Star-UBB Seminar Series in Gravitation, Cosmology and Astrophysics


Presently we are witnessing a period of rapid and intense change in our understanding of the gravitational force, at a rate that is quickly increasing since the important observational discoveries of the late 1990s. With the advent of new observational techniques, we can see the emergence of important cosmological and astrophysical paradigms that lead to a deep change in our understanding of the Universe. Astronomical observations strongly suggest that at large scales the force of gravity may not behave according to standard general relativity, and that a generalization of the gravitational action, either at the geometric level, or at the matter level, may be required for a full understanding of the gravitational interaction.

The goal of the present seminar is to bring Theoretical Gravitational Physics closer to the observations and experiments, and to discuss current topics in general relativity, high-energy physics, astrophysics and cosmology.

Thursday, October 13 | 11:00 AM → 12:30 PM

Hall in the Sky

Speaker: Dr. Loic Marsot, Centre de Physique Théorique, Aix Marseille Université, Université de Toulon, Marseille, France

Abstract: In this talk, we will explain how it is possible to introduce Hall motions into General Relativity. After reviewing the Hall effect for condensed matter, I will explain the two possibilities of bringing such an effect in GR. I will first show how to do so by considering chiral fermions, and then by using a possible effective description of dynamics on null hypersurfaces with the help of the planar Carroll group and its central extension. At the end, we give an example of Hall motion for „extended” photons on the horizon of a Black Hole, and show that they present a typical rotation motion.

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Thursday, October 20 | 11:00 AM → 12:30 PM

Anomalous Luttinger (Tolman-Ehrenfest) equivalence between temperature and curved spacetime: From black hole’s atmosphere to thermal quenches

Speaker: Prof. Maxim Chernodub, Institut Denis Poisson, Université de Tours, Tours, France

Abstract: Following the idea of Tolman and Ehrenfest that heat has weight and must therefore feel gravity in classical physics, Luttinger established a deep connection between gravitational fields and thermal transport. However, on the quantum level, field fluctuations generate several gravitational anomalies, which, as we show, affect the celebrated Tolman-Ehrenfest and Luttinger relations. While anomalous quantum fluctuations naturally occur in the quantum atmosphere of a black hole, our results reveal that analogous effects are also observable in thermal conductors in the flat-space time, provided local temperature varies strongly. In addition, we show how the gravitational anomalies manifest themselves experimentally in propagating energy waves following a thermal quench, as well as in the energy density of heating Floquet states induced by repeated thermal quenches.

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Thursday, October 27 | 11:00 AM → 12:30 PM

Non-local gravity cosmology

Speaker: Prof. Salvatore Capozziello, Dipartimento di Fisica „E. Pancini”, Universitá di Napoli Federico II, Napoli, Italy

Abstract: Recently the so-called Non-Local Gravity acquired a lot of interest as an effective field theory towards the full Quantum Gravity. In this talk, we sketch its main features, discussing, in particular, possible infrared effects at astrophysical and cosmological scales. In particular, we focus on general non-local actions, including curvature invariants like the Ricci scalar and the Gauss-Bonnet topological invariant, in metric formalism, or the torsion scalar, in teleparallel formalism. In both cases, characteristic lengths emerge at cosmological and astrophysical scales. Furthermore, it is possible to fix the form of the Lagrangian and to study the cosmological evolution considering the existence of Noether symmetries.

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Thursday, November 3 | 11:00 AM → 12:30 PM

Constraining dark matter properties by orbital precession around the Galactic supermassive black hole

Speaker: Dr. Man Ho Chan, Department of Science and Environmental Studies, The Education University of Hong Kong, Hong Kong

Abstract: Theoretical models show that dark matter can concentrate near the Galactic supermassive black hole (SMBH). The dark matter distributing around the SMBH can provide significant gravitational effects to change the stellar orbits surrounding the SMBH. These effects can be simply manifested by the orbital precession angles of the stellar orbits. In this talk, I will outline the methodology and describe how the properties of dark matter can be revealed by the orbital precession features.

