There is a supermassive black hole inside every galaxy, in particular, at the centre of the galaxies, also known as the active galactic nucleus (AGN). Surrounding many of these giants are accretion disks, where stellar-mass black holes can form. These smaller black holes can then merge and create ripples of spacetime, known as gravitational waves (GW), which we can detect. When these GWs encounter a supermassive black hole, their trajectories will be bent; in effect, these waves are “lensed” by the supermassive black hole. This implies that if an observer is at the right location, they can observe the GWs from the smaller merging black holes being lensed by the supermassive black hole at the centre of the galaxy. Now, suppose the orientations of each galaxy are randomly distributed; one can expect that, on average, a certain fraction of these galaxies will produce a lensed GW event.
The LIGO-Virgo-KAGRA collaboration has detected approximately 90 GW events to date, but none have been identified as lensed events. This raises the question: What is the probability of not detecting any lensed events thus far? Furthermore, this suggests that there cannot be too many of these galaxies in our Universe; otherwise, we would have observed one of these remarkable lensed events. This also enables us to establish an upper limit on the abundance or population of these AGNs in our Universe.
In our latest research, published in the Astrophysical Journal Letters, we show that current observations can only place weak limits on these AGNs if all GWs come from places very close to the supermassive black hole, about 10 Schwarzschild radii. In the future, with the next-generation detectors and the numerous GW events that they will detect, we will be able to place much stronger bounds on the population of AGNs and extend to a much farther radius.
This time, we adopted the typical techniques used in particle physics, the non-observation of a phenomenon to place constraints on the abundance of a particle. We promoted this technique to put limits on large-scale structures, namely, the abundance of galaxies. We also believe that if such a lensed GW is observed in the future, it will be able to inform us immensely about the host environment.
Our article has recently been featured in AAS Nova as well!
See more:
Constraining Binary Mergers in Active Galactic Nuclei Disks Using the Nonobservation of Lensed Gravitational Waves
Samson H. W. Leong, et. al.
The Astrophysical Journal Letters, Volume 979, Number 2