Astronomers see a clue to the background of gravitational waves in the universe

Gravitational wave astronomy is still in its infancy. LIGO and other observatories have opened a new window to the universe, but their gravitational view of the cosmos is limited. To broaden our vision, we have the North American Nanohertz Observatory for Gravitational Waves (NANOGrav).

Gravitational waves are created by the movement of massive objects. Most of the gravitational waves we have detected come from the fusion of black holes. In their dying moments, binary black holes orbit very rapidly, producing fast and strong gravitational waves. But most of the gravitational waves that ripple through the universe are neither fast nor strong. It’s the faint echoes from orbiting black holes that aren’t about to merge. Their slow orbits create a background of gravitational waves. A single wave from any of these sources can take years to cycle completely.

To detect these gravitational waves, NANOGrav observes radio pulses from rapidly spinning neutron stars called millisecond pulsars. Most of these pulsars are very regular, so a shift in their pulse rate is caused by a change in their motion relative to Earth. Essentially, NANOGrav is like LIGO but on the scale of our galaxy. But since these background gravitational waves oscillate so slowly, it takes years to observe a due displacement of the pulsars.

NANOGrav has been watching pulsars for over a dozen years and has just published some first results. In the study, the team looked at 45 millisecond pulsars that they know have very stable pulse rates. Some of them have been observed for 12.5 years, but all have been observed for at least 3 years. When they filtered out the effects of extraneous noise, they found what appears to be a gravitational wave background signal with an oscillation period of about a year. They cannot prove that gravitational waves are the source of this signal, but they have ruled out other possibilities, including any bias in their data.

While a decade of observations may seem like a long time, this is only a moment in time for many of these gravitational waves. To better understand them, we will have to continue to monitor them for much longer.

Reference: Arzoumanian, Zaven et al. “The NANOGrav 12.5 Year Dataset: Searching for an Isotropic Stochastic Gravitational Wave Background.” Letters from the astrophysical journal 905.2 (2020): L34.