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Thursday, November 10 | 11:00 AM → 12:30 PM

Power-law Cosmology in Weyl type $f(Q,T)$ modified gravity

Speaker: Prof. P.K. Sahoo, Department of Mathematics, Birla Institute of Technology and Science-Pilani, Hyderabad Campus, Hyderabad, India

Abstract: Gravity is attributed to the spacetime curvature in classical General Relativity (GR). But, other equivalent formulation or representations of GR, such as torsion or non-metricity have altered the perception. We consider the Weyl-type $f(Q, T)$ gravity, where $Q$ represents the non-metricity and $T$ is the trace of energy momentum temsor, in which the vector field $\omega_{\mu}$ determines the non-metricity $Q_{\mu \nu \alpha}$ of the spacetime. In this work, we employ the well-motivated $f(Q, T)= \alpha Q+ \frac{\beta}{6k^{2}} T$, where $\alpha$ and $\beta$ are the model parameters. Furthermore, we assume that the universe is dominated by the pressure-free matter, i.e. the case of dust $(p=0)$. We obtain the solution of field equations similar to a power-law in Hubble parameter $H(z)$. We investigate the cosmological implications of the model by constraining the model parameter $\alpha$ and $\beta$ using the recent 57 points Hubble data and 1048 points Pantheon supernovae data. To study various dark energy models, we use statefinder analysis to address the current cosmic acceleration. We also observe the $Om$ diagnostic describing various phases of the universe. Finally, it is seen that the solution which mimics the power-law fits well with the Pantheon data better than the Hubble data.

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Thursday, November 24 | 11:00 AM → 12:30 PM

Relativity forever?

Speaker: Prof. Zurab Silagadze, Novosibirsk State University, Russia

Abstract: The main part of the talk will be devoted to the special theory of relativity (SR), which has radically changed our understanding of space and time. After a brief review of the experimental basis of SR, we discuss the status of superluminarity in SR. If Lorentz symmetry is a low-energy emergent phenomenon, superluminal objects, which we call elvisebrions, can exist without violating our existing physical theories. Next, I indicate some drawbacks of the standard approach to special relativity as practiced in textbooks, and argue that a better pedagogical practice would be to base the presentation of SR from the very beginning on the four-dimensional formulation of Minkowski, and thus remove from the theory all the paradoxical connotations that invariably accompany the usual presentation, and especially the concept of time. In the final part, I will briefly mention some of the interesting directions where Lorentz symmetry breaking can lead.

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Thursday, December 8 | 11:00 AM → 12:30 PM

End of time evolution in self-interacting vector theories

Speaker: Dr. Fethi Mubin Ramazanoglu, Department of Physics, Koc University, Istanbul, Turkey

Abstract: Non-linear extensions of Proca theory are pathological at large field amplitudes. I will explain why. I will also discuss a separate criterion for breakdown proposed in the literature, and why it does not correspond to a real physical problem.

Joint work with: dr. Andrew Coates, Department of Physics, Koc University, Istanbul, Turkey

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Thursday, December 15 | 11:00 AM → 12:30 PM

Curing conical singularities with scalar hair

Speaker: Dr. Eugen Radu, Department of Mathematics of the University of Aveiro, Centre for Research and Development in Mathematics and Applications (CIDMA), Aveiro, Portugal

Abstract: A number of well-known black object solutions in (electro)vacuum general relativity (GR) are plagued by conical singularities which provide the force balance that allows the existence of such configurations. The basic example is the Bach-Weyl (or double-Schwarzschild) solution, which describes two static black holes in four dimensional vacuum GR, with a deficit angle along the section in between the black holes. A different case is provided by the five dimensional static Emparan-Reall black ring, which contains a conical singularity in the form of a disc that sits inside the ring, supporting it against collapse. We argue that the situation in (electro)vacuum GR is not generic, with the possible existence of balanced solutions i) in more general theories or ii) for different spacetime asymptotics. Focusing on the first case, we consider the static black ring in Einstein-Maxwell-gauged scalar field model and argue that the charged scalar hair can balance it, yielding solutions that are singularity-free on and outside the horizon. Moreover, the four dimensional static two black hole system can be balanced by a real scalar field with an appropriate self-interaction.

