Relativity, torque and spin PROFESSOR DAVID MERRITT DAVID PROFESSOR

Professor David Merritt explains his interest in astrophysics and the focus of his current research. Describing the improvements made to the computational algorithms, he highlights the importance of taking into account the effects of relativity in these calculations

sometimes reveal modes of behaviour that can be inefficient when applied to galactic are strikingly simple yet totally unexpected. nuclei, because motion is dominated by the presence of the SBH. A star or stellar remnant Your current project focuses on the dynamical will complete millions of orbits around evolution of dense clusters of stars and the SBH before coming close enough to stellar remnants around supermassive black another star to be perturbed, but it is those holes (SBHs) at the centre of galaxies. Why is perturbations that are driving the collective this important? evolution. The computer code needs to follow the unperturbed orbit about the SBH with A star like the Sun would be tidally torn apart extremely high precision: otherwise the effects if it came sufficiently close to an SBH. Partly of the small perturbations will be swamped by because of tidal effects, the only objects numerical errors. that we expect to find very near an SBH are ‘compact remnants’: neutron stars and Why is it important to take into account stellar-mass black holes, the dense leftovers the effects of relativity when simulating the from the evolution of massive stars. These evolution of nuclear star clusters? remnants can interact through ‘gravitational encounters’: close passages that exchange One insight that comes from my group’s research energy and gradually change orbits. From time is that the effects of relativity can be crucial even Could you describe what led to to time, such interactions cause a remnant for stars that spend most of their time far away your interest in modelling complex to be scattered into the SBH. The resulting from SBHs. The effects of relativity depend less on astrophysical systems? capture event will change the mass and spin of the size of an orbit than on its distance of closest the SBH, but it can also result in an observable approach to the SBH. Very eccentric orbits bring I’ve long been fascinated by the connection burst of gravitational waves. Detecting a star near to the SBH once each orbital period, between the microscopic and the gravitational waves is currently a major focus even if that star spends most of its time much macroscopic in physics. A persistent feature for theoretical physicists, who would like to use farther away. of many complex systems is the emergence the observed properties of the waves to test of coherent (macroscopic) behaviour from Einstein’s theory of gravity. What is the significance of the extreme mass the seemingly random interactions of ratio inspiral (EMRI) problem? microscopic ‘agents’ – molecules in a gas, Is there anything novel about the individuals in a population, stars in a galaxy, computational algorithms that you are using EMRI refers to a particularly interesting mode etc. The questions we pose as scientists are to simulate clusters of stars around SBHs? of stellar remnant capture by an SBH. If the usually at the macroscopic level (eg. how remnant is scattered onto a highly eccentric does a galaxy evolve?), but the best way Standard N-body algorithms work very orbit about the SBH, gravitational waves we have to address them is via simulations well when close encounters between stars emitted during times of closest approach will based on the microscopic physics (eg. how are rare and when the simulation time is extract energy from the orbit. The prospect do individual stars move?). Such simulations limited to a few orbital periods. But they of detecting gravitational waves from the 22 INTERNATIONAL INNOVATION PROFESSOR DAVID MERRITT Modelling galactic nuclei

A team led by a researcher from the Rochester Institute of Technology has secured significant funding to conduct a three- year research project into the evolution of galactic nuclei. The aim is to better understand the interactions of supermassive black holes with stars and stellar remnants, and their role in galaxy evolution

THE ORIGIN AND evolution of the Universe funds have allowed the group to embark upon and its components – stars, planets and galaxies a three-year research project studying the – has long fascinated scientists, resulting in evolution of galactic nuclei. considerable research. Today, our knowledge of the Universe is rapidly advancing, enabled FOCUSING ON THE INFREQUENT by generous funding and expertise behind exploration efforts. One project in particular is Many different types of algorithm are employed attempting to elucidate the processes involved in the modelling of stellar interactions. in the formation of galaxies and specifically their Traditionally, a set of algorithms called N-body central structures. codes have been used: “Standard N-body codes follow the evolution of N stars as they The centers of galaxies contain supermassive move in response to their mutual gravitational black holes (SBHs), which are believed to provide attraction,” explains Merritt. These codes are the gravitational energy underlying quasars and good for understanding infrequent interactions other energetic phenomena. But galactic nuclei between stars within a limited simulation period, have remained largely mysterious, shrouded in a but the conditions in the centres of galaxies are cloak of dust or simply too far away to resolve. far different, necessitating a new approach.

