From The University of Copenhagen [Københavns Universitet](DK) and The Niels Bohr Institute [Niels Bohr Institutet](DK) via phys.org: “Black hole billiards in the centers of galaxies may explain black hole mergers”
From The University of Copenhagen [Københavns Universitet](DK)
and
The Niels Bohr Institute [Niels Bohr Institutet](DK)
via
Illustration of a swarm of smaller black holes in a gas disk rotating around a giant black hole. Credit: J. Samsing/Niels Bohr Institute
Researchers have provided the first plausible explanation to why one of the most massive black hole pairs observed to date by gravitational waves also seemed to merge on a non-circular orbit. Their suggested solution, now published in Nature, involves a chaotic triple drama inside a giant disk of gas around a supermassive black hole in another galaxy.
Black holes are one of the most fascinating objects in the universe, but our knowledge of them is still limited—especially because they do not emit any light. Up until a few years ago, light was our main source of knowledge about our universe and its black holes, until the Laser Interferometer Gravitational Wave Observatory (LIGO) in 2015 made its breakthrough observation of gravitational waves from the merger of two black holes.
“But how and where in our universe do such black holes form and merge? Does it happen when nearby stars collapse and both turn into black holes, is it through close chance encounters in star clusters, or is it something else? These are some of the key questions in the new era of Gravitational Wave Astrophysics,” says assistant professor Johan Samsing from the Niels Bohr Institute at the University of Copenhagen, lead author of the paper.
He and his collaborators may have now provided a new piece to the puzzle, which possibly solves the last part of a mystery that astrophysicists have struggled with for the past few years.
Unexpected discovery in 2019
The mystery dates back to 2019, when an unexpected discovery of gravitational waves was made by the LIGO and Virgo observatories. The event, named GW190521, is understood to be the merger of two black holes that were not only heavier than previously thought physically possible, but had also produced a flash of light.
Possible explanations have since been provided for these two characteristics, but the gravitational waves also revealed a third astonishing feature of this event—namely that the black holes did not orbit each other along a circle in the moments before merging.
“The gravitational wave event GW190521 is the most surprising discovery to date. The black holes’ masses and spins were already surprising, but even more surprising was that they appeared not to have a circular orbit leading up to the merger,” says co-author Imre Bartos, professor at the University of Florida.
But why is a non-circular orbit so unusual and unexpected?
“This is because of the fundamental nature of the gravitational waves emitted, which not only brings the pair of black holes closer for them to finally merge but also acts to circularize their orbit.” explains co-author Zoltan Haiman, a professor at Columbia University.
This observation made many people around the world, including Johan Samsing in Copenhagen, wonder.
“It made me start thinking about how such non-circular (known as ‘eccentric’) mergers can happen with the surprisingly high probability as the observation suggests,” says Samsing.
It takes three to tango
A possible answer would be found in the harsh environment in the centers of galaxies harboring a giant black hole millions of times the mass of the sun and surrounded by a flat, rotating disk of gas.
“In these environments the typical velocity and density of black holes is so high that smaller black holes bounce around as in a giant game of billiards and wide circular binaries cannot exist,” points out co-author professor Bence Kocsis from the University of Oxford.
But as the group further argued, a giant black hole is not enough.
“New studies show that the gas disk plays an important role in capturing smaller black holes, which over time move closer to the center and also closer to one other. This not only implies they meet and form pairs, but also that such a pair might interact with another, third, black hole, often leading to a chaotic tango with three black holes flying around, ” explains astrophysicist Hiromichi Tagawa from Tohoku University, co-author of the study.
However, all previous studies up to observation of GW190521 indicated that forming eccentric black hole mergers is relatively rare. This naturally brings up the question: Why did the already unusual gravitational wave source GW190521 also merge on an eccentric orbit?
Two-dimensional black hole billiards
Everything that has been calculated so far was based on the notion that the black hole interactions are taking place in three dimensions, as expected in the majority of stellar systems considered so far.
