What if a single cosmic collision, half a billion trillion kilometres away, could help settle one of the loudest arguments in modern physics? Two neutron stars did exactly that when they smashed together in 2017, and astronomers have just squeezed a fresh number out of the wreckage.
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The direct answer
Astronomers used the 2017 neutron star merger GW170817 to measure the Hubble constant, the universe’s expansion rate, at 65.5 ± 4.4 km/s/Mpc. This gravitational-wave “standard siren” is a late-universe method, yet its value sits within half a sigma of the early-universe Planck figure and 1.7 sigma from the higher local-distance value. That hints the Hubble tension may not demand exotic new physics after all.
A collision recorded in gravitational waves, radio jets and Hubble images becomes a ruler for the whole cosmos.
How does a neutron star merger measure the Hubble constant?
A neutron star merger measures the Hubble constant by acting as a self-calibrating distance marker in space. The gravitational waves from the collision reveal how far away it happened, straight from Einstein’s general relativity, with no chain of assumptions stacked on top. Pair that distance with how fast the host galaxy is receding, and the expansion rate falls out. A team led by Dr Kelly Gourdji of CSIRO and Australia’s OzGrav centre applied this idea to GW170817 and reported H0 = 65.5 ± 4.4 km/s/Mpc in the peer-reviewed study in The Astrophysical Journal.
Here the megaparsec (Mpc) is the unit of cosmic distance, roughly 3.26 million light-years. So the number says that for every 3.26 million light-years further out you look, galaxies rush away about 65.5 kilometres per second faster. The collaboration behind the work spans Swinburne University of Technology, CSIRO, Tel Aviv University, the University of Queensland, IIT Kanpur and Caltech, blending gravitational-wave data with radio and Hubble observations.
What is the Hubble constant, and why do two numbers refuse to agree?
The Hubble constant is the present-day rate at which the universe expands, named for Edwin Hubble and Georges Lemaître, who worked out the expanding cosmos in the 1920s. The trouble is that two excellent ways of measuring it give answers that stubbornly disagree, a standoff cosmologists call the Hubble tension.
One camp reads the early universe. The Planck satellite mapped the cosmic microwave background, the faint afterglow of the Big Bang, and derived an expansion rate of 67.4 ± 0.5 km/s/Mpc. The other camp works in the nearby, late universe, climbing the cosmic distance ladder from parallax to Cepheid stars to exploding white dwarfs. That route, refined by the SH0ES team, gives 73.04 ± 1.04 km/s/Mpc. The two are separated by several sigma, far too wide to shrug off as bad luck.
Only two escape routes exist. Either one measurement hides an error that grows as the methods reach deeper into space, or our physics of the cosmos is missing a piece. That is why any fully independent third method is so prized, and why a neutron star merger is worth all this effort. If you want the wider backdrop, see our closer look at whether dark energy itself is evolving, another crack in the standard picture.
What is a “standard siren”?
A standard siren is a gravitational-wave source whose signal reveals its true distance on its own, through general relativity alone. Astronomers borrowed the phrase from the older “standard candle”, a star of known brightness. A candle tells you distance from how dim it looks, so it needs careful brightness calibration. A siren instead tells you distance from the exact shape of its gravitational-wave chirp, and needs no ladder of calibrated steps beneath it.
GW170817 is special because it was a bright siren. The collision also flared in light, from a gamma-ray burst to a glowing kilonova, which let astronomers pin the exact host galaxy: NGC 4993, some 130 million light-years away. Knowing the host means knowing its recession speed, and that is the second ingredient the expansion rate needs. It remains the only merger of its kind with a firmly identified home galaxy, which is why one 2017 event still drives so much cosmology.
Why does the tilt of the merger matter so much?
The tilt matters because the gravitational-wave distance is tangled up with the angle we view the merger from. Face-on or edge-on, the two orientations can fake each other in the signal, and that ambiguity smears the distance, which then smears the Hubble constant. Break the tilt free and the whole measurement tightens.
