The Hubble Constant, the value of the expansion rate of the Universe, is crucial to our understanding of the cosmos. There is just one problem: it is not constant. The two best methods used to reliably measure the value end up with two different values, suggesting something is wrong with our theory of the universe, called the standard cosmological model.

A third method was recently put to the test, but although it gave us more insight, it didn’t solve the issue. Now, researchers have proposed a completely new test for the constant that employs the gravitational waves produced by neutron star collisions. The details of this approach are published in a paper in Physical Review Letters.

"The Hubble Constant is one of the most important numbers in cosmology because it is essential for estimating the curvature of space and the age of the universe, as well as exploring its fate," co-author Professor Hiranya Peiris, from University College London, said in a statement. "We can measure the Hubble Constant by using two methods – one observing Cepheid stars and supernovae in the local universe, and a second using measurements of cosmic background radiation from the early universe – but these methods don't give the same values, which means our standard cosmological model might be flawed."

Gravitational waves are ripples in space-time caused by the interaction of massive objects, in this case the merger of neutron stars. Using the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo experiments, researchers measured these small vibrations in order to work out how far away these events happened.

Although LIGO and Virgo can also measure black hole collisions using gravitational waves, astronomers would need their light emissions too in order to determine the Hubble Constant, and so far we have only seen light in the mergers of neutron stars. Using the light, researchers can work out at what speed the object seems to be receding from us. By combining speed and distance, they can then estimate the Hubble Constant.

"We've calculated that by observing 50 binary neutron stars over the next decade, we will have sufficient gravitational wave data to independently determine the best measurement of the Hubble constant. We should be able to detect enough mergers to answer this question within 5-10 years," said lead author Dr. Stephen Feeney from Flatiron Institute in New York City.

"This in turn will lead to the most accurate picture of how the universe is expanding and help us improve the standard cosmological model," concluded Professor Peiris.

Many researchers are trying to find different methods to solve this tension and finally explain just what exactly is going on in cosmology.