inspirals was one In the central parts of galaxies, stars and stellar OBSERVATION AND SIMULATION of the primary remnants such as neutron stars orbit an SBH motivations behind Using both snapshots in time produced millions of times before a close interaction occurs the proposed laser- by instruments such as the Hubble Space with another stellar body. As these interactions interferometer space Telescope and advanced computational are believed to be the driving force behind the antenna (LISA). LISA modelling algorithms, scientists are gaining evolution of galactic centres and are poorly could have detected important insights into the physical processes understood, they are the main theme of the group’s low-frequency that shape galactic centres. Understanding the modelling efforts. One of their key collaborators gravitational waves physical principles that underlie the dynamical has helped the group to make headway on this from EMRIs occurring evolution of galactic nuclei is the ambition of problem by creating a computational code almost anywhere in the Professor David Merritt from the Rochester which accurately tracks normal orbits of stellar observable Universe. Institute of Technology (RIT) in New York and bodies around SBHs and focuses on moments Unfortunately, NASA his colleagues. of interaction which drive the holistic evolution chose to withdraw of the galaxy: “My collaborator Seppo Mikkola financially from the LISA Merritt, an experienced astrophysicist, shares at Tuorla Observatory developed a new scheme project in 2011, but design the now commonly held belief that the centres called ‘algorithmic regularisation’ specifically for work is continuing on a of galaxies contain one or more supermassive scaled-down version. black holes (SBHs). He and his colleagues want to understand how clusters of stars evolve and Do you anticipate any new congregate near the middle of galaxies under directions for your research? the influence of central SBHs. In an attempt to understand this behaviour, the group has Astrophysicists believe that SBHs developed advanced algorithms that can are rapidly spinning. According to simulate these processes. general relativity, a spinning black hole exerts a torque on matter orbiting Finance is central to all modern, large-scale around it. There is also an equal and research efforts. Merritt and his team have opposite back-reaction: the orbiting matter been highly successful in attracting funding, exerts a torque on the black hole, causing having secured US $458,000 in the form of a its spinaxis to precess, in much the same grant from the NASA Astrophysics Theory way that a spinning top precesses due to the programme – which awarded funding to torque exerted by its weight. This mutual spin- only 28 projects from a pool of 181 orbit interaction has a number of important submissions – and an award of US consequences for the joint evolution of SBHs $252,200 from the National and galactic nuclei, but many of the details Science Foundation. These RIPPLES IN THE CURVATURE OF SPACE-TIME ©NASA remain unresolved. N-body algorithms are an ideal tool for exploring this complex problem. WWW.RESEARCHMEDIA.EU 23 INTELLIGENCE

