Dynamical interactions between black holes are thought to be extremely rare, and the gravitational wave data obtained so far by LIGO and Virgo would appear to support this. Nevertheless, two black holes met by chance, and it created something never seen before.
The ripples in space-time caused by colliding black holes have revealed a great deal about these mysterious objects.
Black hole characteristics such as their masses, the pattern of their spiraling inwards toward one another, spins, and orientations are all encoded in these gravitational waves.
Scientists deduced from this that black holes in binary systems have been involved in the majority of the collisions that have been seen. The two black holes once existed as a binary of massive stars that coalesced into black holes, spiraled inward, and ultimately merged.
One merging, out of the roughly 90 thus far discovered, stands out as being extremely odd. GW19052, observed in May 2019, generated ripples in space-time unlike anything else.
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“Its morphology and explosion-like structure are very different from previous observations,” says astrophysicist Rossella Gamba of the University of Jena in Germany.
She adds, “GW190521 was initially analyzed as the merger of two rapidly rotating heavy black holes approaching each other along almost circular orbits, but its special features led us to propose other possible interpretations.”
The short, sharp duration of the gravitational wave signal, in particular, proved difficult to explain.
The actual merger of two black holes generates gravitational waves to be produced, similar to how a rock dropped into a pond causes ripples. However, they are also produced by the binary inspiral, and when two black holes inexorably approach one another, the powerful gravitational interaction emits lesser ripples.
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“The shape and brevity – less than a tenth of a second – of the signal associated with the event lead us to hypothesize an instantaneous merger between two black holes, which occurred in the absence of a spiraling phase,” explains astronomer Alessandro Nagar of the National Institution for Nuclear Physics in Italy.
There are several possible outcomes that could lead to two black holes gravitationally interacting.
The first is that they had been together for a very long period, possibly since the creation of the first stars from the same portion of the universe’s molecular cloud.
The other occurs in a dynamical encounter, which is when two objects traveling through space pass one another close enough to interact gravitationally.
Gamba and her colleagues developed simulations to verify their theory since they believed that this might have occurred with GW190521. In an effort to duplicate the strange gravitational wave signal discovered in 2019, they collided two black holes while adjusting variables including trajectory, spin, and mass.
Their findings imply that the two black holes did not initially form a binary but instead became entangled in one another’s gravitational web, whizzing past one another twice in a crazy, eccentric loop before colliding to form a single, bigger black hole. Furthermore, neither of the hypothetical black holes was spinning.
“By developing precise models using a combination of state-of-the-art analytical methods and numerical simulations, we found that a highly eccentric merger in this case explains the observation better than any other hypothesis previously put forward,” says astronomer Matteo Breschi of the University of Jena.
“The probability of error is 1:4,300!”
This scenario, according to the study, is more probable in a crowded area of space, like a star cluster, where such gravitational interactions are more probable.
This aligns with earlier findings about GW190521. The mass of one of the merging black holes was estimated to be about 85 times that of the Sun.
According to our present theories, black holes with masses greater than 65 solar masses cannot develop from a single star; the sole way such a black hole can originate is through mergers of two lower-mass objects.
Gamba and her colleagues discovered that the masses of the two black holes in the collision are approximately 81 and 52 solar masses, respectively; this is slightly lower than prior calculations, but one of the black holes remains outside the single star core collapse creation path.
It is unclear whether our models need to be tweaked, but hierarchical mergers – in which larger structures grow through the constant merging of smaller items – are more likely in a cluster setting with a high population of objects.
Dynamical interactions between black holes are thought to be extremely rare, and the gravitational wave data obtained so far by LIGO and Virgo would appear to support this. However, rare does not imply impossible, and the current research implies that GW190521 could be the first we have discovered.
And since this is the first, there may be more in the future. The gravitational wave observatories are now being renovated and maintained, but will reopen for a further observing session in March 2023. This time, LIGO’s two detectors in the United States and Italy’s Virgo detector will be joined by KAGRA in Japan for increased observing power.
More discoveries like GW190521 would be fantastic.