Gravitational Waves Could Reveal How Fast Universe is Expanding

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The universe has been expanding since it first exploded into existence 13.8 billion years ago. It is dragging along with it hundreds of billions of galaxies and stars, much like raisins in a rapidly rising dough. Hubble constant, which describes the rate at which the universe is expanding can be determined by the Gravitational waves emitted by a rare system, a hugely energetic pairing of a spiralling black hole and a neutron star.

Astronomers have pointed telescopes to certain stars and other cosmic sources to measure their distance from Earth and how fast they are moving away from us two parameters that are essential to estimate the Hubble constant, a unit of measurement that describes the rate at which the universe is expanding. But to date, the most precise efforts have landed on very different values of the Hubble constant, offering no definitive resolution to exactly how fast the universe is growing.

This new information could shed light on the universe’s origins, as well as its fate, and whether the cosmos will expand indefinitely or ultimately collapse. Scientists from MIT and Harvard University have proposed a more accurate and independent way to measure the Hubble constant, using gravitational waves emitted by a relatively rare system: a black hole-neutron star binary.

As these objects circle in toward each other, they should produce space-shaking gravitational waves and a flash of light when they ultimately collide, the flash of light would give scientists an estimate of the system’s velocity, or how fast it is moving away from the Earth. The study is published in the journal Physical Review Letters.

The emitted gravitational waves, if detected on Earth, should provide an independent and precise measurement of the system’s distance. Even though black hole-neutron star binaries are incredibly rare, researchers calculate that detecting even a few should yield the most accurate value yet for the Hubble constant and the rate of the expanding universe. 

Two independent measurements of the Hubble constant were made recently, one using NASA’s Hubble Space Telescope and another using the European Space Agency’s Planck satellite.  More optimistically, if black hole-neutron star binaries were slightly more common but still rarer than neutron star binaries, the former would produce a Hubble constant that is four times as accurate.