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, May 30 | 11:00 AM → 12:30 PM

Parametric resonance of Alfven waves driven by ionisation-recombination waves in the weakly ionised solar atmosphere

Speaker: Prof. Istvan Ballai, Plasma Dynamics Group, School of Mathematics and Statistics, University of Sheffield, Sheffield, UK

Abstract: Parametric coupling of waves is one of the most efficient mechanisms of energy transfer that can lead to the growth or decay of waves. This transfer occurs at frequencies close to their natural frequencies. In partially ionised solar plasma there are a multitude of waves that can undergo this process. In my presentation I will present results of a recent study on the parametric coupling of Alfven waves propagating in a partially ionised solar plasma with ionisation-recombination waves that appear in a plasma in ionisation non-equilibrium. Depending on the parameters that describe the plasma (density, temperature) the coupling can lead to a parametric resonance. Our study determines the occurrence conditions of parametric resonance, by finding the boundaries between stable and unstable regions in the parameter space. Our results show that collisions and non-equilibrium recombination can both contribute to the onset of unstable behaviour of parametrically resonant Alfven waves.

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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, Department of Physics, Babeș-Bolyai University, Cluj, Romania

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, February 23 | 11:00 AM → 12:30 PM

Tensions in cosmology and implications for the standard model

Speaker: Prof. Eleonora Di Valentino, School of Mathematics and Statistics, University of Sheffield, Sheffield, United Kingdom

Abstract: The scenario that has been selected as the standard cosmological model is Lambda Cold Dark Matter (ΛCDM), which provides a remarkable fit to the bulk of available cosmological data. However, discrepancies among key cosmological parameters of the model have emerged with different statistical significance.
While some portion of these discrepancies may be due to systematic errors, their persistence across probes can indicate a failure of the canonical ΛCDM model.
I will review these tensions, showing some interesting extended cosmological scenarios that can alleviate them.

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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 & Wigner Research Centre for Physics, Budapest, 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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Thursday, March 30 | 11:00 AM → 12:30 PM

On the Nonrelativistic Expansion of GR

Speaker: Dr. Jelle Hartong, School of Mathematics and Maxwell Institute for Mathematical Sciences, University of Edinburgh, Edinburgh, United Kingdom

Abstract: In this talk I will review the covariant 1/c expansion of GR where c is the speed of light. When expanding the Einstein equations and assuming the weak field approximation this leads to Newtonian gravity at leading order and to post-Newtonian corrections at higher orders in 1/c. However, one does not need to assume weak fields and this leads to a novel regime of strong nonrelativistic gravity. I will discuss how the covariant 1/c expansion can be used to revive the classical approach to the study of post-Newtonian corrections and how they are useful for the study of quantum systems in fixed gravitational backgrounds.

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Thursday, April 6 | 11:00 AM → 12:30 PM

Dimensionally-dependent uncertainty relations, or why we (probably) won’t see micro-black holes at the LHC, even if large extra dimensions exist

Speaker: Dr. Matthew J. Lake, National Astronomical Research Institute of Thailand, Chiang Mai, Thailand

Abstract: We present a simple gedanken experiment in which a compact object traverses a spacetime with three macroscopic spatial dimensions and n compact dimensions. The compactification radius is allowed to vary, as a function of the object’s position in the four-dimensional space, and we show that the conservation of gravitational self-energy implies the dimensional dependence of the mass-radius relation. In spacetimes with extra dimensions that are compactified at the Planck scale, no deviation from the four-dimensional result is found, but, in spacetimes with extra dimensions that are much larger than the Planck length, energy conservation implies a deviation from the normal Compton wavelength formula. The new relation restores the symmetry between the Compton wavelength and Schwarzschild radius lines on the mass-radius diagram and precludes the formation of black holes at TeV scales, even if large extra dimensions exist. We show how this follows, intuitively, as a direct consequence of the increased gravitational field strength at distances below the compactification scale. Combining these results with the heuristic identification between the Compton wavelength and the minimum value of the position uncertainty, due to the Heisenberg uncertainty principle, suggests the existence of generalised, higher-dimensional uncertainty relations. These relations may be expected to hold for self- gravitating quantum wave packets, in higher-dimensional spacetimes, with interesting implications for particle physics and cosmology in extra-dimensional scenarios.

