CfAR Seminar Series
Seminars take place on Wednesday's at 15.00 unless otherwise specified
Semester 1 - 2020
Date |
Speaker |
Title & Abstract |
Room |
January 22nd |
Cormac Purcell Research Centre for Astronomy, Astrophysics and Astrophotonics, Macquarie University. |
Sharks, Tombs and Cosmic Bubbles: How machine learning spans diverse scientific fields Science in the 21st century is a vast undertaking and most of us at the coal-face specialise in very narrow fields. Universities love the idea of innovative collaborations across disciplines, but it is often difficult to form such teams in practice. Here I talk about my personal experience widening my horizons as an astronomer, to embrace and accelerate cross-disciplinary research. |
XG22 |
February 12th |
Ciara Maguire Trinity College Dublin |
Shock waves in the solar corona The Sun can produce large-scale energetic events such as solar flares and coronal mass ejections (CMEs) which can excite shock waves that propagate from the low solar atmosphere into interplanetary space. Such shocks can result in electron acceleration and intense bursts of radio emission. These accelerated particles can cause a variety of potentially dangerous ‘space weather’ effects, including damage to ground and space based technologies at Earth. To date, the shock kinematics responsible for particle acceleration and emission at radio wavelengths is poorly understood. This talk will present a variety of methods to determine shock kinematics, allowing us to better understand the evolution of shock properties and how they are related to particle acceleration and radio emission. Identifying the relationship between CME eruption and energetic particle acceleration allows us to better understand how shocks may accelerate particles. A more precise understanding of shock properties is essential for space weather forecasting and subsequent Earth impacts. |
HG22 |
February 26th |
Ian Jubb DIAS |
The problem of superluminal signalling in (naive) QFT In quantum theory ideal measurements are described via the projection postulate. The naive application of the projection postulate to QFT appears to violate locality by enabling superluminal signalling. In this talk I will introduce the protocol by which superluminal signals can be sent in the simpler setting of non-relativistic quantum mechanics. To build up to the relativistic QFT setting, I will then discuss signalling in the context of the 1+1 Destri-de Vega spacetime lattice model for fermionic QFT, the continuum limit of which is the massive Thirring model. Finally, I will discuss the signalling protocol in continuum QFT, and its implications for the set of measurable observables and/or the projection postulate. |
XG20 |
Semester 2 - 2019
Date |
Speaker |
Title & Abstract |
Room |
October 9th @ 14.00 |
Masha Chernyakova CfAR |
Multi-wavelength observations of gamma-ray loud binaries Gamma-ray-loud binary systems are a newly identified class of X-ray binaries in which either accretion onto the compact object (a neutron star, or a black hole), or interaction of an outflow from the compact object with the wind and radiation emitted by the massive companion star lead to the production of very-high energy gamma-ray emission at GeV and TeV energies. The number of these peculiar objects is very limited (less than 10), so it is very important to study them in order to find the conditions required for efficient particle acceleration. In my talk I will review multi-wavelength properties of known gamma-ray-loud binaries, and and try to explain the physics governing the emission in these systems. |
XG22 |
October 23rd |
Carolina Susana Villarreal D'Angelo TCD |
The outflowing atmospheres of Jupiter like planets outside our Solar System Many planets outside our Solar System have been discovered in the last 20 years. These fascinating discoveries showed that our Solar System seems to be unique since we have not found a similar planetary system yet. More surprising is the existence of planetary systems with only one Jupiter size planet orbiting very close to its host star, with orbital periods that can go shorter than 1 day. Because they live so close to their star, these planets receive a huge amount of high energy radiation that causes the heating and expansion of their gaseous envelope. If the planetary atmosphere is heated enough, the gas can become gravitationally unbound from the planet and escape in the form of a wind. The wind of these so call hot-Jupiters will interact with the wind and the radiation from their host star creating signatures that can be observed. In this seminar, I will describe the observations that led to the discover of escaping material from the atmosphere of hot-Jupiters and introduce the theory of photoevaporation in planetary atmospheres. I will explain the numerical tools that help us to model these systems and how we can use them to constrain the characteristics of the stellar wind or the presence of a planetary magnetic field, among other system parameters. I will present the results from the numerical modeling of two well-known exoplanetary systems: HD209458b and GJ436b I have carried out so far. |
XG22 |
November 6th |
Erik Curiel Munich Center for Mathematical Philosophy |