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Thursday, January 5 | 11:00 AM → 12:30 PM

Constraining the Concordance Model of Cosmology with the Large-Scale Structures

Speaker: Dr. Benjamin L’Huillier, Department of Physics and Astronomy, Sejong University, Seoul, Korea

Abstract: Despite great predictive power and its successes in the last decades, the concordance LCDM cosmological model suffers from both observational (H0 & S8 tensions, …) and theoretical issues (nature of dark energy, dark matter, inflation, coincidence problem, …). Therefore, it is important to further test the model and its underlying hypotheses. In this talk, I will discuss how the study of the large-scale structures can help shed light on some fundamental questions such as the nature of dark energy, gravity, or the early Universe, in the context of a new generation of surveys such as Euclid, DESI, or LSST. I will focus on two different aspects: (i) modelling the nonlinear regime of structure formation through N-body simulations, in particular beyond LCDM, and (ii) applying advanced statistics, in particular model-independent methods, to state-of-the-art cosmological data to test different aspects of the concordance such as the metric, gravity, or the nature of dark energy.

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Thursday, January 12 | 11:00 AM → 12:30 PM

The kinetic gas Universe

Speaker: Prof. Nicoleta Voicu, Faculty of Mathematics and Computer Science, Transilvania University, Brasov, Romania

Abstract: A description of many-particle systems, which is more fundamental than the fluid approach, is to consider them as a kinetic gas. In this approach, the dynamical variable encoding the properties of the system is the so-called 1-particle distribution function (1PDF), which is a scalar density on the space of allowed particle positions and velocities – i.e. on the tangent bundle of the spacetime manifold. Yet, when the gravitational field of a kinetic gas is derived via the Einstein-Vlasov equations, the information about the velocity distribution of the gas particles is averaged out and therefore lost. We argue that a more appropriate theory of gravity, that fully takes the velocity distribution into account, must be also modeled on the tangent bundle; the most natural mathematical framework for this task is thus Finsler spacetime geometry. Following this line of thought, we construct a coupling between the kinetic gas and a recently proposed Finsler geometric extension of general relativity. Also, in view of applications to cosmology, we briefly discuss spatially homogeneous and isotropic Finsler spacetimes.

Joint work with: C. Pfeifer and M. Hohmann

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Thursday, January 19 | 11:00 AM → 12:30 PM

Evidence for strong progenitor age bias in supernova cosmology

Speaker: Prof. Young-Wook Lee, Department of Astronomy, Yonsei University, Seoul, South Korea

Abstract: Supernova (SN) cosmology is based on the assumption that the width-luminosity relation (WLR) and the color-luminosity relation (CLR) in the type Ia SN luminosity standardization would not show zero-point offsets with progenitor age. Unlike this expectation, recent age datings of stellar populations in host galaxies have shown significant correlations between progenitor age and Hubble residual (HR). Here we show that this correlation originates from a strong progenitor age dependence of the zero-points of the WLR and CLR, in the sense that SNe from younger progenitors are fainter each at given light-curve parameters x1 and c. This 4.6 sigma result is reminiscent of Baade’s discovery of the zero-point variation of the Cepheid period-luminosity relation with population age, and, as such, causes a serious systematic bias with redshift in SN cosmology. Other host properties show substantially smaller and insignificant offsets in the WLR and CLR for the same dataset. We illustrate that the differences between the high-z and low-z SNe in the WLR and CLR, and in HR after the standardization, are fully comparable to those between the correspondingly young and old SNe at intermediate redshift, indicating that the observed dimming of SNe with redshift may well be an artifact of over-correction in the luminosity standardization. When this systematic bias with redshift is properly taken into account, there is little evidence left for an accelerating universe, posing a serious question to one of the cornerstones of the concordance model.

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Thursday, January 26 | 11:00 AM → 12:30 PM

Minimal symmetry breaking of the cosmological principle and the Lemaitre-Hubble diagram

Speaker: Prof. Thomas Schucker, Aix Marseille Université, Université de Toulon, Marseille, France

Abstract: We present our definition of minimal symmetry breaking of the cosmological principle. For comoving dust, the definition leads uniquely to the axial Bianchi IX universes with positive curvature and their flat Bianchi I limits. We discuss the direction dependent Lemaitre-Hubble diagram of these universes and its future observational tests. In view of promised promising data from the Vera Rubin Observatory, the James Webb Space Telescope and the Chinese Space Station Telescope, we plead for an immediate preparation of a joint analysis with data from SuperNovae 1a, Cosmic Microwave Background, drift of radio sources and Baryonic Acoustic Oscillations.

