Unlike NASA’s Webb Space Telescope, large ground based gravitational wave detectors are never going to generate pretty pictures. But detections of gravitational waves —- perturbations of spacetime itself —- are one of the few ways to track both the mergers of massive stellar black holes and potentially even massive galaxies over cosmic time.
These stellar mass black holes are not the supermassive black holes thought to be at the center of most grand spiral galaxies like our own Milky Way. But rather they are stellar mass black holes originate from the most massive and short-lived stars in the universe. Rare O-type stars with masses some twenty times that of our Sun end their lives as black holes after undergoing a so-called core collapse supernova.
And if two such black holes end up circling each other and eventually merge, ground-based gravitational wave detectors can sometimes pinpoint them even at great cosmological distances.
We’re thinking to use the mergers of massive black holes as tracers of the mergers of much bigger things like galaxies, Astrid Lamberts, a gravitational wave astronomer at France’s Nice Observatory, told me during a sit-down interview in her office.
Einstein’s theory of General Relativity predicts that any accelerated mass would produce these spacetime ripples, informally known as gravitational waves. But the problem is detecting them.
Yet in a paper just accepted for publication in the journal Astronomy & Astrophysics, lead author Tristan Bruel, a doctoral student at France’s Observatoire de la Cote D’Azur, and Lamberts, one of the paper’s co-authors, note that major mergers of galaxies appear to drive the formation of massive clusters which, in turn, result in the formation of merging binary black holes.
Astronomers have connected the most massive black hole binary mergers with extreme cases of star formation, which occur when two galaxies like our own Milky Way merge, says Lamberts, a science team member of the ground-based LIGO-Virgo-KAGRA Collaboration. LVK is a long running international collaboration of ground-based gravitational wave detectors based in Europe, the U.S. and Japan.
We’ve been using simulations of these collisions of galaxies to realize that the merger of massive galaxies leads to intense star formation and the formation of massive clusters, says Lamberts. In these massive clusters, you can have these many random pairings of black holes, so two black holes merge together, and then again merge with other black holes, she says.
Even so, Lamberts says the team doesn’t expect to detect any such black hole mergers from within our own Milky Way Galaxy. We would have to be extremely lucky to detect something like that happening in the Milky Way, she told me.
As For Practical Astronomical Applications?
This could be used to pinpoint specific galaxies. billions of light years away, says Lamberts.
The collaboration has been detecting black hole mergers. But they are puzzled by the fact that some of these black holes appear to be so massive, as much as 80 times the mass of the Sun.
We think that they could be what we call ‘second generation’ mergers of black holes, says Lamberts. Do that again a second or third time and these two things can merge again and make a really massive black hole, she says.
How Is This Measured?
When a gravitational wave passes through the detectors, it creates tiny disruptions in spacetime which literally changes the distance between the detector’s two test mass objects. This disruption is signaled by the way the light pattern changes along perpendicular arms of the detectors.
If the light pattern changes, we know that the distance has changed, says Lamberts.
How Does This Work?
Once you have a black hole, not much happens, it only interacts gravitationally with whatever is around it, says Lamberts. But if you have star clusters, you can have millions of stars and black holes in a rather small volume, she says.
If you have two black holes at the right distance from each other, they will circle each other and emit gravitational waves, says Lamberts. And by emitting the waves, they lose a little bit of energy, she says. So that brings them a little closer and the closer they get, the more gravitational waves they generate, which in turn makes them closer still, says Lamberts.
As for how long it takes these black holes to merge?
Two massive black holes circling each other with an orbital period of only a day may last in that phase for millions of years, says Lamberts. If you take an hour period, it’s probably going to spend 100,000 years in that phase, she says. And with an orbital period of only a second, it’s only a matter of minutes before it merges, says Lamberts.