What is the Einstein Field Equation, and why does it matter for Quantum Gravity? I discuss the ways that different understandings of the nature of the Einstein Field Equation (EFE) implicitly shape approaches to QG, and why some of those seem to me to be prima facie more unpromising and less fruitful than others. My arguments rest upon some simple, though seemingly not widely appreciated, mathematical and physical points about the difference between on the one hand viewing the Einstein Field Equation as a fully four-dimensional point-by-point relation between curvature and stress energy and, on the other, viewing it as a dynamical set of equations of evolution, as in a 3+1 canonical formulation (as is required in Shape Dynamics). They are also based in part on arguments to the effect that GR is more naturally considered as a framework within which physical theories are formulated rather than as a physical theory in its own right. On this view, it would be as sensible to quantize the framework of Lagrangian mechanics itself, and think one were quantizing a physical theory with fixed, determinate physical content, as it would be to quantize general relativity. |
XG20 |
November 20th |
Kate Maguire TCD |
The ever-changing Universe: Exploding stars and catastrophic collisions Supernovae are the incredibly luminous deaths of stars that play vital roles in chemical enrichment, galaxy feedback mechanisms, and stellar evolution. In particular, Type Ia supernovae, the explosions of white dwarf stars in binary systems, were instrumental in the discovery of dark energy. However, what are their progenitor systems, and how they explode, remains a mystery. There is increasing observational evidence that there are multiple ways in which white dwarfs can explode. I will review the status of what we know about the stellar systems that produce Type Ia supernovae, as well as discuss the recently discovered zoo of peculiar transients that are also predicted to result from the explosions of white dwarfs, such as He-shell mergers, tidal disruption events, violent mergers. Distinguishing between these explosion scenarios and understanding their diversity is vital for producing the best samples for future precision measurements of the cosmological parameters. |
XG20 |
December 4th |
Aindriu Conroy CfAR, DCU |
Bouncing Cosmologies in Alternative Theories of Gravity Einstein’s remarkable theory of General Relativity has withstood rigorous scrutiny in the face of precise experimental physics and observational data. Most recently, the detection of gravitational waves by LIGO fully agrees with the predictions of GR, highly constraining alternative theories of gravity. Despite the success of GR, Einstein’s theory remains incomplete in the classical sense in that it breaks down at short distances, generically predicting spacetime singularities, such as the initial Big Bang singularity Bouncing cosmologies arising from alternative theories of gravity seek to alleviate this cosmological singularity problem by modifying the gravitational action in such a way that geodesic congruences may be made past-complete, while a complete quantum theory of gravity would be expected to be similarly free from singularities. In this talk, I will give a potted history of non-singular, alternative theories of gravity with reference to my past research before discussing my current research project which aims to investigate (or indeed constrain) alternative theories by looking at the interplay between quantum mechanics and classical theories, specifically in relation to quantum phenomena as Hawking radiation and Unruh radiation. |
XG20 |
Semester 1 - 2019
Date |
Speaker |
Title & Abstract |
Room |
January 15th |
Dr. Eoin O'Colgain |
de Sitter Swampland, H0 tension & holography Few convincing constructions of de Sitter vacua in string theory exist. Prompted by this observation, it has recently been suggested that de Sitter vacua inhabit a "Swampland" of inconsistent low-energy theories coupled to gravity. Since de Sitter is an attractor for Lambda-CDM, this appears to rule it out. Separately, Macri, Riess, et al. are working towards a determination of the Hubble constant with 1% error, which is in tension with Lambda-CDM. We introduce an alternative late-time cosmology based on a holographic argument, one with the same number of late-time parameters as Lambda-CDM, which relieves the tension and predicts a turning point in the Hubble parameter at small positive redshift. |
X146 |
February 13th |
Dr. Peter Taylor (CfAR) |
Subtracting Infinity: Conceptual and Practical Aspects of Renormalization in QFT in Curved Spacetimes Quantum gravity remains one of the most important outstanding problems in physics. In the absence of a full theory, one must rely on approximations. One particularly important approximation is semi-classical gravity, which is the treatment of quantum fields interacting with a classical spacetime metric via the semi-classical Einstein equations. Solving the semi-classical Einstein equations is notoriously difficult. The first major obstacle one encounters is that the source term in these equations is divergent. These infinities are, in fact, ubiquitous in quantum field theories and removing them in a controlled way, a procedure called renormalization, is necessary to make sense of these theories. After a pedagogical introduction to the concept of renormalization in Quantum Field Theory in Curved Spacetimes, I will discuss some new and efficient methods for numerically computing renormalized quantum expectation values in black hole spacetimes. |
XG22 |
February 27th |
Dr. Créidhe O'Sullivan (Maynooth University) |