Joint work with Galliano Valent and Andre Tilquin

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Thursday, February 2 | 11:00 AM → 12:30 PM

Machine Learning Cosmic Inflation

Speaker: Dr. Savvas Nesseris, Instituto de Física Teorica UAM-CSIC, Universidad Autonoma de Madrid, Madrid, Spain

Abstract: We present a novel approach, using machine learning, to analytically reconstruct the inflationary potential directly from the CMB data. We also perform feature searches, and we find a dramatic improvement over traditional methods based on simple oscillatory models.

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Thursday, February 9 | 11:00 AM → 12:30 PM

Cosmological QUOKKA – Quasar Observations on the KVN from Korea to Australia

Speaker: Dr. Jeffrey Adam Hodgson, Department of Physics and Astronomy, Sejong University, Seoul, Korea

Abstract: Measuring distances vs redshift (more commonly known as a Hubble Diagram) is one of the most fundamental yet difficult observations that can be made in astronomy. Every time the diagram has been extended to greater and greater distances (or higher redshifts), important discoveries have been made. Most recently this was the accelerating expansion of the universe, for which the 2011 Nobel Prize was awarded. I am asking the simple question: „what happens if we look further?”. The discovery of the accelerated expansion of the universe was accomplished using Type Ia supernovae, but they are based on the ‘distance ladder’ and can only be seen to a redshift of ~2. Active Galactic Nuclei (AGN) are unique objects as they can be seen at both low redshift (z~0) and high redshift (z~6). For this reason, they have long been sought as a distance measure with limited success. It is within this context that we are beginning the Cosmological QUOKKA project. The core assumption is that the radio variability observed is constrained by the speed of light. Multiplying the timescale of variability by the speed of light gives a linear size estimate which can then be compared against the apparent size using Very Long Baseline Interferometry (VLBI). In this presentation, I will describe the method, some early results (including a measurement of the Hubble Constant) and the details of our observational project using the Korean VLBI Network and the Mopra telescope in Australia and potentially South Africa.


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Thursday, February 16 | 11:00 AM → 12:30 PM

Buchdahl-inspired spacetimes: How to break a six-decades-old impasse

Speaker: Dr. Hoang Ky Nguyen, Baltimore, Maryland, USA

Abstract: In this talk I shall revitalize an obscure program that Hans Buchdahl ventured circa 1962 in search of static spherically symmetric vacua in pure $R^2$ gravity. He proved therein that the vacua generally have non-constant scalar curvature. Unfortunately, he deemed the final step in his calculations insurmountable and suspended his further pursuit – prematurely so – and his work has largely become a “lost art” nowadays. I recently broke this six-decades-old impasse and accomplished his goal. I shall present three sets of vacua that are either derived from, or inspired by, the Buchdahl program: (i) An exhaustive class of metrics with non-constant scalar curvature in exact compact form for pure $R^2$ gravity; (ii) A non-Schwarzschild asymptotically flat metric in exact closed analytical form also for pure $R^2$ gravity; and (iii) A perturbative metric with non-constant curvature for $R+R^2$ gravity. These metrics project anomalous behaviours yet encountered before. I shall also go over their implications in various directions, ranging from the thermodynamics of $R^2$ black holes to the Lu-Perkins-Pope-Stelle ansatz for the full quadratic action, $R+R^2+Weyl$.


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Thursday, March 2 | 11:00 AM → 12:30 PM

Teleparallel Cosmology – From theory to observational constraints

Speaker: Prof. Jackson Levi Said, Department of Physics, University of Malta, Malta

Abstract: Standard gravity expresses gravitational effects by curving spacetime about matter. It does this through the Levi-Civita connection which is what produces the curvature feature. However, this is not the only choice, there also exists the teleparallel connection through which gravitation exhibits a torsional behaviour. Teleparallel gravity can be constructed to be dynamically equivalent to regular general relativity (GR), known as the teleparallel equivalent to general relativity (TEGR). As in standard GR, one can modify the gravitational theory and consider beyond GR theories. While GR and TEGR are dynamically equivalent, their modifications are not for the most part. In this way, modifications of TEGR are novel and have shown to produce behaviour different to the traditional modifications of gravity. In cosmology, these proposals have shown to produce models beyond GR that can compete with GR in a number of important ways that range from their gravitational wave polarization signatures to their ability to produce the correct expansion dynamics across the cosmological history of the Universe, among others. In this presentation, we will cover the salient features of teleparallel gravity and its key observational predictions in the cosmological regime. We will discuss the confrontation of teleparallel models with the most recent observational surveys and the corresponding constraints on the corresponding beyond GR models. We will close with a discussion on areas to expect the theory to be further developed in and where it can become even more competitive with GR in the coming years.


