Second only to black holes, neutron stars - incredibly dense star remnants - are the densest objects in the universe . When neutron stars collide, they create ripples in the fabric of space and time in a way that we can detect on Earth.
We can then use these ripples to measure one of the universe's most fundamental but elusive properties - how fast it is expanding. This is called the Hubble constant, and scientists have pursued and debated it for decades.
In recent years, the debate has intensified . Scientists using different techniques keep arriving at divergent answers. This growing problem , known as the "Hubble tension", has become one of the biggest challenges in modern cosmology .
In a study published in The Astrophysical Journal today, we have developed a new approach in an attempt to pin down the Hubble constant. To do so, we re-examined gravitational waves caused by a dramatic collision of two neutron stars.
We believe our results provide the most precise measurement of the Hubble constant to date using the gravitational wave method.
Why do we need the Hubble constant?
Knowing how fast the universe is expanding is fundamental for astronomers. It sets the cosmic scale for our measurements, enabling us to determine the true distance and size of astrophysical objects. Even more profoundly, it tells us about how the universe began and how it could end.
Given its importance, we've been trying to measure the Hubble constant with several methods. According to the standard theory of how the universe evolves, all methods should find the same answer.
For years, the various methods appeared to broadly agree, but with large margins of error. Within the last couple of decades, astronomers have made great strides to increase precision and reduce uncertainty.
Some teams achieved precise measurements using leftover light from the Big Bang. This method is described as using the "distant universe", because the light has travelled for nearly the entire age of the universe to reach us.
Other teams have used light from much nearer objects, such as pulsating stars and supernovae, to make their own precise measurements. These are known as "nearby universe" measurements .
Shockingly, these two sets of high-precision measurements disagree - by a lot. The distant universe measurement puts the Hubble constant at 67-68km/s per megaparsec (a vast unit of astronomical distance: 3.26 million light years). The near universe result is higher - around 72-74km/s per megaparsec.
This is the Hubble tension.
What does it mean? Could it be something has gone awry in one or both methods? Despite intense scrutiny, nobody has found any mistakes. Alternatively, our understanding of how the universe evolves may be missing something fundamental and we need "new physics" to resolve it.
To settle this cosmic debate, new and independent methods of measuring the Hubble constant are highly sought after.
Enter gravitational waves
Gravitational waves offer an entirely independent way to measure the expansion of the universe. These large ripples in the fabric of space-time are produced when extremely dense objects - such as black holes - collide.
Neutron stars are the dense remains of stars that exploded into supernovas, but were not quite heavy enough to collapse into black holes afterward. The first gravitational waves were detected from colliding black holes just over a decade ago .
In 2017, scientists made history when they also detected gravitational waves from a neutron star collision , labelled GW170817. Unlike a black hole collision, it produced a glow of light, enabling astronomers to identify the nearby galaxy where it occurred.
By combining that information with the gravitational wave signal, researchers could make a new measurement of the Hubble constant based directly on Einstein's theory of gravity. Problem solved? Unfortunately, no. The measurement was not as precise as those that make up the Hubble tension.
In fact, it fell right in between the competing measurements, much to everyone's frustration.
Over the last nine years, astronomers have worked to improve the precision of the GW170817 measurement. The best results came from tracking the aftermath of the collision using a worldwide network of radio telescopes. When the two neutron stars collided and merged, they produced an ultra-fast jet of charged particles. The telescopes revealed the motion and structure of this jet's afterglow.
This data reduced the uncertainty, but the measurements remained consistent with both sides of the Hubble tension.
What we found
In our new study, we found several ways to improve on earlier analyses, including more sophisticated models, improved statistical techniques, and a careful treatment of key sources of uncertainty.
By reanalysing the extraordinarily precise telescope observations of the merger's aftermath in greater detail, we found that models commonly used in earlier studies struggled to match the data.
We believe this has produced the most accurate Hubble constant measurement yet from GW170817: 61-70km/s per megaparsec.
Intriguingly, our result agrees more closely with measurements from the distant universe than those based on the nearby universe - despite our method also relying on the nearby universe.
This suggests there may not be something wrong with our understanding of the universe. Instead, the tension may arise from subtle calibration issues affecting other nearby universe methods.
Our result is still four times less precise than the leading nearby-universe measurements . We will need to detect more neutron star collisions to definitively settle the Hubble tension using gravitational waves. Such events are rare, so it may be a while - but for now, our study provides an important new clue in one of astronomy's biggest problems.
Kelly Gourdji receives funding from the Australian Research Council.