QUBIC – a telescope to measure the polarisation of the cosmic microwave background In this presentation I will discuss the origin and properties of the cosmic microwave background (CMB) radiation what observations of the CMB can tell us about the early Universe. In particular I will describe a ground-based telescope called QUBIC that researchers in Maynooth are building as part of a European-Argentinian collaboration. QUBIC aims to make extremely sensitive measurements of the polarisation of the CMB using a technique called bolometric interferometry. We plan to deploy a QUBIC demonstrator module in Argentina later this year. |
XG22 |
March 13th |
Prof. John Wise (Georgia Tech, US) |
The First Stars, Black Holes, and Galaxies in the Universe Cosmic structure forms hierarchically through smooth accretion and dark matter halo mergers. As a consequence, all galaxies are the product of the dozens of mergers over billions of years. However, one can ask, "What were the first stars and galaxies in the universe?" I will review the current state-of-the-art simulations of early galaxy formation, starting with the formation of the first stars, which are initially devoid of elements heavier than lithium and are suggested to have a characteristic mass of tens of solar masses. I will then present results from a suite of cosmological radiation hydrodynamics simulations that focus on the transition from the first stars to the first galaxies. Each simulation captures the radiative and chemical feedback from 10,000 first stars, leading to the formation of a 10^9 solar mass galaxy only 500 million years after the Big Bang, which is detectable with the upcoming James Webb Space Telescope. I will also highlight how some of the earliest massive black holes form during these early epochs that could be the seeds of supermassive black holes that exist at the centers of all massive galaxies today. |
XG22 |
March 27th |
Dr. Pavel Ivanov (Astro Space Center, Moscow) |
A twisted accretion disc formed after tidal disruption of a star by a rotating black hole In this talk I am going to consider misaligned accretion discs formed after tidal disruption events occurring when a star encounters a supermassive rotating black hole. I will discuss the disc shape when the stream produced by the disrupted star provides a source of mass and angular momentum that is misaligned with that of the black hole's. It will be shown that at early times the disc inclination with respect to black hole equatorial plane is nearly uniform at small radii where the aspect ratio is large. However, since torques arise from the Lense-Thirring effect and the stream there is non-uniform precession. At these times a disc with small viscosity parameter alpha=0.01 exhibits a new feature. An inclined hot inner region joins an outer low inclination cool region via a thin transition front propagating outwards with a speed exceeding that of bending waves in the cool region. These waves accumulate where the propagation speeds match producing an inclination spike separating inner and outer discs. At late times a sequence of quasi-stationary configurations approximates disc shapes at small radii. I'll discuss observational implications of our results. |
XG22 |
April 10th |
Dr. James Edholm (University of Lancaster) |
Modifying General Relativity without introducing ghosts We know that General Relativity breaks down at short distances, when we try and apply it to black holes and the Big Bang. Attempts to fix this by modifying GR generically lead to ghosts, but by introducing infinite derivatives to the Lagrangian, we can generate a ghost-free theory: Infinite Derivative Gravity. I will discuss how this affects the Newtonian potential, show that we can avoid singularities and how IDG can also generate the cosmological inflation which we observe in the early universe. |
XG22 |
Semester 2 - 2018
Date |
Speaker |
Title & Abstract |
Room |
August 23rd |
Tilman Hartwig, University of Tokyo |
Stellar Archeology as a Time Machine to the First Stars Understanding the nature of the first stars is a major challenge of modern cosmology. Despite their importance for the formation of subsequent stars and galaxies, their mass distribution is still uncertain due to a lack of direct observations. Extremely metal-poor stars in the Milky Way allow us to constrain the mass of their progenitor supernovae and thereby provide precious information about the first generation of stars. I will give a general introduction to stellar archaeology, the main open questions, and discuss several of our recent results: I will present a new diagnostic to reliably distinguish mono-enriched from multi-enriched metal-poor stars, based on their elemental abundances. We apply this diagnostic to recently observed stars from the TOPoS survey, which allows us to better understand the nature of the first stars. Finally, I will present a novel scenario for the formation of carbon-enhanced metal-poor stars. |
X130 |
Oct 3rd |
Joost Slingerland, Maynooth University |
Topology in Physics: from Ether Knots to Topological Order Topology describes structures that are stable under deformation. This applies to geometrical objects - for example, you can distinguish donuts from pretzels by counting holes, regardless of the detailed shape. It also plays a role in physics. Topology has been used (with much varying success) in attempts to explain the stability of atoms, the quantization of electric charge and the existence of bosons and fermions. It also plays a role in finding stable excitations of (super)fluids, and monopoles and even different stable vacua in gauge theories like QCD. Recently, there has been a lot of excitement in condensed matter physics over topological phases of matter, which can be distinguished through topological properties but not in the traditional way through symmetry. The physics of these phases can also be applied to fault tolerant quantum information processing. |