This is where the jet comes in. When the two neutron stars merged, they fired a narrow jet of matter at almost the speed of light, aimed slightly away from Earth. The jet points along the same axis as the pair’s orbital spin, so measuring the jet’s direction hands you the viewing angle for free. Watching how the jet appeared to move across the sky, radio astronomers could read that angle off the sky itself rather than guessing it from a model.
How did the jet’s afterglow crack the problem?
The afterglow cracked the problem because it stayed visible long enough to be tracked with extreme precision. The jet fires for barely a second or two, but as it ploughs into surrounding gas it lights up for months. Professor Adam Deller of Swinburne, who led the radio work, notes that these jets “glow for months afterwards” once they slam into that gas, giving observers a slow-moving beacon to follow.
The team stitched together almost a year of watching: two epochs from the High Sensitivity Array of radio dishes across the United States and Europe, a sharper global radio image, and a Hubble Space Telescope position of the kilonova. Across those images the radio glow drifted in a way that looked faster than light, a projection trick called superluminal motion that betrays a jet tilted close to our line of sight.
What is new here is the analysis, not just the data. Earlier studies tracked only the moving centre point of the glow. Gourdji’s team instead built a Bayesian framework that fits full hydrodynamic jet models straight to the raw radio measurements, using every scrap of spatial information including the changing size of the afterglow. That directness is what let them fold gravitational-wave data, galaxy velocities and radio geometry into one honest measurement of the expansion rate. The same hunt for merger flashes is now being industrialised by instruments like the BlackGEM telescopes built to catch the light from colliding neutron stars.
What does 65.5 km/s/Mpc mean for the Hubble tension?
The value of 65.5 km/s/Mpc means this late-universe method is leaning toward the early-universe answer, which is a genuine surprise. The merger belongs to the local, late-time family of measurements, the same broad camp as the high SH0ES number. Yet its peak lands within 0.5 sigma of the early-universe Planck value and a full 1.7 sigma away from SH0ES.
To see why that turns heads, it helps to line up the GW170817 measurements made over the years as methods and data improved. The table below is our own synthesis of the key results, each pairing a jet viewing angle with an expansion rate.
| Study (year) | What was added | Viewing angle | H0 (km/s/Mpc) |
|---|---|---|---|
| Abbott et al. (2017) | First bright siren, gravitational waves only | Loosely constrained | 70.0 (+12.0 / −8.0) |
| Hotokezaka et al. (2019) | Added radio jet proper motion | About 17° to 19° | 68.9 (+4.7 / −4.6) |
| Howlett & Davis (2020) | Averaged over many galaxy-velocity models | Inherited from above | 64.8 (+7.2 / −7.3) |
| Mooley et al. (2022) | Added Hubble kilonova position and later radio data | 21.9° (+3.3 / −2.9) | 71.5 ± 4.6 |
| Gourdji et al. (2026) | Full fit to raw radio data, 28 velocity models | About 17° to 19° | 65.5 ± 4.4 |
Notice how the answers swing with the viewing angle and the galaxy-velocity choices, not with random noise. A late-universe siren that agrees with Planck is awkward for the tidiest fixes on offer, especially models where dark energy changes over cosmic time to force early and late measurements apart. For you as a reader, the stakes are real: whichever way the tension breaks decides whether our standard model of the universe survives intact or needs rebuilding, and that in turn shapes what we can say about the ultimate fate of everything.
How solid is this result, really?
The result is solid enough to matter but too loose to close the case, and the team is refreshingly open about that. The single biggest source of doubt is not the exotic physics at all. It is the ordinary motion of the host galaxy through its neighbourhood, the so-called peculiar velocity, which muddies how much of NGC 4993’s recession is pure cosmic expansion.
Ranking the ingredients of the roughly six per cent total uncertainty makes the point clearly.
