Science Gazette

Black hole billiards in galaxies’ cores

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Researchers provide the first possible explanation for why one of the most massive black hole pairs ever seen by gravitational waves seemed to combine on a non-circular orbit. Their proposed solution, which has just been published in Nature, includes a chaotic triple drama within a vast disk of gas around a super enormous black hole in a galaxy far, far away.

Black holes are one of the most interesting things in the Universe, but our understanding of them is still restricted, owing to the fact that they do not produce light. Until a few years ago, light was our only source of information about our universe and its black holes, until the Laser Interferometer Gravitational Wave Observatory (LIGO) made the ground-breaking detection of gravitational waves from the merging of two black holes in 2015.

“But how and where do such black holes arise and merge in our Universe? Is it caused by neighboring stars collapsing and turning into black holes, by accidental interactions in star clusters, or by something else? These are some of the most important problems in the new era of gravitational wave astronomy “Assist. Prof. Johan Samsing of the Niels Bohr Institute at the University of Copenhagen, the paper’s primary author, states

He and his colleagues may have now added a new component to the jigsaw, perhaps solving the last piece of a problem that astrophysicists have been grappling with for some years.

In 2019, an Unexpected Discovery

The mystery began in 2019, when the LIGO and Virgo observatories announced an unexpected detection of gravitational waves. The event known as GW190521 is considered to represent the merging of two black holes that were not only heavier than previously thought physically feasible, but also created a burst of light.

Possible explanations for these two traits have subsequently been proposed, but gravitational waves also revealed a third astounding aspect of this event: the black holes did not orbit each other in a circle in the seconds before merging.

“The most startling finding to date is the gravitational wave incident GW190521. The masses and spins of the black holes were already remarkable, but what was much more shocking was that they did not seem to be in a circular orbit leading up to the merger “Imre Bartos, a professor at the University of Florida, is a co-author.

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But what makes a non-circular orbit so strange and unexpected?

“This is due to the basic structure of the gravitational waves released, which not only pulls the pair of black holes closer for them to eventually merge but also works to circularize their orbit,” says co-author and Columbia University Professor Zoltan Haiman.

This remark piqued the interest of many individuals throughout the globe, including Johan Samsing in Copenhagen.

“It got me wondering about how such non-circular (also known as “eccentric”) mergers might happen with such an unexpectedly high likelihood,” explains Johan Samsing.

Tango Requires Three

A potential solution might be found in the hostile environment of galaxies’ cores, where a gigantic black hole millions of times the mass of the Sun is surrounded by a flat, revolving disk of gas.

“In these settings, the usual velocity and density of black holes is so great that smaller black holes bounce about like billiard balls and broad circular binaries cannot form,” says University of Oxford co-author Prof. Bence Kocsis.

However, as the researchers went on to explain, a massive black hole is insufficient “According to new research, the gas disk plays a crucial role in catching smaller black holes, which migrate closer to the center and also closer to one another over time. This indicates that they not only meet and form couples, but that such a pair may interact with another, third, black hole, frequently resulting in a chaotic dance with three black holes floating about “explains co-author and astronomer Hiromichi Tagawa of Tohoku University.

However, all previous research prior to the discovery of GW190521 showed that the formation of eccentric black hole mergers is extremely uncommon. This begs the question, why did the already odd gravitational wave source GW190521 additionally combine on an eccentric orbit?

Black Hole Billiards in Two Dimensions

Everything that has been estimated so far has been predicated on the assumption that black hole interactions occur in three dimensions, as predicted in the bulk of star systems studied thus far.

“But then we wondered what would happen if the black hole interactions took place on a flat disk, which is closer to a two-dimensional environment. Surprisingly, we discovered that in this limit, the likelihood of producing an eccentric merger jumps by up to a hundred times, implying that almost half of all black hole mergers in such disks may be eccentric “Johan Samsing goes on to say:

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“And that finding matches amazingly well with the 2019 observation, which all leads in the direction that the otherwise remarkable features of this source aren’t that weird anymore, assuming it was generated in a flat gas disk encircling a super big black hole in a galactic center.”

This probable answer also contributes to a century-old mechanical issue “The interaction of three things is one of the oldest problems in physics, which Newton, myself, and others have researched extensively. That this now seems to play a critical part in how black holes merge in some of our Universe’s most extreme locations is tremendously exciting “Nathan W. Leigh, Professor at the Universidad de Concepción in Chile, is a co-author.

Gaseous Disks with Black Holes

The gas disk idea also accords with other researchers’ explanations of GW190521’s other two perplexing features. The huge masses of the black hole were achieved by consecutive mergers inside the disk, whereas the emission of light might be caused by the surrounding gas.

“We have now demonstrated that there can be a significant difference in the signals emitted by black holes that merge in flat, two-dimensional disks versus those we commonly consider in three-dimensional stellar systems, which tells us that we now have an additional tool that we can use to learn about how black holes are created and merge in our Universe,” says Johan Samsing.

However, this research is only getting started “People have been working for many years to understand the structure of such gas disks, but the challenge is complex. Our findings are sensitive to the flatness of the disk and the movement of the black holes inside it. When we have a larger population of black hole mergers, including more odd examples like GW190521, we will be able to understand more about these disks. To do so, we must expand on our recently published finding and see where it takes us in this new and interesting area “Zoltan Haiman, co-author, concludes.

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