# Scientists revisited a famous neutron star collision and found a sharper clue to the universe’s expansion

> A study in The Astrophysical Journal has revisited the famous 2017 GW170817 analysis and produced the most precise measurement yet of the Hubble constant from that single gravitational-wave event. The result gives astronomers a sharper independent check on one of cosmology's most...

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Byline: The Astrophysical Journal
Published: 2026-07-18T13:15:02+00:00
Categories: News, Space

![Artistic depiction of a neutron star surrounded by a glowing magnetic field](https://www.argo.net/wp-content/uploads/2026/07/neutron_star_collision.jpg)

A study in **The Astrophysical Journal** has revisited the famous 2017 [GW170817 analysis](https://iopscience.iop.org/article/10.3847/1538-4357/ae706c) and produced the most precise measurement yet of the Hubble constant from that single gravitational-wave event. The result gives astronomers a sharper independent check on one of cosmology's most stubborn questions, how fast the universe is expanding.

The new analysis places the **Hubble constant** between 61 and 70 kilometers per second per megaparsec. That range overlaps more closely with values inferred from the early universe than with several nearby-universe measurements. It adds a fresh data point to the long-running **Hubble tension**, the disagreement between two leading ways of measuring cosmic expansion.

GW170817 has held a special place in astronomy since August 17, 2017. It marked the merger of two neutron stars and scientists detected both gravitational waves and light from the event. That rare combination allowed researchers to connect the ripples in space-time with a real galaxy in the sky.

## A landmark merger gets a second look

The 2017 signal came from a **neutron star merger**, a collision between two ultra-dense stellar remnants. Neutron stars pack more mass than the Sun into a sphere roughly the size of a city. When two of them spiral together, they shake space-time itself.

Those ripples are called **gravitational waves**. They stretch and squeeze space by tiny amounts as they pass through Earth. Instruments such as LIGO and Virgo can detect those distortions when massive objects collide across the cosmos.

GW170817 became especially valuable because astronomers also saw electromagnetic light from the same merger. That light helped identify the host galaxy, NGC 4993. Once the galaxy was known, scientists could compare the distance from gravitational waves with the galaxy's motion away from us.

That pairing turned GW170817 into a cosmic measuring tool. The original measurements showed the promise of the method, although the uncertainty was large. The new study returned to the same event with improved modeling and a deeper look at the aftermath.

## Gravitational waves become a cosmic ruler

Astronomers often describe this method as a **standard siren**. The name echoes the older idea of a standard candle, an object whose known brightness helps measure distance. In a standard siren, the gravitational-wave signal itself carries information about distance.

The basic idea is rooted in **Einstein's theory of gravity**. As two dense objects orbit and merge, their gravitational-wave pattern reveals how far away the system is. A nearby event produces a stronger signal. A more distant event appears weaker.

To estimate the Hubble constant, scientists also need to know how fast the host galaxy is moving away. That motion is tied to the expansion of space. With distance from gravitational waves and recession speed from the galaxy, researchers can calculate the expansion rate.

This approach gives cosmologists a measurement that stands apart from the two best-known routes. One route uses the **cosmic microwave background**, the ancient afterglow of the Big Bang. Another uses nearby pulsating stars and supernovas to build a distance ladder across space.

The importance of GW170817 comes from its independence. It relies on a nearby merger and gravity itself. That makes it a useful cross-check in a debate where small differences have grown harder to dismiss.

## A fast jet sharpens the measurement

After the neutron stars merged, the event produced a narrow jet of charged particles. This jet moved at tremendous speed and gave astronomers another way to understand the viewing angle of the merger. That angle matters because it affects how the gravitational-wave signal is interpreted.

A global network of **radio telescopes** tracked the jet in fine detail. The new paper revisited those observations at **milliarcsecond scale**, a level of precision that can separate tiny apparent motions on the sky. For distant cosmic events, that kind of detail can make a large difference.

The researchers used more sophisticated jet models and updated statistical tools. They also treated sources of uncertainty more carefully. According to the study summary, several earlier models struggled to reproduce the observations as well as the revised analysis.

That matters because the jet's structure helps narrow the geometry of the merger. If scientists know the viewing angle better, they can better separate distance from orientation in the gravitational-wave signal. This is one reason the updated analysis improves the Hubble constant estimate from GW170817.

The result is a tighter value from the same historic event. The team reports **61 to 70 kilometers per second per megaparsec**, a range that improves the precision achieved by previous studies of GW170817.

## A new number enters the Hubble tension

The Hubble constant measures how quickly galaxies move away from one another as space expands. A megaparsec equals about 3.26 million light-years. A value of 70 means that for every megaparsec of distance, expansion adds about 70 kilometers per second of recession speed.

The trouble is that different measurement methods give different answers. Studies based on the early universe often place the Hubble constant around 67 to 68 kilometers per second per megaparsec. Measurements built from the **nearby universe**, including Cepheid variable stars and supernovas, often land closer to 72 to 74.

The new gravitational-wave result sits closer to the early-universe side of that divide. Because GW170817 occurred in the relatively nearby universe, that alignment is interesting. It suggests that gravitational waves may offer a separate way to test whether the tension points to measurement issues or deeper physics.

The result also comes with a clear limitation. The authors emphasize that the gravitational-wave value remains about four times less precise than the leading nearby-universe measurements. A single merger, even one as rich as GW170817, can only carry the field so far.

Even with that caveat, the updated number strengthens the role of gravitational-wave astronomy. It shows that old events can keep yielding new science when better models and sharper analysis are brought to the data.

## More neutron star mergers could change the debate

Future detections will be the key step. Each additional merger with a known host galaxy can add another independent measurement of cosmic expansion. Over time, a collection of standard sirens could reduce the uncertainty enough to test the Hubble tension directly.

Events like GW170817 are especially powerful when gravitational waves and light are both detected. The light points astronomers toward the host galaxy. The gravitational waves provide a distance tied to the merger itself.

More events will also help scientists understand the diversity of neutron star mergers. Jets may vary in shape, brightness and viewing angle. Better samples will show how much those details affect the Hubble constant.

For now, the 2017 collision remains a landmark with unfinished scientific value. It launched multimessenger astronomy into a new era and nearly a decade later, it is still helping scientists measure the size and history of the universe.

The new analysis gives cosmologists a sharper gravitational-wave ruler. As detectors improve and more neutron star mergers are found, that ruler could become one of the clearest ways to probe cosmic expansion.