Astronomers Use a Neutron Star Merger to Measure Cosmic Expansion
For roughly a century, scientists have known that our Universe is in a constant state of expansion — a discovery so profound that it fundamentally reshaped our understanding of space, time, and the cosmos itself. Known as the Hubble-Lemaître Law, in honor of astronomers Edwin Hubble and Georges Lemaître, who independently demonstrated this phenomenon in the late 1920s, this principle forms one of the cornerstones of modern cosmology. Yet despite nearly a century of refinement, one critical question remains stubbornly unresolved: exactly how fast is the Universe expanding?
The rate of cosmic expansion, encapsulated in a value known as the Hubble-Lemaître Constant (H₀), has been revised numerous times as astronomers have developed more powerful instruments and peered ever deeper into the cosmos. Pinning down this constant is far more than an academic exercise — it determines the Universe's age, geometry, and ultimate fate, while also bearing directly on some of the deepest mysteries in physics, including the nature of Dark Matter and Dark Energy, the invisible components thought to make up roughly 95% of the Universe's total energy content.
Now, in a recent study published in The Astrophysical Journal, an international team of researchers led by scientists at Swinburne University of Technology (SUT) and Australia's Commonwealth Scientific and Industrial Research Organization (CSIRO) has brought a fresh and exciting perspective to this long-standing debate. By observing the violent collision of two neutron stars and combining telescope observations with gravitational wave data, they have produced a new, independent measurement of the Hubble-Lemaître Constant — one that may prove pivotal in resolving one of cosmology's most heated controversies.
The Hubble Tension: A Crisis in Cosmology
To understand why this new measurement matters, it is essential to grasp the problem it aims to address: the Hubble Tension. This refers to a significant and growing disagreement between two independent methods of measuring the Universe's expansion rate, a discrepancy so persistent that some researchers believe it may signal the need for entirely new physics.
Cosmologists rely on a hierarchy of distance measurements called the Cosmic Distance Ladder — a sequence of overlapping techniques that each extend our reach farther into the Universe. The first and second "rungs" of this ladder involve:
- Parallax measurements of nearby stars, using the apparent shift in a star's position as Earth orbits the Sun to determine its distance directly.
- Cepheid Variable stars, which pulsate with a period directly related to their intrinsic luminosity, making them reliable "standard candles" for measuring distances up to tens of millions of light-years.
- Type Ia Supernovae, extraordinarily bright stellar explosions that occur with a consistent peak luminosity, allowing astronomers to measure distances hundreds of millions to billions of light-years away.
Using these rungs, calibrated in large part by the venerable Hubble Space Telescope, astronomers have calculated an expansion rate of approximately 252,000 km/h per megaparsec (Mpc) — roughly equivalent to 70 km/s/Mpc, where one megaparsec spans about 3.262 million light-years. This is the so-called "late Universe" measurement, derived from observations of the relatively nearby cosmos.
The final rung of the ladder, however, tells a different story. Measurements of the Cosmic Microwave Background (CMB) — the faint thermal echo of radiation left over from the Big Bang, when the Universe was only about 380,000 years old — yield a slightly but significantly lower expansion rate. ESA's Planck satellite, which produced the most detailed map of the CMB to date, returned an estimate of approximately 244,000 km/h per Mpc (roughly 67.4 km/s/Mpc). This is the "early Universe" measurement.
The gap between these two values — now statistically significant at more than five standard deviations (5σ), the conventional threshold for a discovery in physics — constitutes the Hubble Tension. As cosmologist and lead author Dr. Kelly Gourdji of CSIRO and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) explained in a Swinburne University press statement:
"One method uses data from the very early Universe — the cosmic microwave background radiation — to make the measurement, while the other uses measurements from relatively nearby supernovae, making it data from the late Universe. Our independent measurement using gravitational waves is a late Universe method, but the result is more consistent with the early Universe value."
Fundamentally, only two broad explanations can account for this tension: either one (or more) of the existing measurement chains contains a systematic error that has yet to be identified, or our standard model of cosmology — the so-called ΛCDM model (Lambda Cold Dark Matter) — is incomplete or incorrect. Both possibilities carry enormous scientific implications, which is why independent measurement techniques are so critically needed.
Gravitational Waves: A New Kind of Cosmic Ruler
The breakthrough potential of gravitational waves (GWs) as a tool for measuring cosmic expansion was first recognized theoretically in the 1980s by physicist Bernard Schutz. Unlike conventional distance measurements that rely on luminosity comparisons, gravitational wave sources — particularly binary neutron star mergers — can serve as "standard sirens", a cosmic analogy to standard candles.
When two neutron stars spiral inward and collide, the ripples they send through the fabric of spacetime encode precise information about the distance to the source. Combined with optical observations that reveal the recession velocity of the host galaxy through its redshift, this provides a clean, independent measurement of H₀ that bypasses the entire Cosmic Distance Ladder and its accumulated uncertainties.
The landmark event GW170817 — the first detected binary neutron star merger, observed in August 2017 by the LIGO and Virgo gravitational wave observatories — provided the first such measurement. However, the precision of that initial estimate was limited by uncertainties in the merger's geometry, particularly the angle of inclination of the binary system's orbital plane relative to our line of sight.