“But then we started thinking about what would happen if the black hole interactions were instead to take place in a flat disk, which is closer to a two-dimensional environment. Surprisingly, we found in this limit that the probability of forming an eccentric merger increases by as much as a 100 times, which leads to about half of all black hole mergers in such disks possibly being eccentric,” says Johan Samsing and continues:
“And that discovery fits incredibly well with the observation in 2019, which all in all now points in the direction that the otherwise spectacular properties of this source are not so strange again, if it was created in a flat gas disk surrounding a supermassive black hole in a galactic nucleus.”
This possible solution also adds to a century-old problem in mechanics,
“The interaction between three objects is one of the oldest problems in physics, which both Newton, myself, and others have intensely studied. That this now seems to play a crucial role in how black holes merge in some of the most extreme places of our universe is incredibly fascinating “, says co-author Nathan W. Leigh, professor at Universidad de Concepción, Chile.
Black holes in gaseous disks
The theory of the gas disk also fits with other researchers’ explanations of the other two puzzling properties of GW190521. The large masses of the black hole have been reached by successive mergers inside the disk, while the emission of light could originate from the ambient gas.
“We have now shown that there can be a huge difference in the signals emitted from black holes that merge in flat, two-dimensional disks, versus those we often consider in three-dimensional stellar systems, which tells us that we now have an extra tool that we can use to learn about how black holes are created and merge in our universe,” says Samsing.
But this study is only the beginning.
“People have been working on understanding the structure of such gas disks for many years, but the problem is difficult. Our results are sensitive to how flat the disk is, and how the black holes move around in it. Time will tell whether we will learn more about these disks, once we have a larger population of black hole mergers, including more unusual cases similar to GW190521. To enable this, we must build on our now published discovery, and see where it leads us in this new and exciting field,” concludes co-author Zoltan Haiman.
See the full article here .
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The Niels Bohr Institute [Niels Bohr Institutet](DK) is a research institute of the The University of Copenhagen [Københavns Universitet][UCPH] (DK). The research of the institute spans astronomy, geophysics, nanotechnology, particle physics, quantum mechanics and biophysics.
The Institute was founded in 1921, as the Institute for Theoretical Physics of the Københavns Universitet [UCPH](DK), by the Danish theoretical physicist Niels Bohr, who had been on the staff of the University of Copenhagen since 1914, and who had been lobbying for its creation since his appointment as professor in 1916. On the 80th anniversary of Niels Bohr’s birth – October 7, 1965 – the Institute officially became The Niels Bohr Institutet (DK). Much of its original funding came from the charitable foundation of the Carlsberg brewery, and later from the Rockefeller Foundation.
During the 1920s, and 1930s, the Institute was the center of the developing disciplines of atomic physics and quantum physics. Physicists from across Europe (and sometimes further abroad) often visited the Institute to confer with Bohr on new theories and discoveries. The Copenhagen interpretation of quantum mechanics is named after work done at the Institute during this time.
On January 1, 1993 the institute was fused with the Astronomic Observatory, the Ørsted Laboratory and the Geophysical Institute. The new resulting institute retained the name Niels Bohr Institutet (DK)).
The University of Copenhagen [Københavns Universitet][UCPH] (DK) is the oldest university and research institution in Denmark. Founded in 1479 as a studium generale, it is the second oldest institution for higher education in Scandinavia after Uppsala University [Uppsala universitet](SE) (1477). The university has 23,473 undergraduate students, 17,398 postgraduate students, 2,968 doctoral students and over 9,000 employees. The university has four campuses located in and around Copenhagen, with the headquarters located in central Copenhagen. Most courses are taught in Danish; however, many courses are also offered in English and a few in German. The university has several thousands of foreign students, about half of whom come from Nordic countries.
The university is a member of the International Alliance of Research Universities (IARU), along with University of Cambridge (UK), Yale University, The Australian National University (AU), and The University of California-Berkeley(US), amongst others. The 2016 Academic Ranking of World Universities ranks the University of Copenhagen as the best university in Scandinavia and 30th in the world, the 2016-2017 Times Higher Education World University Rankings as 120th in the world, and the 2016-2017 QS World University Rankings as 68th in the world. The university has had 9 alumni become Nobel laureates and has produced one Turing Award recipient.