XG22 |
Oct 17th |
Aline Vidotto, TCD |
Stellar magnetic activity and their effects on exoplanets In this talk I will review some recent works on the magnetic activity of Sun-like stars, their winds and potential impact on surrounding exoplanets. Stellar magnetic fields drive space weather on (exo)planets, being responsible for the ejection of stellar winds and bursty events, such as flares and coronal mass ejections. The dramatic differences in the Physical properties of the host stars compared to the properties of our Sun, in addition to the extreme architecture of most of the known exoplanetary systems, can give rise to physical interactions that might not even be recognised in the solar system. These interactions can generate observable signatures, thus providing other avenues for characterising exoplanetary systems, which would otherwise remain unknown. |
XG22 |
Oct 31st |
Peter Coles, Maynooth University |
Cosmic Anomalies Data from the Wilkinson Microwave Anisotropy Probe (WMAP) and, more recently, the Planck Mission, have provided us with unprecedented quantitative information that has helped to establish the current `concordance' cosmology. These observations have also revealed a number of features that we do not understand so well, prompting suggestions of departures from the standard framework. In this talk I will review a few of these interesting such anomalies, touching on the methods used to identify them and what their physical origin might be if they are not simply experimental artifacts of some sort. |
XG22 |
Nov 14th |
Rebeca Garcia-Lopez, DIAS |
The inner regions of protoplanetary disks A first step towards understanding planetary formation is the characterisation of the structure and evolution of protoplanetary discs. Although the large scale disc is understood in some detail, very little is known about the inner 5 au (i.e. within a radius corresponding to Jupiter's orbit) in which the main physical processes take place: accretion, ejection and planetary formation. Until recently, it was widely accepted that turbulence drove the gas dynamics of this region transporting most of the angular momentum outwards and allowing matter to accrete onto the young star. However, new studies raised doubts about this scenario, proposing MHD winds as the main mechanism for angular momentum removal in protoplanetary disks. To discern whether turbulence or MHD winds govern the gas dynamics has major implications for planet formation as both processes have different effects on the dust grain growth and settling, the collisional evolution of planetesimals, and planet growth and migration. In this talk, I will review the latest findings in the field of high angular and spectral resolution observations of the inner regions of protoplanetary disks, and how these studies can help us to constrain the inner disk structure and evolution, and thus the formation of planets. |
XG22 |
Nov 28th |
Barry Wardell, UCD |
Gravitational Backreaction on Cosmic Strings Cosmic strings are a fascinating potential source of gravitational waves. Arising as a generic feature from physical processes including early-universe phase transitions, field theory models and string theory, their discovery through gravitational wave detection would vastly improve our understanding of the fundamental physics that governs our universe. However, existing searches for gravitational wave signals from cosmic strings are based on simplified models that ignore the potentially important effect of gravitational backreaction. In this talk I will present a method for computing the linearized gravitational backreaction for Nambu-Goto strings using a fully covariant formalism. The method makes use of equations of motion expressed in terms of a higher dimensional analog of the geodesic equation subject to self-generated forcing terms. The approach allows arbitrary spacetime and worldsheet gauge choices for the background and perturbation. As an illustration of the method, I will give numerical results for the self-force on the worldsheet of several cosmic string loops with kinks, cusps and self-intersections. For field points on smooth parts of the worldsheet the self-force is finite. As the field point approaches a kink or cusp the self-force diverges, but is integrable in the sense that the displacement of the worldsheet remains finite. |
XG22 |
Dec 13th |
Eric Curiel, Munich Center for Mathematical Philosophy |
What is the Einstein Field Equation, and why does it matter for Quantum Gravity? I discuss the ways that different understandings of the nature of the Einstein Field Equation (EFE) implicitly shape approaches to QG, and why some of those seem to me to be prima facie more unpromising and less fruitful than others. My arguments rest upon some simple, though seemingly not widely appreciated, mathematical and physical points about the difference between on the one hand viewing the Einstein Field Equation as a fully four-dimensional point-by-point relation between curvature and stress energy and, on the other, viewing it as a dynamical set of equations of evolution, as in a 3+1 canonical formulation (as is required in Shape Dynamics). They are also based in part on arguments to the effect that GR is more naturally considered as a framework within which physical theories are formulated rather than as a physical theory in its own right. On this view, it would be as sensible to quantize the framework of Lagrangian mechanics itself, and think one were quantizing a physical theory with fixed, determinate physical content, as it would be to quantize general relativity. |
