Similar to people, black holes exist in pairs and singles. Furthermore, the three-body dynamics are frequently chaotic when a third black hole joins an already-existing pair. As in interpersonal connections once again.
A supermassive black hole may be found in the heart of most galaxies. This holds true for both the Andromeda galaxy, which is our closest neighbor, and our Milky Waygalaxy. There is a collision path between these two galaxies. Before the Sun dies, in a few billion years, the two galaxies will unite. I co-authored the initial study predicting this merger to a larger galaxy, which I named “Milkomeda,” in 2008 with my former postdoctoral fellow T.J. Cox.
It is anticipated that the supermassive black holes from Andromeda and the Milky Way will settle to the core of Milkomeda, each adorned with a dense star cluster. The black holes are predicted to merge when they reach a distance less than the solar system due to gravitational radiation emission, which is caused by dynamic friction on nearby stars and gas. Similar to the waves created when two sticks move in a circle on a pond’s surface, the two black holes’ spacetime ripples are unavoidable.
Not only can mergers occur locally, but they also occur in the young cosmos. A recent preprint revealed that 740 million years after the Big Bang, the Webb telescope has found an offset quasar that was probably part of a merging black-hole pair. Galaxy mergers throughout cosmic history are expected to produce an abundant population of black hole couples, as I have shown in 17 articles with my former PhD student, Laura Blecha.
A third galaxy might join a black-hole duo in a galaxy like Milkomeda, pushing out the lightest black hole, and disrupting the pair. Ten years ago, Girish Kulkarni and I calculated in a study that a significant portion of supermassive black-hole pairs may have a third black hole in galactic nuclei disrupting them. My former doctoral student Loren Hoffman and I computed the chaotic dynamics of the triple black hole system in a previous publication, which we utilized to explain the properties of a particular kicking quasar in a different study.
Whether or not pairings of black holes at such distances may release all of their energy as gravitational radiation before merging is the central topic. Alternatively, the absence of any material that might propel them into the gravitational emission phase could prevent them from merging at all, or friction on a massive mass of gas and stars could speed up the final merging phase. Regardless of the scenario, there will be less gravitational wave emission than if all partners finish the last stage of their merger by releasing gravitational radiation.
Is the predicted cosmic background of gravitational waves consistent with the census of all black hole pairings at vast separations? Interestingly, both amounts were measured recently.
Initially, a study conducted last month using data from the Webb telescope revealed that tens of percent of quasars are found in pairs. This significant proportion suggests that, unlike synchronized swimmers, quasars light simultaneously during a merger, or else we would only be able to observe one at a time. This conclusion makes sense since it is anticipated that massive amounts of gas would be thrust simultaneously into each merging black hole during galaxy mergers. Since both black holes receive their feeding from the same gas reservoir, the length of the merger phase during which the feeding is effective must be equivalent to the lifespan of quasars for a high proportion of dual quasars.
Second, new research has measured the cumulative gravitational-wave background from all such couples across cosmic history using a variety of pulsars, or spinning neutron stars, as clocks. How well does this measurement match the black hole pair census at vast separations?
Working along with the eminent scientist Ham sa Padmanabhan, we demonstrated that the pulsar timing arrays’ cosmic backdrop of gravity waves is not as strong as anticipated, according to the Webb telescope’s census of black hole pairings at great distances. This implies that pairings merge more quickly in the last stages. Considering the turbulent dynamics and feedback in merging galaxies, the outcome is not surprising.
Furthermore, by ejecting stars into space, black hole couples function similarly to pinball machines. In a work with my former postdoc James Guillochon, I calculated that some of these stars surpass the speed of light. Interstellar travel businesses will have to charge top dollar for tickets to visit any habitable planets that these relativistic stars may have.
These journeys begin in the middle of a merging galaxy, such as Milkomeda, and travel through interstellar space within it until arriving to breathtaking outside vistas of the galaxy in intergalactic space. Occasionally, nature presents a spectacle that surpasses any conceivable spacecraft we could envision.
