A new analysis of the neutron star merger known as GW170817 has delivered the most precise measurement yet of the Hubble constant using gravitational waves. Published in The Astrophysical Journal, the research adds another independent data point to one of the biggest debates in modern cosmology.
The Hubble constant measures how fast the universe is expanding. Astronomers use it to calculate the distances and sizes of objects across the cosmos, making it one of the cornerstones of modern astronomy. Yet despite decades of work, scientists still cannot agree on its exact value.
The disagreement, known as the Hubble tension, has only grown as measurement techniques have become more precise. One method relies on the faint glow left behind by the Big Bang, while another uses nearby objects such as pulsating stars and supernovas. Instead of converging on the same answer, the two approaches continue to produce different results.
A Groundbreaking Collision Still Has More To Tell
Gravitational waves have opened a completely different way to measure the universe’s expansion. These ripples in space-time are produced when extremely dense objects, including neutron stars and black holes, collide.
GW170817 became a landmark event in 2017 when astronomers detected both gravitational waves and light from the merger of two neutron stars. That combination made the event especially valuable because it allowed researchers to identify the galaxy where the collision occurred.
The study explains that combining the gravitational-wave signal with observations of the host galaxy made it possible to calculate the Hubble constant using a method based directly on Einstein’s theory of gravity. The first estimate, though, landed between the two competing values at the heart of the Hubble tension, leaving the mystery unresolved.
A More Precise Measurement
Astronomers have continued studyingGW170817 ever since its discovery. A global network of radio telescopes tracked the fast-moving jet of charged particles produced after the merger, revealing its motion and structure in remarkable detail.
The new paper revisited those observations using more sophisticated models, improved statistical methods and a more careful treatment of uncertainty. The researchers also found that several models used in earlier analyses struggled to reproduce the observational data.
Their revised analysis produced what the team describes as the most accurate gravitational-wave measurement yet from GW170817. The Hubble constant was measured at 61 to 70 kilometers per second per megaparsec, improving the precision achieved by previous studies based on the same event.
A New Clue in the Hubble Tension
The new measurement does not settle the debate, but it adds another independent result to the discussion. Measurements based on the cosmic microwave background continue to place the Hubble constant between 67 and 68 kilometers per second per megaparsec. Observations of nearby pulsating stars and supernovas, by contrast, produce values of around 72 to 74 kilometers per second per megaparsec.
The authors note that the revised gravitational-wave measurement is more consistent with the value obtained from the distant universe, even though the method itself relies on a relatively nearby event. They also point out that this could reflect subtle calibration issues affecting other nearby-universe techniques.
Even so, the researchers emphasize that their result is still about four times less precise than the leading nearby-universe measurements. More neutron star mergers will need to be detected before gravitational waves alone can determine the Hubble constant with enough precision to resolve the Hubble tension.
