The existence of gravitational waves, or ripples in the space-time, is no more just a speculation but a firm truth, after the recent direct detection of such waves from at least two pairs of merging black holes by the LIGO gravitational-wave detector. In such a binary system, two black holes orbit each other at a close separation, nearly at the speed of light, whirling the spacetime in their neighbourhood. Such a disturbance propagates throughout the Universe along its fabric of spacetime, carrying away energy from the binary, which causes the black holes to spiral towards each other and ultimately merge into a single black hole. But how are such tight binary black holes formed? The surest way involves star clusters: the Universe’s gravity reservoirs.
Most, if not all, of the stars in the Universe form in densely-packed spherical groups or clusters, which are held together by the constituent stars’ mutual gravitational pull. When their hydrogen fuel is exhausted, the brightest stars in a cluster collapse into black holes. These black holes, being still the heaviest entities in a cluster, sink and densely gather at the cluster’s center. The close-packed black holes there move randomly and undergo energetic interactions with each other, often forming tight black hole binaries. In fact, the powerful interactions within the tiny ensemble of black holes in a cluster’s stomach make it serve as the entire cluster’s powerhouse. Following the first LIGO detections, there is now a renewed momentum to understand this process better than ever, which would prove critical in interpreting the forthcoming LIGO detections.
In a recent study conducted by Dr. Sambaran Banerjee of the University of Bonn, most detailed and self-consistent computer simulations to date of this scenario have been performed. Even until recently, such simulations have typically been done either with black holes of similar weights (about ten times heavier than the Sun) or by following approximate numerical strategies, so that the calculations were deficient in one way or the other. In reality, the weight can vary widely from black hole to black hole; typically being from about ten to about 100 times that of the Sun. At the same time, it is important to do the calculations as realistically and accurately as possible, since the success of the scenario and its outcome depend on how well the intricate interactions among the black holes are treated. In the latest study by Dr. Banerjee, both of these aspects are well taken care of. Here, models of star clusters with realistic details are evolved in supercomputers. Each such cluster is assembled in a computer with up to hundreds of thousands of stars. Each star’s internal evolution, its orbit, and all sorts of encounters that it undergoes are tracked closely, which is done for all stars simultaneously. After their birth through collapse of massive stars, the black holes are also treated in a similarly elaborate manner, taking into account of the relativistic nature of the interactions among them. The models also vary in the chemical composition of the stars, that affects the formation and the masses of the black holes. That way, the evolution of the model clusters are followed in the highest possible detail until they become as old as the Universe. These comprise the most advanced computer simulations of the “black hole engine”. From these calculations, the type of binary black hole mergers detected by the LIGO could be explicitly reproduced.
“these simulations, for the first time, reveal that many of such gravitational-wave emitting black hole binaries could actually be parts of triple black holes”
Moreover, these simulations, for the first time, reveal that many of such gravitational-wave emitting black hole binaries could actually be parts of triple black holes (or possibly even a binary black hole orbited by a normal star) located inside the clusters. In contrast, until now, it was largely thought that such mergers mainly happen in binaries which are kicked out of the clusters. This is, firstly, a newer insight into the phenomenon that has not been properly realized until now. This might also affect the nature of the mergers seen by the LIGO; for example, binary black hole mergers driven by triples can possibly show up with residual eccentricities and atypical amplitude modulation. Furthermore, since such triple-driven mergers happen right within the clusters, they are going to tell not only about the black holes and their binaries but also about their host clusters, e.g., how dense they are or their chemical composition. Of course, for this, one needs to pinpoint the location of the merging binary black hole in the cosmos, which will hopefully become possible within a decade when there will be a network of gravitational-wave detectors over the globe.
The new study predicts that some tens to hundreds of such mergers should be detected yearly by the LIGO, as the instrument keeps improving. Now that the future space-based gravitational-wave observatory LISA has just been approved by the European Space Agency as an official mission, which will be able to probe the binaries in much greater detail, it is only the dawn of such studies.