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Thursday, March 9 | 11:00 AM → 12:30 PM

Stability analysis of spin evolution in compact binaries with black hole, neutron star, gravastar, or boson star components

Speaker: Prof. Laszlo Arpad Gergely, Institute of Physics, University of Szeged, Szeged, Hungary

Abstract: I will discuss the spin dynamics and stability of precessing compact binaries on eccentric orbit with gravastar, black hole, neutron star, or boson star components. This is based on a closed system of first-order differential equations characterising the secular evolution to second post-Newtonian–order, with leading-order spin-orbit, spin-spin, and mass quadrupole-monopole contributions included.

Among the interesting effects are i) new, quadrupole-induced spin flip-flops occurring when the neutron star with dominant spin has a quadrupolar parameter ≈ 3 and ii) a swinging-type azimuthal evolution in the case of equal mass binaries with the black hole companion being either a gravastar or another black hole (the spins of the components periodically overpassing each other), as opposed to their conserved sequence, when the companion is a boson star.

Both the aligned and more generic coplanar configurations of the spins and orbital angular momentum have fixed points. We mapped the parameter regions with stable fixed points of the spins. They include the binary configurations with both spins aligned to the orbital angular momentum (with the exception of certain quadrupolar parameter ranges of neutron stars and boson stars), both spins antialigned, also the extremal mass ratio. For equal masses, the configurations of one of the spins aligned and the other antialigned is stable for gravastar binaries, for neutron star binaries in the high quadrupolar parameter range, and for boson star binaries. We identified a new transitional instability regime during the inspiral of certain gravastar, neutron star, or boson star binaries with opposing spins. For coplanar configurations instabilities occur only for the gravastar – gravastar, boson star – boson star and black hole – boson star binaries.

In summary, both the spin dynamics and the stability of fixed points heavily depend on the nature of the binary components.


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Thursday, March 16 | 11:00 AM → 12:30 PM

Probing general relativistic spin-orbit coupling with strongly lensed gravitational waves

Speaker: Dr. Marius Oancea, Faculty of Physics, University of Vienna, Boltzmanngasse 5, Vienna, Austria

Abstract: Spin Hall effects represent a diverse class of physical phenomena related to the propagation of wave packets carrying intrinsic angular momentum. Such wave packets generally experience spin-orbit couplings between their internal and external degrees of freedom, resulting in wave packets following spin-dependent trajectories. These effects have been experimentally observed in optics and condensed matter physics, but they are also expected to occur for wave packets propagating in gravitational fields. In this talk, I will introduce the equations of motion describing the gravitational spin Hall effect and I will discuss some of their properties. Based on this, I will present recent results (arXiv:2209.06459) regarding the strong lensing of gravitational waves generated in hierarchical triple systems. These results suggest that there are plausible astrophysical scenarios in which the gravitational spin Hall effect has a significant imprint for gravitational waves and could lead to experimental observation.


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Thursday, March 23 | 11:00 AM → 12:30 PM

Pulsars could be strangeon stars

Speaker: Renxin Xu, School of Physics, Peking University, Beijing, China

Abstract: Human beings are curious about the material world since civilization, to link macro-phenomena to micro-statistics. Beyond question, the Fermi-Dirac statistics of electrons is the key, which was soon applied successfully to understand the nature of matter, and it is not surprising that the subject of condensed matter physics dominates in the physics department at any university. Such so familiar stuff is condensed by the electromagnetic force. Nevertheless, is there a kind of matter by the elementary strong force? Certainly, atomic nuclei are “drops” of strong matter, with building units of nucleons. What about the basic units of a “gigantic nucleus” with baryon number, A, larger than a thousand? My answer is “strangeon”, and strong matter with large A, from maybe 10^5 to 10^57, could then be strangeon matter. I would like to explain the physics and astrophysics of strangeon matter in this talk, focusing on strangeon stars governed by gravity in this multi-messenger era, especially of gravitational waves.


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