-wave data with months of radio telescope observations across the United States and Europe, plus imagery from the Hubble Space Telescope. Their result: a Hubble constant of 65.5 kilometers per second per megaparsec, sitting within half a standard deviation of the early-Universe value and nearly two standard deviations from the supernova-based measurement.
What distinguishes this approach is its independence. Traditional cosmic measurement builds a distance ladder, stacking layers of astronomical objects to estimate how far away things are. Gravitational waves need no such ladder — the physics of the merger itself encodes distance directly. The team tracked the radio afterglow of jets produced by the collision for months, applied improved statistical methods and relativistic jet models, and carefully corrected for the host galaxy's own drift through space, which had previously been a major source of error.
Lead author Kelly Gourdji was measured in her interpretation: the result favors the early-Universe rate, but one event cannot resolve a decade-old argument. The team estimates that around a dozen comparable mergers, analyzed with similar precision, could reduce uncertainty to roughly 2% — enough to either vindicate the current cosmological model or force a reckoning with it. New gravitational-wave detectors coming online in the years ahead are expected to deliver exactly that volume of events, turning what is now a single compelling data point into a verdict the cosmos can no longer withhold.
For more than a decade, astronomers have been stuck on a question that should have a single, definitive answer: how fast is the Universe actually expanding? The problem isn't that they don't know how to measure it. The problem is that two different methods, both extraordinarily precise, keep giving different answers. One approach looks back at the oldest light in the cosmos—the cosmic microwave background, a kind of fossil radiation left over from the Big Bang itself. The other measures nearby exploding stars. The gap between these two measurements, known as the Hubble tension, has become one of cosmology's most stubborn puzzles, and some researchers have begun wondering whether the standard model of the Universe itself might need revision.
Now a team led by Swinburne University of Technology and Australia's CSIRO has weighed in with what may be the strongest independent measurement yet, and it breaks decisively in one direction. They revisited GW170817, a neutron star merger detected in 2017—the first time humans ever caught such an event in both gravitational waves and light. By combining fresh gravitational-wave data with nearly a year of radio observations from telescope networks spanning the United States and Europe, plus optical data from the Hubble Space Telescope, they calculated the Hubble constant to be 65.5 kilometers per second per megaparsec, with an uncertainty of plus or minus 4.4.
That number matters because it sits much closer to what the Planck mission measured from the early Universe than to what the SH0ES collaboration found by studying nearby supernovae. In statistical terms, the new gravitational-wave result falls within 0.5 standard deviations of Planck's value but 1.7 standard deviations away from SH0ES. It's not a knockout blow—the uncertainty is still too large to declare the debate settled—but it represents the clearest signal yet from gravitational waves that the lower expansion rate may be the correct one.
What makes this measurement so valuable is its independence. Traditional methods build a cosmic distance ladder, stacking multiple types of astronomical objects to triangulate how far away things are. Gravitational waves work differently. They encode distance information directly in the physics of the merger itself. When two neutron stars collide, they send ripples through spacetime that carry a signature of the event's geometry. By analyzing those waves alongside the aftermath—in this case, jets of material moving near light speed that plowed into surrounding gas and glowed in radio wavelengths for months—researchers can measure cosmic distances without relying on the usual ladder of assumptions.
Professor Adam Deller of Swinburne, who led the radio observations, explained the technical challenge: the jets themselves lasted only seconds, but their collision with surrounding material created a radio afterglow that persisted for months. The team tracked this glow using radio telescope arrays and the Hubble Space Telescope, then applied improved statistical methods and advanced computer models of relativistic jets to extract maximum information from the data. They also accounted for the host galaxy's own motion through space—a random drift separate from the Universe's overall expansion—which had been one of the largest sources of uncertainty in previous studies.
Lead author Kelly Gourdji emphasized what the result suggests: "Our independent measurement using gravitational waves is a late Universe method, but the result is more consistent with the early Universe value." If that pattern holds as more events are observed, it would suggest the cosmological model is sound and that new physics isn't needed to explain the discrepancy. But Gourdji and the team were careful not to overstate their case. One event, no matter how precisely measured, cannot resolve a decade-old debate. They estimate that roughly a dozen comparable neutron star mergers could reduce the uncertainty in the Hubble constant to around 2 percent—a level of precision that would either confirm or definitively challenge the current model.
GW170817 remains the only neutron star merger observed in sufficient detail across both gravitational waves and the electromagnetic spectrum for this type of analysis. But that is about to change. New generations of gravitational-wave detectors are coming online in the coming years, and astronomers expect to discover many more such events. Each one will provide another data point, another chance to determine whether the Universe is expanding at the rate its earliest light suggests or at the rate its more recent history implies. For now, this measurement adds weight to one side of the scale—but the weighing is far from over.
Notable Quotes
Our independent measurement using gravitational waves is a late Universe method, but the result is more consistent with the early Universe value.— Dr. Kelly Gourdji, lead author
We'll need to examine more neutron star mergers like this one to be sure, but this result adds another data point for cosmologists to consider in the Hubble tension debate.— Dr. Kelly Gourdji
The Hearth Conversation Another angle on the story
Why does it matter which expansion rate is correct? Aren't they both just numbers?
Because if the two methods genuinely disagree, it means something fundamental is wrong—either with how we measure the early Universe, how we measure the nearby Universe, or with the model itself. If the model is broken, physics is broken.
And gravitational waves are somehow more trustworthy than the other methods?
Not more trustworthy exactly, but independent. The other methods build on each other, stacking assumptions. Gravitational waves measure distance directly from the collision itself. It's like checking your answer using a completely different method.
So this one measurement settles it?
No. One event is just one data point. But it points in a direction—toward the lower rate, toward the idea that maybe the model is fine after all.
How many more do they need?
About a dozen comparable events could get the uncertainty down to 2 percent. That would be decisive. And new detectors are coming, so we'll probably get there within a few years.
What happens if the next dozen also favor the lower rate?
Then cosmologists can stop worrying about new physics and start asking why the nearby supernova measurements have been systematically off. That's a different kind of problem to solve.