Kyriakos Destounis – Academic Profile

Throughout my scientific journey, I've delved deep into the frontiers of General Relativity, black holes, and gravitational waves – making impactful contributions that have advanced our understanding of strong–field gravity. My expertise lies in using black–hole perturbation theory, a powerful framework for simulating the trajectories of asymmetric black–hole binaries and the dynamic aftermath of binary remnants. ​

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Black–hole spectroscopy: quasinormal modes and ringdown physics​​

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To unravel the physics of cosmic collisions, I employ both modal and non–modal analytical techniques, bringing precision and insight to the study of perturbed black–hole remnants. The modal tools used form the black–hole spectroscopy program, which, at the moment, is the predominant tool utilized to comprehend the nature of the final remnant. From the early stages of my career, I've concentrated on developing non–modal approaches to black hole perturbations – driven by the inherent limitations that still challenge traditional modal analyses. Due to some caveats that still plague this program, I have has focused, from the early stage of my career, on non–modal tools that do not depend on the eigenvalues of perturbed black holes, know as quasinormal modes, but rather the structure of the system as a whole in the complex plane where quasinormal modes live, in an attempt to offer a more holistic and robust understanding of black hole dynamics. A powerful tool for unveiling the non–modal structure of perturbed systems is the mathematical notion of the pseudospectrum, which offers a striking topographic view of quasinormal–mode behavior as they migrate through the complex plane – shedding light on dynamics that traditional spectral analyses often miss, such as pseudoresonances and transient effects. This map provides information regarding the existence of disproportionate migration of quasinormal modes when the system is slightly perturbed by the black–hole environment – a phenomenon know as spectral instability. This phenomenon could imprint itself on gravitational–wave signals, arising from the wide array of possible perturbations to the black–hole background, ranging from accretion disks and matter halos to dark matter overdensities and other complex astrophysical environments.

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Black holes in galaxies and dark matter

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We are currently witnessing an unprecedented influx of data from diverse astrophysical experiments, all pointing toward the existence of non–luminous dark matter – an invisible field scaffolding the Universe and holding galaxies together – and revealing the complexity of environments in active galactic nuclei. To unlock the full potential of high–precision astrophysics with current and future gravitational–wave detectors like LISA, it is crucial to accurately model how these astrophysical environments influence both the generation and propagation of gravitational waves. In this context, I have made a significant contribution by building the first, fully–relativistic, exact black hole solution to the Einstein equations embedded within a specific dark matter halo. This marks a major milestone in non–vacuum black–hole perturbation theory, as prior studies typically introduced environmental effects heuristically, superimposing them on vacuum geometries using post–Newtonian techniques. The construction of such solutions has now been numerically streamlined in order for static, spherically symmetric black holes with different matter halo properties to be investigated.​​

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Extreme–mass–ratio inspirals: gravitational waves and environmental effects

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Another active direction in my research involves using perturbation theory to model the evolution of currently elusive binary systems composed of a primary supermassive black hole and a secondary stellar–mass compact object. These titanic black holes – often weighing millions of times the mass of the Sun – are strongly hinted to dwell in the hearts of galaxies. Remarkably, the upcoming LISA mission is poised to detect the gravitational waves emitted by such systems, known as extreme–mass–ratio inspirals (EMRIs), offering an unprecedented window into strong–field gravity and galactic dynamics. A core strength of my work in LISA Science lies in the development of a universal perturbation framework for any static, spherically symmetric EMRI – unveiling both sectors of gravitational energy fluxes, alongside the resulting, coupled, matter fluctuations of the astrophysical environment that the EMRI occupies. Interestingly, a quite similar scheme is used to study the gravitational and matter quasinormal modes of astrophysical black holes, thus contributing further in the black–hole spectroscopy program. A powerful window into the rich and varied astrophysical environments surrounding EMRIs – and supermassive black holes more broadly – has been opened, with the ambitious goal of rigorously assessing how environmental effects might alter parameter estimation during both the inspiral and ringdown stages. Crucially, this work also aims to determine whether these insights can be extended to the more intricate case of rotating supermassive black–hole primaries.