XG22 |
|
Date |
Speaker |
Title & Abstract |
Room |
September 27th |
Ko Sanders, C-fAR |
Black hole radiation and other hot stuff I will give a brief introduction to black hole thermodynamics and discuss some of the questions and problems that it raises. After reviewing the simplest model of a quantum field in curved space-times and of the temperature of its states, I will discuss some precise mathematical results that address two of these questions. The first concerns the existence of so-called Hartle-Hawking-Israel states, which afford a simplified description of Hawking's black hole radiation. The second is about the concept of local temperature, which aims to clarify in which sense a radiating black hole is hot. |
XG19 |
October 11th |
Alberto Sesana, Birmingham |
Massive black holes along the cosmic history: formation, dynamics and gravitational waves Following detection by advance LIGO, gravitational wave (GW) stocks are on the rise. The low frequency GW Universe is likely dominated by signals emitted by a cosmological population of massive black hole binaries (MBHBs). I will review several aspect of MBH astrophysics, including their formation, evolution, interaction with their environment and GW emission. I will then discuss prospect of GW detection with pulsar timing arrays and/or future space based interferometers such as the laser interferometer space antenna (LISA). |
XG19 |
October 25th |
Jose Groh, TCD |
The Surprising look of Massive Stars Before Death Stars more massive than about 8 Msun end their lives as a supernova (SN), an event of fundamental importance Universe-wide. The physical properties of massive stars before the SN event is very uncertain, both from theoretical and observational perspectives. In this talk, I will review recent efforts to couple stellar evolution and atmosphere modeling of stars in the pre-SN stage. These models are able to predict the high-resolution spectrum and broadband photometry, which can then be directly compared to the observations of core-collapse SN progenitors. I will discuss the surprising predictions of spectral types of massive stars before death. Depending on the initial mass and rotation, single star models indicate that massive stars die as red supergiants, yellow hypergiants, luminous blue variables, and Wolf-Rayet stars of the WN and WO subtypes. I will finish by discussing how a close companion profoundly affects the fate of massive stars, and assessing the detectability of the different types of SN progenitors. |
XG19 |
November 8th |
Asaf Pe'er, UCC |
Frontiers in the study Gamma-ray bursts Gamma ray bursts (GRBs) are the most violent explosions in the universe. While being studied extensively since the 1990's, a major event took place a few weeks ago, with the first confirmed detection of a GRB associated with the gravitational wave (GW) signal that results from the merger of binary neutron star (NS) system, namely GW/GRB170817a. In the first part of the talk I will give broad overview of our current understanding of GRB physics, followed by a description of the GW/GRB170817a event and its implications. In the second part of the talk (as time permits), I will discuss the recent development in understanding the origin of the prompt spectra, and in particular the role of thermal emission in it. I will discuss some novel effects and theoretical ideas relevant for the study of many astronomical objects. As a few examples, I will show how emission from the photosphere can be observed to have high degree of polarization; how it can be used to infer the jet magnetization; and more. |
X130 |
November 22nd |
Peter Gallagher, TCD |
From the Leviathan to LOFAR Astronomical objects such as planets, stars and galaxies emit light across the electromagnetic spectrum. This has led astronomers to develop instruments to observe these objects at wavelengths ranging from gamma-rays to radio waves. For over 70 years from the middle of 19th century, the largest optical telescope in the world - the Leviathan - was located at Birr, Co. Offaly, and enabled astronomers to resolve the spiral arms of galaxies for the first time. Now, a consortium of Irish universities have constructed a new telescope at Birr to explore the universe at radio wavelengths. In this talk I will describe the construction of the Irish Low Frequency Array (I-LOFAR; www.lofar.ie), describe how it operates, and discuss some first-light observations. |
X130 |
December 6th |
Chris Watson, Queens |
