Neutron Star Collision Just Settled a Decades-Old Cosmology Debate: The Hubble Constant
Featured image: Artist’s impression of two neutron stars colliding, with jets and gravitational waves; credit: Carl Knox/OzGrav/Swinburne University
A team of astronomers has used the collision of two neutron stars to produce one of the most consequential measurements of the universe’s expansion rate, casting decisive weight in favor of the standard cosmological model. The result, published in The Astrophysical Journal, suggests that the long-running “Hubble Tension” may be a measurement problem rather than a sign that our understanding of physics is fundamentally broken.
The Hubble-Lemaitre Constant describes how fast the universe is expanding. But measurements have stubbornly disagreed for more than a decade. The early universe method, based on the cosmic microwave background recorded by the Planck satellite, yields a value of roughly 244,000 kilometers per hour per megaparsec. The late universe method, using Cepheid variable stars and Type Ia supernovae observed by the Hubble Space Telescope, gives a higher value of around 252,000 kilometers per hour per megaparsec. The mismatch has persisted across hundreds of independent measurements, leading some cosmologists to propose modifications to the standard model.
A Kilonova as a Cosmic Ruler
The new measurement exploits a completely independent technique. The team, led by Dr. Kelly Gourdji of CSIRO and OzGrav, observed the aftermath of GW170817, a binary neutron star merger detected by LIGO and Virgo in 2017. When two neutron stars collide, they produce a kilonova, a violent explosion that launches a narrow jet of energetic particles into space.
By combining gravitational wave data with radio observations from the High Sensitivity Array, a global network of radio telescopes, and astrometry from the Hubble Space Telescope, the team tracked the jet’s motion for nearly a year after the merger.
“These jets are launched for only a couple of seconds, but as they slam into the surrounding gas, they glow for months afterwards,” said Prof. Adam Deller of Swinburne University, who led the radio observations.
Consistency with the Standard Model
The team’s Hubble Constant value is closer to the early universe measurement derived from the cosmic microwave background than to the late universe supernova value. While the new measurement is not as precise as established methods, it is more accurate than any previous attempt using gravitational waves alone, and it provides the strongest evidence yet that gravitational wave astronomy can help resolve the tension.
Crucially, the result argues against proposals that both measurements could be correct under a modified understanding of cosmology. “Some astronomers had proposed ways in which both measurements could be correct if our understanding of cosmology was changed,” Deller said. “But our measurement argues quite strongly against that solution.”
Gourdji was more measured: “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.”
The Road Ahead
With LIGO, Virgo, and the Japanese KAGRA detector now operating in their most sensitive configuration yet, the rate of neutron star merger detections is expected to climb sharply. Each new merger offers an opportunity to repeat the measurement and tighten the error bars. If the pattern holds, the Hubble Tension, the most stubborn puzzle in modern cosmology, may finally give way to a consensus.
For now, the standard model of cosmology survives intact, and the collision of two dead stars 130 million light-years away has provided one of the sharpest votes yet.
Source: 1ban.news – Space Desk