Scientists say rare black hole-neutron stars could identify rate of universe's expansion

Scientists say that the universe has been expanding ever since it came into existence about 13.8 billion years ago. As the universe expands the planets, galaxies, and stars have been carried along with it. Scientists have long tried to measure the rate of expansion in the universe.

One way they have tried to measure the expansion is by aiming telescopes at stars and other objects to try and measure their distance from Earth to tell how fast the expansion is happening. This is all in an attempt to measure something called the Hubble constant, which is a unit of measurement describing the rate of expansion in the universe.

The catch is that so far, the most precise efforts of the scientific community have generated very different values for the Hubble constant. Scientists at MIT and Harvard believe they have a more accurate way of measuring expansion. They want to use gravitational waves emitted by a rare system- a black hole-neutron star binary.

That system is described as hugely energetic pairing with a spiraling black hole and a neutron star that circle in towards each other. As they do they create gravitational waves and a flash of light when they collide. That flash of light would give scientists on Earth an estimate of the system's velocity or how fast it is moving away from Earth.

Such binary systems are incredibly rare, but the scientists think that if they could detect just a few it would allow them to generate the most accurate Hubble constant and rate of expansion ever generated. The team would use the LIGO or Laser Interferometry Gravitational-Wave Observatory to detect these gravitational waves. Scientists on the team say that these waves provide a "very direct and easy way of measuring the distances of their sources." One major challenge is finding more of the binary systems to observe.