- Peculiar velocity of NGC 4993. The largest slice. The galaxy’s local drift is hard to separate from cosmic expansion, so the team averaged over 28 plausible velocity maps rather than trusting one.
- Gravitational-wave signal strength. A smaller contribution, set by how loudly the 2017 chirp was detected.
- Viewing angle. The smallest slice now, thanks to the radio jets, a reversal from the early days when tilt dominated everything.
Two honest caveats remain. The team’s gravitational-wave distance of 44.0 ± 1.6 Mpc comes out a touch larger than the 40.7 Mpc distance measured to NGC 4993 by other means, a mild discrepancy worth watching. And the very late radio glow, past roughly a year, shines brighter than standard jet models predict, from a source no one has fully explained. Neither issue overturns the headline, but both are reminders that one merger cannot end a debate. The authors estimate that about a dozen similar bright sirens would be needed to reach two per cent precision on the expansion rate. This is the same expanding-universe story that Georges Lemaitre first argued for a century ago, still being written one measurement at a time.
The bigger picture
Step back and marvel at the method. Two dead stars collided before humans had cities, their ripples in spacetime crossed 130 million light-years, and we caught them with laser detectors, radio dishes and a space telescope. From that we read the pace of the entire expanding cosmos and weighed in on a debate about the fate of the universe. The merger did not silence the argument, but it added a clear, independent voice, and it leaned toward the early-universe side.
This article was written specifically for you by FreeAstroScience.com, where we explain complex scientific principles in simple terms. We want you never to switch off your mind, because the sleep of reason breeds monsters. Stay curious, keep questioning, and come back soon, there is always more universe to explore.
Written by Gerd Dani, President, FreeAstroScience Science and Cultural Group.
Frequently asked questions
What is GW170817?
GW170817 was the collision of two neutron stars detected on 17 August 2017 through gravitational waves and light. It happened in the galaxy NGC 4993, about 130 million light-years away, and remains the only merger whose host galaxy has been pinned down, making it uniquely useful for cosmology.
How can a neutron star merger measure the Hubble constant?
A neutron star merger emits gravitational waves that encode its distance directly, with no cosmic distance ladder needed. Pair that distance with the host galaxy recession speed and you get the Hubble constant. Radio images of the merger jet fix the viewing angle, which sharpens the distance and the final value.
Why do early and late universe measurements of the Hubble constant disagree?
Early universe methods read the cosmic microwave background and predict expansion of about 67 km/s/Mpc. Late universe methods use nearby exploding stars and give roughly 73 km/s/Mpc. The gap of several sigma is the Hubble tension. Either a measurement hides an error, or our physics of the cosmos is incomplete.
What is a standard siren in astronomy?
A standard siren is a gravitational wave source whose signal reveals its true distance through general relativity alone. Unlike standard candles such as supernovae, it needs no brightness calibration. When the merger also produces light that identifies its host galaxy, it becomes a bright siren, ideal for measuring cosmic expansion.
Does this new measurement solve the Hubble tension?
No single measurement can settle the Hubble tension yet. The new value of 65.5 km/s/Mpc leans toward the early universe camp and challenges models where dark energy varies with time. Still, its error bars are wide. The team estimates that about a dozen similar mergers are needed for a firm verdict.
Sources
- Kelly Gourdji, Adam T. Deller, Chris Flynn, Taya Govreen-Segal, Cullan Howlett, Kunal P. Mooley and Ehud Nakar, “Revisiting GW170817 at Milliarcsecond Scale: High-precision Constraints on Jet Geometry and H0”, The Astrophysical Journal, 1005:93 (2026). DOI: 10.3847/1538-4357/ae706c.
- Matthew Williams, “Astronomers Use a Neutron Star Merger to Measure Cosmic Expansion”, Universe Today, 10 July 2026.
- Swinburne University of Technology, press statement on the GW170817 standard siren measurement, 2026.
