On August 17, 2017 a short ripple, traveling across space-time, betrayed the cataclysmic merger of two neutron stars out in the cosmos to scientists here on Earth.
Known as a gravitational wave, the initial signal was caught by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo detector. Shortly after, 70 other observatories across the world confirmed the colossal merger. The fact that our detectors had managed to make multiple observations of one cosmic collision was unprecedented — and priceless to astronomy.
By combining all the data gathered from this neutron-star merger, a group of scientists has made the most accurate measurement yet of a fundamental property of the universe: The rate at which the cosmos are expanding — the Hubble Constant.
Their results are detailed in a study published Thursday in the journal Science.
Since the Big Bang birthed the universe around 13.7 billion years ago, the cosmos has continued to expand. The universe expansion rate is measured through the Hubble Constant, which is calculated in one of two ways:
- Tracking the distances between galaxies and the Earth over time.
- Looking at the light from nearby variable stars, or a form of radiation called the cosmic microwave background, in different galaxies.
But these two measures are at odds with each other — both methods produce different results, and scientists are not sure which was right.
Tim Dietrich, a professor of Theoretical Astrophysics at Potsdam University and lead author of the new study, tells Inverse that his team set out to resolve the debate over which of the two measurements is the real Hubble Constant once and for all.
"These two measurements are in disagreement, and one of them might simply be wrong," Dietrich says.
To solve the mystery, Dietrich and his team turned to a unique cosmic object: Neutron stars.
Neutron stars are the super dense remains of stars which exploded in supernovae. These stars have a mass about 1.4 times that of the Sun, but it is all packed tightly into a small, dense orb perhaps the size of a city.
Scientists have never been able to recreate in a lab matter as dense as that found in neutron stars. As a result, observing explosive mergers between these stellar beings is the only way scientists can observe the properties of matter in this strange state.
The team behind this study analyzed data gathered from the merger GW 170817.
These two neutron stars spiraled around each other, attracted by each other's gravitational pull, and emitted gravitational waves that lasted for about 100 seconds. As the two stars collided, they emitted a flash of light in the form of gamma rays, which was observable from Earth around two seconds after the gravitational waves hit our detectors, according to LIGO.
Scientists around the world also made X-ray, ultraviolet, optical, infrared and radio waves observations of the merger, adding to the data trove.
Using this host of data, the researchers were able to measure the mass and radius of each neutron star, and determined how far they were from Earth.
With these measurements, they were then able to compare the stars' distance to the apparent rate of recession away from Earth.
So, where did their measurement land between the two existing methods of measuring the Hubble Constant? Unfortunately, more work will need to be done before astronomers can say for sure which measure — if any — is the most accurate.
"Our result is not tight enough to rule one or the other out," Dietrich says. But the results do favor one measure over the other: The cosmic microwave background method.
The researchers hope to observe more similar events of neutron star mergers in order to settle the debate.
"This doesn’t solve the tension, it’s just one way of getting a new data point to understand the Hubble Constant," Dietrich adds.