Unearthing Earth-like planets cloaked in noise. Clues from the HARPS-N solar telescope? Planets. They are everywhere. Yet the discovery of the much coveted Earth-2.0 (an Earth-size planet in an Earth-like orbit around a Sun-like star) has so far eluded us. First I will outline some of the key discoveries to date, and the principal techniques (the transit and Doppler wobble methods) employed for finding planets and then characterising their most basic properties such as mass and radius. I will then highlight some of the upcoming space missions and next generation instrumentation that seemingly brings the potential for discovering Earth-2.0 into the realms of reality for the first time, before cruelly dashing everyone’s hopes by throwing stellar activity into the scene. Stellar surface inhomogeneities (manifestations of which include spots, plage, convective blueshift suppression, and granulation) induce radial velocity (RV) perturbations that can mask or even mimic the Doppler wobble effect induced by an orbiting planet. Understanding and correcting for this stellar noise is crucial for pushing exoplanet RV detection limits towards Earth analogue exoplanet confirmation using the next generation of stabilised spectrographs. I will present some highlights from the HARPS-N solar telescope consortium. This telescope feeds full-disk sunlight into the HARPS-N spectrograph, and thus enables observing the Sun as a star with high RV precision. In particular, I will show that the manifestations of solar activity can be tracked in individual absorption lines even on approach to the minimum of the solar cycle. Such signatures are also clearly visible in HARPS spectra of alpha Cen b. The ability to trace such signals in stellar spectra may provide an important route to directly tracking the impact of stellar noise, and thus developing optimal correction techniques to uncloak Earth analogue planets from the noise of their host stars. |
X130 |
February 7th |
Tristan McLoughlin, TCD |
AdS/CFT and integrability - Exact Results in Gauge/Gravity Duality The mathematical description of the fundamental interactions governing the vast majority of observable physical phenomena has been long known in terms of quantum field theory. For example, the properties of most materials is essentially determined by the well understood electromagnetic interactions. However, when there are large numbers of particles and particularly when they strongly interact, the emergent physics shows a remarkable diversity of behaviour - from strongly correlated electrons in matter to the substructure of protons and neutrons - much of which can’t be understood with existing theoretical methods. In this talk we will discuss a novel method to study such problems, the AdS/CFT correspondence, and in particular will discuss the use of methods from integrable systems to exactly calculate correlation functions. |
XG20 |
February 21st |
Alan Fitzsimmons, Queens |
Chasing the Alien: What we know about the first Interstellar Object. On 19 October 2017 an Interstellar Object was discovered passing through our Solar system, the first of its kind to be discovered. With little warning, astronomers had only a few days to study it before it became too faint for detailed investigation, even when using the world’s largest telescopes. Imaging and spectroscopy by astronomers at QUB and others have revealed a body that matched some of our expectations, but that differed significantly in other aspects. These studies have given considerable insight into our alien visitor, now named ‘Oumuamua, but many questions remain. In this seminar I will explain why we expected to see something like ‘Oumuamua at some point, what we have discovered about the first object to visit us from another star, and what we might expect to find in the future. |
XG19 |
March 7th |
Eleni Kontou, University of York |
Singularity theorems, energy conditions and quantum inequalities Singularities, boundary points of spacetime beyond which no extension is possible, continue to intrigue both mathematicians and physicists since they are places where our current understanding of physical law breaks down. The question whether they exist in physical situations is still an open one. Fifty years ago, Hawking and Penrose developed the first general, model independent, singularity theorems. These theorems showed that singularities have to exist in any spacetime that satisfies certain properties, some of which are mild assumptions but others, called energy conditions, depend on matter content and are more problematic. For both classical and quantum fields, violations of these conditions can be observed in some of the simplest of cases. Focusing on the strong energy condition, I will present an effort to develop theorems with weaker restrictions, namely energy conditions averaged over an entire geodesic and quantum inequalities; weighted local averages of energy densities. |
HG23 |
March 21st |
Donna Rodgers-Lee, University of Hertfordshire |
The extreme physical conditions of protoplanetary disks: magnetic fields and low energy cosmic rays Protoplanetary disks, the birthplace of planets, have been the focus of much research in recent years since the first light of the Atacama Large Millimeter Array. One of the main focuses of protoplanetary disk studies is to investigate the mechanism responsible for angular momentum transport which allows accretion onto the young star to occur. While the magnetorotational instability is thought to operate in certain regions of protoplanetary disks, which can facilitate the transport of angular momentum, a more global view including non-ideal magnetohydrodynamic simulations is still missing. The simulations I will discuss initially are some of the first to study the global nature of protoplanetary disks including all three non-ideal effects in a region where the Hall effect is dominant. I will show how our results using a multifluid approach are comparable with those of the single fluid approximation but also point out some important differences. |
HG23 |
April 6th |
Nguyen Viet Dang, Universite Claude Bernard Lyon I |
- |
HG23 |
April 18th |
Brien Nolan, CfAR |
