Astronomers see a hint of the gravitational-wave background of the universe
Astronomy of gravitational waves is still in its infancy. LIGO and other observatories have opened a new window to the universe, but their gravitational view of the universe is limited. To broaden our view, we have the Nanohertz Gravitational Wave Observatory in North America (NANOGrav).
Gravitational waves are created by the movement of massive objects. Most of the gravitational waves we detect come from merging black holes. In their final moments, binary black holes rotate around each other very quickly, producing fast and strong gravitational waves. But most of the gravitational waves rippling through the universe are neither fast nor strong. They are faint echoes of rotating black holes that are not about to merge. Their slow orbits create a background of gravitational waves. A single wave from one of these sources can take years to form a complete cycle.
To detect these gravitational waves, NANOGrav detects radio pulses from rapidly rotating neutron stars known as millisecond pulsars. Most of these pulsars are very regular, so the change in their pulse rate is caused by a change in their motion relative to Earth. Basically, NANOGrav is similar to LIGO but the size of our galaxy. But because the background gravitational waves oscillate so slowly, it takes years to notice the pulsars shift due to them.
NANOGrav has been watching pulsars for over a dozen years, and they’ve just released some preliminary results. In the study, the team looked at 45 milliseconds of pulsars that they know have very stable pulse rates. Some have been observed for 12.5 years, but all have been observed for at least 3 years. When they filtered out the effects of the false noise, they found what appeared to be a background signal of gravitational waves with an oscillation period of about a year. They cannot prove that gravitational waves are the origin of this signal, but they have ruled out other possibilities, including any biases in their data.
While a decade of observations seems like a long time, it is only a moment in time for many of these gravitational waves. To understand it better, we’ll need to keep watching longer.
reference: Arzumanian, Zavin, et al. “12.5 year NANOGrav dataset: Search for the background of isotropic random gravitational waves. ” The Astrophysical Journal Letters 905.2 (2020): L34.
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