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

Chemical Enrichment History of Galaxies

Speaker: Dr. Randa Asad, American University of Sharjah, Physics Department, Sharjah, United Arab Emirates

Abstract: The star formation and chemical enrichment histories of Local Group galaxies are traditionally studied by analysing their resolved stellar populations. The integrated light of star clusters however, can be studied using ground‐based telescopes to much larger distances. My talk will discuss the first chemical composition measurement obtained using the IR spectra we observed with Xshooter/VLT for a newly discovered young stellar cluster in the central region of our galaxy as well the first detailed chemical abundances that we obtained for three star-clusters in the Large Magellanic Cloud (LMC), using integrated-light optical spectroscopic observations obtained with the Magellan Echelle spectrograph on Magellan Baade telescope.

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

Cosmological Parametrization: A Model-Independent approach in Cosmological
Modelling and Reconstructing Cosmic Evolution

Speaker: Dr. Shibesh Kumar Jas Pacif, Thanu Padmanabhan Centre for Cosmology, SGT University, Haryana, India

Abstract: Gravity best describes the large-scale structure of the Universe and Einstein general theory of relativity is the most successful theory till date even though axed so many times in its journey of hundred years. After the formulation of Einstein Field Equations (EFEs), the task is to solve the system of equations, which are complicated set of ordinary differential equations. The discovery of cosmic acceleration brought a paradigm shift in recent times. In Einstein’s theory, an extra degree of freedom has to be incorporated in the matter sector in order to explain the late-time acceleration. With this extra degree of freedom (dubbed dark energy) in EFEs, one more variable comes into picture and in order to make the system consistent, one more equation is needed besides the equation of state of ordinary and cold dark matter. This extra equation can be chosen in a model-independent way i.e. by parametrizing any physical or geometrical parameter involved in the field equations. Since, the nature of dark energy is a mystery; there is freedom to choose the equation of state (EoS) in different functional forms such as Polytropic gas, logotropic gas, van der Waals gas equation of state etc. Similarly, there is some freedom to choose the functional form of physical of geometrical parameters mathematically (generally termed as cosmological parametrization). The assumption does not violate the background physics rather provide a simple mathematical way to reconstruct the cosmic evolution. In the past few decades, a number of functional forms of various cosmological parameters have been considered for obtaining a consistent solution of the EFEs. In this talk, the idea of the model independent approach to the cosmological parametrization will be discussed in some detail.

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

Hyperfluids and Non-Riemannian Effects in Cosmology

Speaker: Dr. Damianos Iosifidis, Laboratory of Theoretical Physics, Institute of Physics, University of Tartu, Tartu, Estonia

Abstract: We investigate the cosmological aspects of the most general parity preserving Metric-Affine Gravity theory quadratic in torsion and non-metricity in the presence of a cosmological hyperfluid. The latter is a generalization of the usual Perfect Fluid notion that includes also a non-vanishing hypermomentum tensor that is compatible with the Cosmological Principle and encodes the microscopic characteristics of matter. Then, the equations of motion are obtained by varying the action with respect to the metric and the independent affine connection. Subsequently, considering a Friedmann-Lemaître-Robertson-Walker background, we derive the most general form of the modified Friedmann equations for the full quadratic theory. We then focus on a characteristic sub-case involving only two quadratic contributions given in terms of torsion and non-metricity vectors. In this setup, studying the modified Friedmann equations along with the conservation laws of the perfect cosmological hyperfluid, we provide exact solutions both for purely dilation and for purely spin hypermomentum sources. We then discuss the physical consequences of our model and the prominent role of torsion and non-metricity in this cosmological setup.

