Gravitational-wave astronomy is still in its infancy. Although LIGO and other observatories have opened a new window into the universe, their gravitational views of the universe are limited. To broaden our perspective, there is the North American Nanohertz Observatory (nanograv) for gravitational waves.
Gravitational waves are created by the movement of large objects. Most of the gravitational waves we find are from the fusion of black holes. At the last moment, the binary black holes orbit each other very fast, creating rapid and strong gravitational waves. But most of the gravitational waves that wave through the universe are not fast or strong. They are faint echoes of orbiting black holes that are not going to merge. Their slow orbit creates a background of gravitational waves. A single wave from one of these sources can take years to build a complete cycle.
To detect these gravitational waves, the nanograph observes radio pulses rapidly rotating from neutron stars known as millisecond pulsars. Most of these pulsars are very regular, so the change in their pulse rate is due to a change in their motion compared to the earth. Basically, nanograms are like LIGO, but on the scale of our galaxy. Because these background gravitational waves oscillate very slowly, it can take years to observe the change of pulsars.
Nanograv pulsars have been around for more than a dozen years, and they have published some preliminary results. In the study, the team looked at 45 millisecond pulsars that they knew had very stable pulse rates. Some of them are observed for 12.5 years, but all of them are observed for at least 3 years. When the false sound effects are filtered out, the background signal of the gravitational waves appears to be about a year of oscillation. They cannot prove that gravitational waves are the origin of this signal, but they rule out other possibilities, including any bias in their data.
Although observations of a decade may seem like a long time, for many of these gravitational waves it is only a moment. We need to pay more attention to understand them better.
Reference: Arzomanian, Saven, et al. “Nanogram 12.5 Year Data Set: Search for an Isotropic Stochastic Gravitational-Wave Background. ” Letters from the Journal of Astronomy 905.2 (2020): L34.
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