# Measuring the expansion of the Universe with gravitational waves

A new study proposes an accurate and independent way of determining the Hubble constant—which accounts for the expansion rate of the Universe—by measuring the gravitational waves emitted by a black hole-neutron star binary system. Pinpointing the exact value of the Hubble constant is central to shedding light on the origin of our Universe, as well as inferring on whether it will end in an indefinite expansion or in an ultimate crunch.

One of the greatest discoveries of modern cosmology is that the Universe is expanding. This expansion follows Hubble’s law: *v = H _{0}D*, where

*v*is the recessional velocity (i.e. the velocity of galaxies which redshift proportionally to their distance from Earth),

*H*is the Hubble Parameter (whose value is the same throughout the Universe for a given comoving time), and

_{0}*D*is the proper distance between the galaxy and the observer (which can change over time, unlike the constant comoving distance). The issue is that to date the different methods used to estimate the Hubble constant have produced different values of

*H*offering no definitive answer to how fast, exactly, the Universe is expanding. This matters because the value of

_{0}*H*could help us determine whether the Universe will end in a Big Crunch (collapsing into a dimensionless singularity, back where the Universe started with the Big Bang) or in Heath Death (expanding continuously to approach absolute zero temperature)—or rather end in any of the many other hypothesized fates (Big Rip, Big Bounce, etc.)

_{0}Two of the most recent estimates of the Hubble constant include one from NASA’s Hubble Space Telescope (based on certain “reference” stars’ distance and velocity) and another from the European Space Agency’s Planck satellite (based on observations of the fluctuations in the cosmic microwave background—the residual heat of creation, afterglow of the Big Bang). Unfortunately while both observations are extremely precise, their respective values for the Hubble constant disagree significantly.

In a paper published this week in Physical Review Letters, scientists from MIT and Harvard University have proposed a more accurate, independent way to measure *H _{0}* using gravitational waves emitted by a rare binary system consisting of a spiraling black hole and a neutron star. The idea is that as they circle in toward each other, they should produce space-shaking gravitational waves which LIGO (the Laser Interferometry Gravitational-Wave Observatory) would be able to detect. “Gravitational waves provide a very direct and easy way of measuring the distances of their sources,” says Salvatore Vitale, assistant professor of physics at MIT and lead author of the paper. “What we detect with LIGO is a direct imprint of the distance to the source, without any extra analysis.”

Due to the spin of the black hole around the neutron star, the binary system is potentially ideal for pinpointing the origin of the emanated gravitational waves hence giving a more accurate estimate of the Hubble Parameter. “Because of this better distance measurement, […] black hole-neutron star binaries could be a competitive probe for measuring the Hubble constant,” Vitale adds. “LIGO will start taking data again in January 2019, and it will be much more sensitive, meaning we’ll be able to see objects farther away. So LIGO should see at least one black hole-neutron star binary, and as many as 25, which will help resolve the existing tension in the measurement of the Hubble constant, hopefully in the next few years.”

#### Carlo Bradac

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