Causality violation in Gödel's universe In 1949, motivated by the implications of relativity theory for Kant's ideas about time and change, Kurt Gödel derived a model of the universe within Einstein's General Relativity that has some very strange chronological features: it allows for an observer to travel into their own past. However, this requires extravagant amounts of fuel to accelerate the observer to the required velocity, which is a significant fraction of (but nevertheless less than) the speed of light. I'll discuss this model as a means of providing an introduction to some key ideas in General Relativity, including the concepts of geodesic motion, space-time symmetry and causality. I'll review what is and isn't possible in relation to causality violation in Gödel's universe, in particular discussing a new result that shows how any observer can send a signal to their own past, without the need for acceleration or unattainably high velocities. |
HG23 |
April 27th |
Chris Power University of Western Australia - International Centre for Radio Astronomy Research (ICRAR) |
Synthetic Universes for Future Surveys (SURFS) - Using cosmological simulations for astrophysical interpretation Within the last 20 years, large galaxy surveys, such as the 2dF Galaxy Redshift Survey and the Sloan Digital Sky Survey, transformed our understanding of cosmology and the properties of the galaxy population in the nearby Universe. The newest generation of galaxy surveys will mark another transformation in our view of the Universe. Radio telescopes such as the Atacama Large Millimetre Array and SKA pathfinders such as the Murchison Widefield Array and the Australian SKA Pathfinder will reveal how star formation within galaxies is fuelled, how super-massive black holes are assembled, and how the first galaxies recognised the Universe. Integral field spectroscopy with MaNGA, SAMI/HECTOR, and future IFU surveys are deconstructing galaxies and revealing how winds and outflows from stars and black holes drive their evolution, while innovative telescopes such as Dragonfly are opening up the low surface brightness Universe and shedding light on galaxy assembly histories. Interpreting these observations within a theoretical framework requires state-of-the-art numerical simulations of galaxy formation, and I describe our current work building the Synthetic Universes for Future Surveys (SURFS) framework. In particular, I will highlight our recent work on the Local Group, which suggests that the MW and M31 aren't as massive as previously thought, and shows us how to get the most robust estimates of galaxy mass from the kinematics of satellites. I will also show how multi-wavelength (e.g. combined IFU and HI) data is essential understand if we are to be better the kinematics and angular momenta of galaxies. |
XG19 |
May 2nd |
Sam Dolan, University of Sheffield |
Higher-order geometrical optics for circularly-polarized electromagnetic waves I will describe the geometrical-optics expansion for circularly-polarized electromagnetic waves propagating on a curved spacetime in general relativity, showing that higher-order corrections to the Faraday and stress-energy tensors may be found via a system of transport equations, in principle. At sub-leading order, the stress-energy tensor possesses terms proportional to the wavelength whose sign depends on the handedness of the circular polarization. Due to such terms, the direction of energy flow is not aligned with the gradient of the eikonal phase, in general, and the wave may carry a transverse stress. I will argue that, heuristically, the flow direction is consistent with a modified phase which includes a correction due to the differential precession arising in parallel-propagated basis on a curved spacetime. The result also appears consistent with the posited existence of an optical Magnus effect, and with a spin-helicity effect in the absorption of electromagnetic waves by a Kerr black hole. |
XG19 |
Seminars 2016/2017
January 2017
No Seminar
February 2017
No Seminar
March 2017
No Seminar
April 2017
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Wednesday 05/04/17
Ab Harte, CfAR, DCU
Title: Metric-independence of electromagnetic fields
Abstract: Almost all measurements of relativistic gravity rely in one way or another upon the behavior of light in curved spacetimes. Gravitational lensing has for example been developed into a routine observational tool, a fact which might lead one to conclude that much of a spacetime's geometry is encoded in some way in electromagnetic fields. I will explain that this is actually false: A single electromagnetic field is actually compatible with an enormous variety of spacetime metrics. Identifying these ambiguities unifies a number of existing results in the literature, allows new solutions to be derived, and also old ones to be better understood. Another consequence is an easy demonstration that electromagnetic and gravitational waves propagating in the same direction cannot interact.
Location: XG20 @ 14.30
Notes: Talk PDF
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Thursday 13/04/17
Matt Kenworthy, Leiden Observatory
Title: Looking for circumplanetary material around a giant exoplanet with bRing - The beta Pictoris b Ring project
Abstract: In 2007, the 20 million year old pre-main sequence star J1407 underwent a complex series of eclipses lasting two months, showing temporal structure down to a timescale of 20 minutes. We interpret this as a giant ring system (some 200 times larger than Saturn's rings) surrounding an unseen secondary companion, J1407b, that is being sculpted by forming exomoons.
