Gravity waves may help resolve Hubble constant conundrum

An artist’s impression of two neutron stars on the verge of merging, generating gravitational waves that distort the fabric of space. Image: NASA/Tod Strohmayer (GSFC)/Dana Berry (Chandra X-Ray Observatory

The emerging field of gravitational wave astronomy may help resolve a vexing cosmological conundrum: a discrepancy between the two foremost techniques for determining the Hubble constant, a measure of how fast the universe is expanding and a key indicator of the size, shape and future evolution of the cosmos.

One technique uses relatively nearby Cepheid variable stars and much more distant type 1a supernovae as “standard candles,” allowing them to measure the current rate of the universal expansion and how it has changed over the history of the universe. Measured in kilometres per second per million parsecs (3.26 million light years), the Hubble constant computed in this manner comes to 73.2.

Another technique involves close study of the microwave background radiation left over from the Big Bang. In 2015, a team of astronomers analysed data collected by the European Space Agency’s Gaia spacecraft and came up with a Hubble constant of 67.8.

Other techniques provide slightly different answers. But if the underlying assumptions about the early universe are correct, they all should agree.

“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,” said Hiranya Peiris, professor of physics and astronomy at London City College. “But these methods don’t give the same values, which means our standard cosmological model might be flawed.”

A new study in Physical Review Letters shows how gravitational waves generated by the mergers of binary neutron stars could be used to come up with a more reliable value for the Hubble constant.

Gravitational waves generated when two neutron stars spiral closer and closer together can be detected by the Laser Interferometer Gravitational-Wave Observatory and the Virgo detectors, providing a measure of how far the system is from Earth. By studying the light from such explosive mergers, the system’s velocity can be determined. From those data, an accurate value for the Hubble constant can be computed.

“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,” said lead author Dr Stephen Feeney of the Center for Computational Astrophysics at the Flatiron Institute in New York City. “We should be able to detect enough mergers to answer this question within 5-10 years.”

Recent grants from the U.S. National Science Foundation, UK Research and Innovation and the Australian Research Council will fund LIGO upgrades expected to be in place by 2024 that will increase the volume of space the gravitational wave observatory can “see” by up to seven times. The upgrades are known as Advanced LIGO Plus.

“With it we expect to detect gravitational waves from black hole mergers on a daily basis, greatly increasing our understanding of this dark sector of the universe,” said David Reitze, executive director of LIGO at Caltech. “Gravitational-wave observations of neutron star collisions, now very rare, will become much more frequent, allowing us to more deeply probe the structure of their exotic interiors.”