The Gravity Chorus

There is a supermassive black hole at the center of our galaxy, and it’s not alone. There is also likely a forest of binary black holes, neutron stars, and white dwarfs. All of these emit gravitational waves as they gradually spiral ever closer together. These gravitational waves are too faint for us to detect at the moment, but future observatories will be able to observe them. This poses an interesting astronomical challenge.

X-ray: NASA/UMass/Q.D. Wang et al.; IR: NASA/STScI

Right now, our gravitational wave observatories can only see the last moments of a black hole or neutron star merger. What is known as the chirp of gravitational waves just before the two bodies collide. Future observatories such as LISA should give us a longer view. We should be able to capture the gravitational waves well before they merge. This is particularly true for binaries that are asymmetrical. If one member of a binary is much larger than the other, or the two are in a very elliptical orbit, the rise and fall of the gravitational signal will be easier to detect. That’s the hope because the longer we can observe an inspiraling binary, the better we will understand the gravitational dynamics.

But there is another source of gravitational waves that could frustrate our observations of binaries. These occur when a neutron star or even a brown dwarf orbits close to the supermassive black hole in our galaxy. A few dozen of these orbit very close to the black hole and will make their own gravitational waves as they do. This is also useful to astronomers because gravitational perturbations of the supermassive black hole allow us to better understand its dynamics.

Both types of systems are things we’d love to observe, but as a recent study shows, the two types of signals overlap.¹ Instead of finding new clear signals, LISA and other future observatories may find a cacophony of gravitational noise.

In this study, the authors show how the “forest” of gravitational sources could drown out binary black hole signals if they have a mass less than 10,000 solar masses or more. But they also demonstrate how this background forest has a statistical profile. With better modeling, we might be able to filter the gravitational background from interesting signals. Another possibility is to use machine learning to distinguish unique signals from within the noise. For sources such as the inspiraling of brown dwarfs, there would likely be radio flares as tidal forces of the supermassive black hole stress the brown dwarf. So multi-messenger observations of light and gravity could further distinguish background signals.

We are still decades away from sensitive gravitational observatories such as LISA, but as this study shows, the limits of observation aren’t our only challenge. We will have to filter that data in creative ways. This second challenge is something we can work on now.