In mid-2017, the Hill sphere of the young exoplanet orbiting the bright southern hemisphere star beta Pictoris will transit, presenting a unique opportunity to search for Hill sphere filling rings and to perform transmission spectroscopy of the circumplanetary material.
I will present our observing campaign for beta Pictoris in 2017, including the first results from our beta Pic b Ring (bRing) cameras that will carry out continuous monitoring of this star throughout the next 14 months.
Location: N115 @ 13.00
Notes: 1 p.m.
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Tuesday 25/04/17
John Regan, CFAR, DCU
Title: Rapid formation of massive black holes in close proximity to star bursting proto-galaxies
Abstract: The observation of massive black holes a billion times the mass of the sun less than 1 billion years after the big bang can not be explained by our current structure formation paradigm. Here I will outline an alternative mechanism where super-massive stars are a natural consequence of early radiation bursts from the first galaxies. Super-massive stars provide the ideal progenitors for the massive black holes powering quasars at high redshift.
Location: X130 @ 14.30
Notes:
The seminar will be streamed from this link
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May 2017
Wednesday 10/05/17
Peter Johansson, University of Helsinki
Title: Accurate modelling of supermassive black hole dynamics in galaxy simulations
Abstract: When massive galaxies merge the supermassive black holes (SMBHs) in their centres will eventually coalesce through a three-step process. First, dynamical friction from gas and stars will reduce the semi-major axis of the black holes to ~1 parsec. Next, the binary will lose further orbital energy by kicking out stars in complex three-body interactions and in the very final steps energy will be lost by the emission of gravitational waves.
In this talk I will present the first results from our new KETJU code, which is the first simulation code able to resolve all the three phases of SMBH merging in one self-consistent simulation. The KETJU code is an extension of the widely used GADGET-3 code and employs algorithmic chain regularisation, which resolves arbitrarily close encounters in the vicinity of the SMBHs. In addition our code includes Post-Newtonian terms in the equations of motions, which enables us to resolve the SMBH binary dynamics accurately down to separations of ~0.005 pc.
We test the code by setting up dynamically stable multi-component (stars + dark matter) progenitor galaxies and study the SMBH binary evolution during galaxy mergers. We study the hardening rate of the SMBH binary and find the result to be resolution-independent for the more realistic models that also include dark matter, whereas the models with only a stellar component show a clear resolution dependency. Our simulations do not suffer from the famous final-parsec problem, which we attribute to the non-spherical shape of the merger remnants. Finally, we show that small differences in the initial eccentricity of the SMBH binary of the order of Delta_e~0.15 can lead to large differences of Delta_t~150 Myr in their final coalescence time.
Location: X130 @ 11.30
Notes: The seminar will be streamed live from here
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Cancelled
Wednesday 24/05/17
Dr. Pavel Ivanov, Lebedev Physical Institute of the Russian Academy of Sciences
Title: Supermassive binary black holes
Abstract: In this talk I am going to review different aspects of astrophysics of supermassive binary black holes. Both theoretical and observational sides of this topic will be discussed including the problem of their formation, emission of gravitational waves, interaction with ambient stars and gas and possible observational candidates.
Location: X130 @ 11.30
Notes: The seminar will be streamed live from here
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Wednesday 31/05/17
Borja Reina, DCU
Title: Perturbed matching of spacetimes with applications to astrophysics
Abstract: The plan for this talk is to briefly review the tools needed to match two spacetimes across a timelike hypersurface, in the exact and perturbative senses. For the latter case, I will introduce the theory of perturbations of hypersurfaces to first and second order constructed by M. Mars in 2005, which we have used to explore the perturbative stationary and axisymmetric matching around a static spherically symmetric background configuration.
This scheme provides us a firm ground where to study the importance (or lack thereof) of the implicit assumptions made to construct Hartle's model for rotating stars, i.e. a rigidly rotating perfect fluid interior matched to an asymptotically flat vacuum exterior. As a further application, we have explored (a part of) the tidal problem in binary systems inside this context. We recover the universal I-Love-Q relations for neutron and quark stars, and complete them including the perturbed mass of the rotating configuration.
Location: X130 @ 11.30
Notes: The seminar will be streamed live from here
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June 2017
July 2017
August 2017
September 2017
October 2017
November 2017
December 2017