Gravitational wave astronomy currently allows us to detect only powerful and rapid events, such as the merger of neutron stars or small black holes with masses of a few stars. We have made significant progress in detecting mergers of stellar-mass black holes, but our long-term goal is to detect the merger of supermassive black holes.
While stellar-mass black holes can have sizes tens of times larger than the Sun, supermassive black holes can reach masses millions or billions of times greater than the Sun. This means that the timescale for the merger of supermassive black holes is not seconds but rather years or even decades. Unlike the rapid gravitational wave signal observed during the merger of stellar-mass black holes, the signal from a supermassive merger is too slow to be directly observed using observatories such as LIGO. Even the planned gravitational wave space telescope, LISA, will not be sensitive enough to observe such large-scale events. The wavelength of gravitational waves in such a case would be too large.
However, a new article published by the NANOGrav project shows how we can observe the merger of supermassive black holes. Instead of building large gravitational wave observatories, NANOGrav studies the radio pulses of millisecond pulsars. These pulsars rotate so rapidly that they emit radio wave pulses almost a thousand times per second. Their pulses are so regular that they can be used as cosmic clocks.
For over a decade, NANOGrav has been observing pulsar pulses with a period of 45 milliseconds, searching for slight shifts in their timing characteristics. The idea is that when long gravitational waves pass through space, they slightly perturb the pulsars, resulting in a synchronization shift of the observed pulses. By analyzing the overall statistical shifts of many pulsars, we can detect the large-scale gravitational wave effect from the merger of supermassive black holes.
However, it is not as straightforward as it may seem because of the so-called “red noise.” Despite the highly regular radio pulses of millisecond pulsars, they still have small variations. Neutron stars can have internal dynamics or thermal oscillations that change the pulse frequency. These variations are common among pulsars, so when we observe many pulsars, the background “red noise” can appear as a gravitational wave shift.
In this study, the team examines how gravitational wave effects can resemble “red noise” at first glance and how we can distinguish it from genuine gravitational waves. The research has not yet detected any gravitational wave pulses, but it places certain constraints on gravitational wave observations. They were only able to show that there were no mergers of black holes with masses billions of times that of the Sun within a distance of 300 million light-years.
Therefore, it is only a matter of time until they can observe mergers on a galactic scale and extract gravitational wave astronomy from the “red noise.”
Previously, we reported on scientists’ attempts to capture gravitational waves from the moment of the Big Bang.