This is precisely where the new research makes its most significant contribution.
The Study: Combining Gravitational Waves with Radio and Optical Observations
The research team — drawn from Swinburne's Centre for Astrophysics and Supercomputing, OzGrav, Tel Aviv University, the University of Queensland, the Indian Institute of Technology Kanpur (IIT Kanpur), and the California Institute of Technology (Caltech) — focused on the same GW170817 event, but brought dramatically improved observational data to bear on the problem.
The collision of the two neutron stars, each roughly 1.4 times the mass of the Sun yet compressed to the size of a city, was so violent that it produced not only gravitational waves but also a brilliant electromagnetic fireworks display. The merger ejected jets of ultra-energetic particles moving at close to the speed of light — a phenomenon known as a kilonova — along with a burst of gamma rays, ultraviolet light, visible light, infrared radiation, X-rays, and radio waves. As Professor Adam Deller of Swinburne, who led the radio observations, described:
"These jets are launched for only a couple of seconds, but as they slam into the surrounding gas, they glow for months afterwards. We analyzed almost a year of observations from the Hubble Space Telescope and two different arrays of radio telescopes spread across the USA and Europe."
The team combined data from three key observational sources:
- The High Sensitivity Array (HSA), a global network of radio telescopes, providing extraordinarily precise astrometry — the measurement of the exact position and motion of the jet over time.
- Hubble Space Telescope optical imaging data, tracking the jet's apparent motion across the sky, a technique known as measuring superluminal motion — the jet's motion appears faster than light due to a geometric projection effect.
- LIGO/Virgo gravitational-wave data, providing the intrinsic distance to the merger event.
By measuring the superluminal motion of the jet with exquisite precision, the researchers were able to constrain the viewing angle of the merger far more tightly than before. This geometric constraint, combined with the gravitational wave distance, allowed a substantially more precise determination of H₀.
Key Findings and Their Implications
The team's new measurement of the Hubble-Lemaître Constant falls closer to the early Universe (CMB-derived) value than to the late Universe (distance ladder) value. While the measurement is not yet precise enough to single-handedly resolve the Hubble Tension — the uncertainty remains larger than that of the established methods — it represents the most compelling gravitational wave-based measurement to date, and its directionality carries important implications.
Crucially, the result argues against one proposed class of solutions to the Hubble Tension: the idea that new or modified cosmological physics could simultaneously reconcile both the early and late Universe measurements. As Professor Deller noted, the team's measurement "argues quite strongly against that solution." Dr. Gourdji further elaborated:
"This would suggest that there is not something wrong with our understanding of cosmology, though we'll need to examine more neutron star mergers like this one to be sure. For now, this result adds another data-point for cosmologists to consider in the lively Hubble Tension debate."
The broader significance lies in the promise of the method itself. As gravitational wave detectors like LIGO, Virgo, and the future LISA (Laser Interferometer Space Antenna) grow more sensitive and detect more binary neutron star mergers, the statistical power of this technique will grow. Astronomers estimate that observing 50 or more such events with well-constrained viewing angles could yield a H₀ measurement precise enough to definitively resolve the Hubble Tension — either confirming a systematic error in existing methods or heralding a revolution in our understanding of the cosmos.
Looking Ahead
The detection of GW170817 opened a new era in multi-messenger astronomy — the practice of studying cosmic events simultaneously across gravitational waves, light, and other signals. This study exemplifies the power of that approach, demonstrating how the combination of radio astrometry, optical imaging, and gravitational wave data can extract information that no single technique could provide alone.
As next-generation observatories come online — including the Einstein Telescope in Europe and the Cosmic Explorer in the United States — the rate of detected neutron star merger events is expected to increase dramatically, potentially to hundreds or thousands per year. Each new merger will offer another independent measurement of H₀, steadily sharpening our picture of the Universe's expansion history.
For now, the work of Dr. Gourdji, Professor Deller, and their international collaborators represents a significant step forward — a demonstration that the Universe's most violent collisions, once considered too chaotic to yield precise measurements, are becoming some of our most powerful cosmological tools. The tension in the Hubble constant may yet be resolved not by choosing between the early and late Universe, but by listening carefully to the gravitational whispers of colliding stars.
Key Takeaways
- The Hubble Tension is a statistically significant (>5σ) disagreement between early- and late-Universe measurements of the cosmic expansion rate.
- Binary neutron star mergers serve as "standard sirens," providing a distance measurement independent of the Cosmic Distance Ladder.
- The new H₀ measurement, derived from GW170817, is more consistent with the early Universe (CMB-based) value than with the late Universe (supernovae-based) value.
- The result argues against modified cosmological models proposed to simultaneously reconcile both existing measurements.
- As more mergers are detected, gravitational wave-based measurements could definitively resolve the Hubble Tension within the next decade.
The study was conducted by researchers from Swinburne University of Technology, CSIRO, OzGrav, Tel Aviv University, the University of Queensland, IIT Kanpur, and Caltech, and was published in The Astrophysical Journal.