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

Stellar and substellar objects in modified gravity: matter properties and tests of gravitational models

Speaker: Dr. Aneta Wojnar, Departamento de Fısica Teorica & IPARCOS, Universidad Complutense de Madrid, Madrid, Spain

Abstract: I will provide a brief overview of the latest research in the physics of (sub-)stellar objects in the context of modified gravity. Our primary focus will be on the matter’s properties of these objects, including the equation of state, specific heats, and other relevant characteristics. Additionally, we will explore the internal processes of these objects, such as light element burning, crystallization, and cooling, and discuss potential methods for testing modified gravity theories using seismic data.

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Thursday, May 18 | 11:00 AM → 12:30 PM

Thermodynamics in f(R) theories of gravity with coupling between matter and geometry

Speaker: Dr. Jun Wang, School of Physics and Astronomy, Yunnan University, Kunming, China

Abstract: The result of the observational data from the type Ia supernovae indicates that our universe presently experiences an accelerating expansion, usually called the late-time cosmic acceleration. It makes the modern cosmology based on general relativity face troubles. In order to shed some lights on this observational issue, f(R) theories of gravity have been taken into consideration, which is a competitive member in modified theories of gravity due to the simplicity and stability. From the theoretical aspect, the present cosmic accelerating expansion can be explained by f(R) theories of gravity. Besides exploring the cosmic accelerating expansion, f(R) theories of gravity have been used to investigate other cosmological issues, in which thermodynamics is an important one. Recently, considering the coupling between matter and geometry, a more general type of f(R) gravitational model has been proposed. In this talk, I would like to discuss the effect of the coupling between matter and geometry on thermodynamics in f(R) theories of gravity, especially of the first as well as generalized second laws of thermodynamics.

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Thursday, May 25 | 11:00 AM → 12:30 PM

The boundary problem in (super)gravity from a geometric perspective

Speaker: Prof. Lucrezia Ravera, Department of Applied Science and Technology, Torino, Italy

Abstract: I will discuss the boundary problem in gravity and supergravity, retracing the geometric construction of four-dimensional (super)gravity Lagrangians with negative cosmological constant on a manifold with boundary. The presence of a spacetime boundary breaks diffeomorphisms (and supersymmetry) invariance of the action. To recover such invariance, it is necessary to modify the theory by adding appropriate boundary contributions to the Lagrangian, which, in the gravity case, correspond to the Euler-Gauss-Bonnet term and, in supergravity, to its supersymmetric extension. The full Lagrangian can be recast in a MacDowell-Mansouri form. Regarding the supergravity side, I will also consider the case of vanishing cosmological constant in the presence of a non-trivial boundary, where the inclusion of boundary terms involving additional fields, which behave as auxiliary fields for the bulk theory, allows to restore supersymmetry. I will eventually report on the application of the results to specific problems regarding cases where the boundary is located asymptotically.

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Thursday, June 1 | 11:00 AM → 12:30 PM

Near horizon soft hair

Speaker: Prof. Daniel Grumiller, Institute for Theoretical Physics, Technische Universitat Vienna, Austria

Abstract: Asymptotic symmetries have been crucial in understanding the holographic principle from a gravity perspective since they coincide with the global symmetries of the dual quantum field theory. A few years ago, we extended this notion to near horizon symmetries with some unexpected technical and conceptual insights, notably the discovery of soft Heisenberg hair. I summarize these developments and give an outlook.

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

On the quasi-stationary expansion in gravity

Speaker: Dieter Van den Bleeken, Physics Department, Bogazici University, Bebek, Istanbul, Turkey

Abstract: After shortly commenting and reviewing the quasi stationary (or adiabatic) approximation/expansion in general, I will discuss some recent applications of it to GR. First, I will discuss how in the setting of GR inside a finite spatial volume, the Einstein equations in the quasi-stationary limit can be re-expressed as geodesic equations on the (infinite dimensional) group of boundary diffeomorphisms. Secondly, I will explain how the quasi-stationary expansion is related to the strong field non relativistic expansion of GR (also referred to as the 1/c expansion). Finally, I will mention some work in progress where the quasi-stationary expansion is used concretely to find an (approximate) solution for a time dependent (i.e. dynamic) black hole in an evolving de Sitter background.

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

Thermodynamics and gravity are more closely related than you may think

Speaker: Prof. Diego Pavon, Departamento de Física, