Gravitational waves will settle cosmic conundrum

Measurements of gravitational waves from ~50 binary neutron stars over the next decade will definitively resolve an intense debate over how fast our universe is expanding, find an international team including UCL and Flatiron Institute cosmologists.

The cosmos has been expanding for 13.8 billion years and its present rate of expansion, known as the Hubble constant, gives the time elapsed since the Big Bang.

However, the two best methods used to measure the Hubble constant do not agree, suggesting our understanding of the structure and history of the universe - called the 'standard cosmological model' - may be wrong.

The study, published in Physical Review Letters, shows how new independent data from gravitational waves emitted by binary neutron stars called 'standard sirens' will break the deadlock between the measurements once and for all.

"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," said Professor Hiranya Peiris (UCL Physics and Astronomy).

"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."

The team developed a universally applicable technique which calculates how gravitational wave data will resolve the issue.

Gravitational waves are emitted when binary neutron stars spiral towards each other before colliding in a bright flash of light that can be detected by telescopes. Indeed, UCL researchers were involved in detecting the first light from a gravitational wave event in August 2017.

Binary neutron star events are rare but invaluable in providing another route to track how the universe is expanding.

This is because the gravitational waves they emit cause ripples in space-time which can be detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo experiments, giving a precise measurement of the system's distance from Earth.

By additionally detecting the light from the accompanying explosion, astronomers can determine the system's velocity, and hence calculate the Hubble constant using Hubble's Law.

For this study, the researchers modelled how many such observations would be needed to resolve the issue in measuring the Hubble constant accurately.

"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 of the Center for Computational Astrophysics at the 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.

Related Links
University College London
The Physics of Time and Space

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