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Gravitational Waves From Black Hole-Neutron Star Collision May Yield Most Accurate Hubble Constant Yet

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A rare collision between a black hole and a neutron star may help experts precisely pin down how much the universe is expanding.

The cataclysmic event would generate gravitational waves, which are ripples in the fabric of space-time created by extremely powerful astronomical events.

These gravitational waves would allow experts to measure the exact distance of these objects from Earth and the rate at which they are moving away. These are the two essential elements that help scientists calculate the Hubble constant, or the rate of the expansion of the universe.

A more accurate Hubble constant would help researchers gain better insight into how the universe began. It will also provide valuable clues as to whether the universe will keep expanding into infinity or if it will implode in on itself.

What Is Hubble's Constant?

Physicists at Massachusetts Institute of Technology and Harvard University propose a new way of measuring the rate of the expansion of the universe.

In a paper published in the Physical Review Letters, the team says measuring gravitational waves from a rare type of binary system will yield better results than previous methods.

Since Edwin Hubble discovered that the universe has been expanding since the Big Bang, scientists have scrambled to measure how fast this expansion happens.

By looking at the distance and velocity of stars observed from Earth, Hubble concluded that the universe was growing at 310 million miles/second/megaparsec, but this figure has been subject to debate ever since. One megaparsec is equivalent to 3.3 million light-years.

As of March 2013, the Hubble constant has been pegged at a much lower rate of 44 million miles/second/megaparsec. However, independent measurements done by two of the world's most advanced instruments have yielded significantly different results.

The measurements from NASA's Hubble Space Telescope were based on observing the distance and velocity of Cepheid variable stars. These are stars that regularly change in brightness every two to 100 days.

On the other hand, the European Space Agency's Planck satellite looked at fluctuations in the electromagnetic afterglow that was left over from the Big Bang.

Gravitational Waves As An Alternative

Team lead Salvatore Vitale, assistant professor of physics at Harvard University, says gravitational waves from a binary system consisting of a black hole and a neutron star can provide a more accurate Hubble's constant.

As both gravitationally strong objects spiral toward each other, they will eventually crash in a catastrophic astronomical event that releases gravitational waves.

Measuring these gravitational waves will allow scientists to determine the exact distance of their source and how fast the source is moving from Earth.

"Gravitational waves provide a very direct and easy way of measuring the distances of their sources," says Vitale.

The team is not the first to propose the use of gravitational waves to measure Hubble's constant. In 2017, scientists detected space-churning gravitational waves coming from the explosive merger of two neutron stars through the National Science Foundation's Laser Interferometer Gravitational-Wave Observatory and its Virgo counterpart in Europe.

However, the Hubble constant derived from this observation had a 14 percent uncertainty, making it far more uncertain than the measurements done by the Hubble and Planck satellites.

The challenge here is collisions between neutron stars produce different types of gravitational waves. Some of them may come directly from the center of the crash, while others may slip out through the edges.

Scientists measure gravitational waves by how clear they are. If the signal comes in loud and clear, it may mean two things. One, the gravitational waves come from the center of a binary system far away. Two, they may come from the edges of a much nearer system.

With a neutron star binary system, scientists find it nearly impossible to distinguish between the two.

Black Hole And Neutron Star System Yields Accurate Measurements

Before the discovery of gravitational waves in 2015, Vitale and his team have been studying black hole and neutron star pairings to see how fast a black hole could spin.

As a side effect of their study, the team found that distance and velocity measurements of this rare binary system were far more accurate than pairs of neutron stars.

"Because of this better distance measurement, I thought that black hole-neutron star binaries could be a competitive probe for measuring the Hubble constant," says Vitale.

The team is confident in their findings that even if they could find only one black hole-neutron star pair among 50 neutron star binaries, the rare system will still yield a more accurate Hubble constant.

MIT and Caltech are poised to bring LIGO back to life in January 2019 with upgraded equipment. Vitale and his team hope that the newly improved, more sensitive instruments will help them find up to 25 black hole and neutron star systems.

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