Its establishment sanctioned by Pope Sixtus IV, the University of Copenhagen was founded by Christian I of Denmark as a Catholic teaching institution with a predominantly theological focus. After 1537, it became a Lutheran seminary under King Christian III. Up until the 18th century, the university was primarily concerned with educating clergymen. Through various reforms in the 18th and 19th century, the University of Copenhagen was transformed into a modern, secular university, with science and the humanities replacing theology as the main subjects studied and taught.
The University of Copenhagen consists of six different faculties, with teaching taking place in its four distinct campuses, all situated in Copenhagen. The university operates 36 different departments and 122 separate research centres in Copenhagen, as well as a number of museums and botanical gardens in and outside the Danish capital. The University of Copenhagen also owns and operates multiple research stations around Denmark, with two additional ones located in Greenland. Additionally, The Faculty of Health and Medical Sciences and the public hospitals of the Capital and Zealand Region of Denmark constitute the conglomerate Copenhagen University Hospital.
A number of prominent scientific theories and schools of thought are namesakes of the University of Copenhagen. The famous Copenhagen Interpretation of quantum mechanics was conceived at the Niels Bohr Institute [Niels Bohr Institutet](DK), which is part of the university. The Department of Political Science birthed the Copenhagen School of Security Studies which is also named after the university. Others include the Copenhagen School of Theology and the Copenhagen School of Linguistics.
As of October 2020, 39 Nobel laureates and 1 Turing Award laureate have been affiliated with the University of Copenhagen as students, alumni or faculty. Alumni include one president of the United Nations General Assembly and at least 24 prime ministers of Denmark. The University of Copenhagen fosters entrepreneurship, and between 5 and 6 start-ups are founded by students, alumni or faculty members each week.
History
The University of Copenhagen was founded in 1479 and is the oldest university in Denmark. In 1474, Christian I of Denmark journeyed to Rome to visit Pope Sixtus IV, whom Christian I hoped to persuade into issuing a papal bull permitting the establishment of university in Denmark. Christian I failed to persuade the pope to issue the bull however and the king returned to Denmark the same year empty-handed. In 1475 Christian I’s wife Dorothea of Brandenburg Queen of Denmark made the same journey to Rome as her husband did a year before. Unlike Christian I Dorothea managed to persuade Pope Sixtus IV into issuing the papal bull. On the 19th of June, 1475 Pope Sixtus IV issued an official papal bull permitting the establishment of what was to become the University of Copenhagen.
On the 4th of October, 1478 Christian I of Denmark issued a royal decree by which he officially established the University of Copenhagen. In this decree Christian I set down the rules and laws governing the university. The royal decree elected magistar Peder Albertsen as vice chancellor of the university and the task was his to employ various learned scholars at the new university and thereby establish its first four faculties: theology; law; medicine; and philosophy. The royal decree made the University of Copenhagen enjoy royal patronage from its very beginning. Furthermore, the university was explicitly established as an autonomous institution giving it a great degree of juridical freedom. As such the University of Copenhagen was to be administered without royal interference and it was not subject to the usual laws governing the Danish people.
The University of Copenhagen was closed by the Church in 1531 to stop the spread of Protestantism and re-established in 1537 by King Christian III after the Lutheran Reformation and transformed into an evangelical-Lutheran seminary. Between 1675 and 1788 the university introduced the concept of degree examinations. An examination for theology was added in 1675 followed by law in 1736. By 1788 all faculties required an examination before they would issue a degree.
In 1807 the British Bombardment of Copenhagen destroyed most of the university’s buildings. By 1836 however the new main building of the university was inaugurated amid extensive building that continued until the end of the century. The University Library (now a part of the Royal Library); the Zoological Museum; the Geological Museum; the Botanic Garden with greenhouses; and the Technical College were also established during this period.