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Orbital resonances and chaos in extreme–mass–ratio inspirals

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An equally intriguing phenomenon in the dynamics of Kerr EMRIs is their adiabatic a passage through orbital resonances, which occur when two of the system's fundamental orbital frequencies lock into a rational ratio. These resonances are not rare – they're ubiquitous in EMRIs – and play a pivotal role in gravitational-wave data analysis due to the significant disruptions they cause in the inspiral–driving fluxes, known as "kicks"; deviations that are absent in idealized, non–resonant models. Depending on the symmetries of the rotating supermassive black hole primary, these resonances can even become prolonged, thus amplifying their impact on the evolution of the system. Intriguingly, certain configurations hint at the onset of astrophysical chaos, leaving indirect imprints in the gravitational–wave signal emitted by perturbed Kerr EMRIs. These investigations – and their extensions into the precise, quantitative effects of resonances in EMRIs – are expected to be central to interpreting the rich data anticipated from future LISA detections, potentially transforming our understanding of strong–field gravitational dynamics.​​​

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Mathematical aspects of General Relativity

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​During my undergraduate studies, I conducted research on the theoretical and dynamical properties of black holes under the supervision of Prof. Vitor Cardoso and Prof. João Lopes Costa. My thesis focused on developing a comprehensive understanding of key mathematical aspects of General Relativity, with particular emphasis on the Strong Cosmic Censorship (SCC) Conjecture. In collaboration with colleagues, we identified what is likely the first – and arguably the most robust – counterexample to the SCC in the context of charged black holes within a Universe possessing a positive cosmological constant, akin to our own. This result was achieved through a combination of black–hole spectroscopy techniques, such as the analysis of all the families of quasinormal modes present in such spacetimes, and (semi–)rigorous mathematical arguments concerning the stability of the hypersurface of maximal globally hyperbolic development of initial data – i.e., the stability of the Cauchy horizon within the black–hole interior, which determines the validity of SCC. The aforementioned work was published as an Editor's Suggestion in Physical Review Letters, was Featured in Physics at the website of the American Physical Society and currently is in the top 5% of all research outputs ever tracked. This line of research, along with the subsequent investigations it inspired, has been instrumental in shaping my development as a rigorous and analytically minded scientist, equipped with a strong foundation in high–performance numerical computing. This combination of theoretical precision and computational expertise has proven to be invaluable in addressing the complex challenges posed by the emerging era of gravitational–wave astronomy.

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LISA Consortium: Working Groups and contributions

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As an active member of the Fundamental Physics, Astrophysics, Waveform Modelling, and LISA Early Career Scientist (LECS) Working Groups, I am deeply committed to advancing our understanding of the LISA detector's capabilities and its extraordinary potential to uncover new gravitational–wave sources. A key aspect of my work focuses on enhancing the modelling of accurate waveform templates – specifically by incorporating the effects of orbital resonances and environmental effects into EMRI signals, ensuring that future detections capture the full complexity of these systems. Furthermore, in collaboration with Greek colleagues, we established the first ever Greek Member Group inside LISA Consortium, named ΛISA–GR, which represents the efforts of Greek scientists, as a whole, across all working groups and within the core of LISA Consortium.  Currently, I serve as the Representative of the ΛISA–GR member group in the LISA Consortium Council.  My ambition is to become one of the key contributors in the global efforts of LISA. At the internal level, my contribution encapsulates decision-making for the community, providing services and facilities in a fair and sustainable way, and plan at the local level for the development and future needs of the area.  At the external level, I want to contribute in pure science, on all Working Group that I am a member of, with the goal of extracting the maximum out of this experiment, and try to collectively revolutionize the way we understand gravitation.

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