Gravitational Waves Hold Key to Knowing How Fast the Universe Is Expanding

"So far, people have focused on binary neutron stars."

Bryant Garcia

The universe has been expanding ever since it began. But the question on every astrophysicist’s mind is: How fast? Scientists have proposed more than a few possible ways to get at an answer, and these have yielded pretty good estimates of the rate of expansion. However, they’re not as precise as they could be. This number, called the Hubble constant, has a big degree of uncertainty, but as a pair of physicists at MIT and Harvard describe in a new Physical Review Letters article, they’ve figured out a way to narrow down the margin of error.

In a paper to be published Thursday, two physicists propose using rare pairs of black holes and neutron stars to nail down a more precise Hubble constant. As these binary systems dance closer and closer, they shed mass in the form of gravitational waves, and when they finally merge, they emit a flash of light.

Salvitore Vitale, Ph.D., an assistant professor at MIT, and Hsin-Yu Chen, Ph.D., a postdoctoral fellow at Harvard’s Black Hole Initiative, think these flashes and gravitational waves could provide humans with a much more precise reference point to measure just how fast the universe is expanding.

A simulation of a neutron star-black hole merger, in 43 milliseconds (43 thousandths of a second). As this occurs, the system lets off a flash of light that scientists say could help them get a better idea of how quickly the universe is expanding.

Classical and Quantum Gravity)

Multiple different teams are working on ways to nail down the Hubble constant, but they all rely on roughly the same idea: By gazing at faraway astronomical objects, astrophysicists can get a sense of just how fast they’re moving away from us, and therefore, how fast the universe is expanding. In this new method, the researchers suggest looking at rare astronomical phenomena in which a black hole and a neutron star orbit one another and eventually merge.

In the paper, they propose that they could use ground- and space-based telescopes to figure out how fast the system is moving. Then, by using the Laser Interferometer Gravitational-Wave Observatory (LIGO) to detect the gravitational waves the system emits, they can measure its distance from Earth. With those two numbers, they should be able to get a precise measure for the Hubble constant.

“So far, people have focused on binary neutron stars as a way of measuring the Hubble constant with gravitational waves,” Vitale says. “We’ve shown there is another type of gravitational wave source which so far has not been exploited as much: black holes and neutron stars spiraling together.”

The biggest problem with this method is that these systems are much less common than black holes. But still, Vitale and Chen hope their increased precision will make up for their rarity.

“Is the fact that every black hole-neutron star binary will give me a better distance going to compensate for the fact that potentially, there are far fewer of them in the universe than neutron star binaries?” wonders Vitale.

LIGO starts gathering data again in January 2019, so perhaps then we will find out whether Vitale and Chen are on the right path.