Between 1842 and 1850 the faculties at the university were restructured. Starting in 1842 the University Faculty of Medicine and the Academy of Surgeons merged to form the Faculty of Medical Science while in 1848 the Faculty of Law was reorganised and became the Faculty of Jurisprudence and Political Science. In 1850 the Faculty of Mathematics and Science was separated from the Faculty of Philosophy. In 1845 and 1862 Copenhagen co-hosted nordic student meetings with Lund University [Lunds universitet] (SE).
The first female student was enrolled at the university in 1877. The university underwent explosive growth between 1960 and 1980. The number of students rose from around 6,000 in 1960 to about 26,000 in 1980 with a correspondingly large growth in the number of employees. Buildings built during this time period include the new Zoological Museum; the Hans Christian Ørsted and August Krogh Institutes; the campus centre on Amager Island; and the Panum Institute.
The new university statute instituted in 1970 involved democratisation of the management of the university. It was modified in 1973 and subsequently applied to all higher education institutions in Denmark. The democratisation was later reversed with the 2003 university reforms. Further change in the structure of the university from 1990 to 1993 made a Bachelor’s degree programme mandatory in virtually all subjects.
Also in 1993 the law departments broke off from the Faculty of Social Sciences to form a separate Faculty of Law. In 1994 the University of Copenhagen designated environmental studies; north–south relations; and biotechnology as areas of special priority according to its new long-term plan. Starting in 1996 and continuing to the present the university planned new buildings including for the University of Copenhagen Faculty of Humanities at Amager (Ørestaden) along with a Biotechnology Centre. By 1999 the student population had grown to exceed 35,000 resulting in the university appointing additional professors and other personnel.
In 2003 the revised Danish university law removed faculty staff and students from the university decision process creating a top-down control structure that has been described as absolute monarchy since leaders are granted extensive powers while being appointed exclusively by higher levels in the organization.
In 2005 the Center for Health and Society (Center for Sundhed og Samfund – CSS) opened in central Copenhagen housing the Faculty of Social Sciences and Institute of Public Health which until then had been located in various places throughout the city. In May 2006 the university announced further plans to leave many of its old buildings in the inner city of Copenhagen- an area that has been home to the university for more than 500 years. The purpose of this has been to gather the university’s many departments and faculties on three larger campuses in order to create a bigger more concentrated and modern student environment with better teaching facilities as well as to save money on rent and maintenance of the old buildings. The concentration of facilities on larger campuses also allows for more inter-disciplinary cooperation. For example the Departments of Political Science and Sociology are now located in the same facilities at CSS and can pool resources more easily.
In January 2007 the University of Copenhagen merged with the Royal Veterinary and Agricultural University and the Danish University of Pharmaceutical Science. The two universities were converted into faculties under the University of Copenhagen and were renamed as the Faculty of Life Sciences and the Faculty of Pharmaceutical Sciences. In January 2012 the Faculty of Pharmaceutical Sciences and the veterinary third of the Faculty of Life Sciences merged with the Faculty of Health Sciences forming the Faculty of Health and Medical Sciences and the other two thirds of the Faculty of Life Sciences were merged into the Faculty of Science.
Cooperative agreements with other universities
The university cooperates with universities around the world. In January 2006, the University of Copenhagen entered into a partnership of ten top universities, along with the Australian National University (AU), Swiss Federal Institute of Technology in Zürich [ETH Zürich] [Eidgenössische Technische Hochschule Zürich](CH), The National University of Singapore [Universiti Nasional Singapura] (SG), Peking University [北京大学](CN), University of California Berkeley (US), University of Cambridge (UK), University of Oxford (UK), University of Tokyo {東京大学](JP) and Yale University (US). The partnership is referred to as the International Alliance of Research Universities (IARU).
The Department of Scandinavian Studies and Linguistics at University of Copenhagen signed a cooperation agreement with the Danish Royal School of Library and Information Science in 2009.
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