DYNAMICAL STUDIES OF THE CENTERS OF GALAXIES The findings made by the Rochester Institute of Technology OBJECTIVES investigators have provided important contributions to the science To study the dynamical evolution of dense of galactic nuclei and their evolution clusters of stars and stellar remnants – white dwarfs, neutron stars and black holes – around super-massive black holes (SBHs) in the centers simulating galactic nuclei,” highlights Merritt. parsecs apart. In order for the two SBHs to of galaxies like the Milky Way. This project Mikkola’s new algorithm is much faster than coalesce into a single body, the system must be will use a new computational algorithm that existing codes, making it more efficient and stripped of huge amounts of energy. Emission of can efficiently simulate systems with larger allowing the research to focus on the events gravitational waves is one mechanism capable numbers of gravitating bodies. The effects of considered most important. of removing this energy, but before gravitational spin of the SBH will be included for the first wave emission can be efficient, the two SBHs time; spin induces a frame-dragging torque on must somehow be brought much closer together the orbits, and an inverse torque exerted by THE PROMISE OF GRAVITATIONAL WAVE the orbiting bodies on the SBH, altering its spin than a parsec. direction. Further simulations will study clusters One way that an orbiting object can be captured containing a distribution of remnant masses and by a SBH is through gravitational perturbations Recent work by Merritt and his team has made clusters containing binary black holes. from other stars, in much the same way that notable inroads into this problem by illustrating Jupiter scatters comets into the Sun. But a more that the interaction between binary SBHs and KEY COLLABORATORS interesting capture channel is called an EMRI, nearby stars is efficient enough to bridge this Tal Alexander, Weizmann Institute, Israel or extreme-mass-ratio inspiral. An EMRI will gap: “This is important, because while every follow a more gradual pattern, spiralling inwards SBH seems to be accompanied by stars, only Seppo Mikkola, Tuorla Observatory, Finland over multiple orbits as it emits gravitational a fraction are associated with appreciable Eugene Vasiliev, Lebedev Physical Institute, waves, causing its orbital energy to decrease. amounts of gas,” he explains. Moscow, Russia The detection of gravitational waves from EMRIs would enable astrophysicists to accurately The findings made by the RIT investigators have Clifford Will, University of Florida, measure the mass and angular momentum of the already provided important contributions to the Gainesville, USA SBH, as well as testing Einstein’s theory of gravity. science of galactic nuclei and their evolution. FUNDING Their contributions to the EMRI hypothesis and Merritt and his colleagues were the first scientists SBH coalescence have also changed the way National Science Foundation – to successfully apply an N-body code to the scientists think about these events. Holding award no. 1211602 EMRI hypothesis. When they did so the results great promise for contributions to tomorrow’s National Aeronautics and Space Administration were startling: “When we adapted Mikkola’s new astrophysics, the work represents the forefront – award no. NNX13AG92G algorithm to this problem in 2011, a striking and of theoretical astrophysics. Despite impressive unexpected phenomenon appeared which we success, the need for technology capable CONTACT call the ‘Schwarzschild barrier’,” he explains. of detecting gravitational waves in deep Professor David Merritt space highlights the necessity of continued Principal Investigator This phenomenon acts in such a way as to investment in both the people and tools of ‘repel’ inspiralling bodies from the eccentric future astrophysics. Only with this in place can School of Physics and Astronomy orbits that would otherwise lead to capture we hope to unlock more of the deep secrets of Rochester Institute of Technology as EMRIs. Critically, the presence of the the Universe. Office: CAR A100 Schwarzschild barrier reduces the frequency of 85 Lomb Memorial Drive predicted EMRI events. Rochester, New York 14623, USA T +1 585 475 7973 Since this discovery, Merritt and his team have E [email protected] worked on understanding the principles behind this new phenomenon: “We believe we now http://ccrg.rit.edu/people/merritt understand each part of this cycle, but we would never have predicted the existence of such an DAVID MERRITT is an American Astrophysicist effect if we had not observed it in the N-body and Professor at the Rochester Institute of Technology (RIT), New York. He received his simulations,” he underlines. PhD in Astrophysical Sciences from Princeton University and held postdoctoral positions at TO COALESCE OR NOT TO COALESCE the University of California, Berkeley and the Canadian Institute for Theoretical Astrophysics Another aim of Merritt’s research is to better in Toronto. His fields of specialisation understand the theorised coalescence of two include dynamics and evolution of galaxies, SBHs in a binary system. These massive binaries supermassive black holes, and computational are predicted to occur during the merger of astrophysics. Merritt is a former Chair of the two galaxies – a relatively common event in Division on Dynamical Astronomy of the the timescale of the Universe. Once the two American Astronomical Society. He is founder galaxies have merged, the two central SBHs THE SCHWARZSCHILD BARRIER (SB) IN A of the Center for Computational Relativity and become locked in orbit a few to a few tens of SIMULATION OF EXTREME-MASS-RATIO INSPIRALS Gravitation at RIT.

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