Science & Technology

Gravitational waves may help answer the cosmic conundrum of Universal expansion

Artist's impression of a neutron star merger and the resulting ripples in time-space which can be measured by LIGO
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Measurements of gravitational waves over the coming decades will settle the debate over how fast the Universe is expanding, new research suggests.

The cosmos has been expanding for 13.8 billion years. 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 — the product of work by Edwin Hubble and Georges Lemaître in the 1920s and one of the most important numbers in cosmology — have conflicting results. This suggests that our understanding of the structure and history of the universe — the “standard cosmological model” — and its rate of expansion, may be incorrect.

As study co-author UCL Professor of Physics & Astronomy Hiranya Peiris, says: “[The Hubble constant] is essential for estimating the curvature of space and the age of the universe, as well as exploring its fate.

“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 study, published today in Physical Review Letters from an international team that includes University College London (UCL) and Flatiron Institute cosmologists, shows how new independent data from gravitational waves emitted by 50 binary neutron stars called “standard sirens” will break the deadlock between the conflicting measurements once and for all.

Dr Stephen Feeney of the Center for Computational Astrophysics at the Flatiron Institute in New York City, the paper’s lead author, says: “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 to 10 years.”

Feeney, Peiris and colleagues developed a universally applicable technique that calculates how gravitational wave data will resolve the issue.

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

Binary neutron star events are rare, but they are invaluable in providing another route to track how the universe is expanding. The gravitational waves they emit cause ripples in space-time that 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 of measuring the Hubble constant accurately.

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

Original research: https://www.simonsfoundation.org/2019/02/14/neutron-stars-hubble-constant/

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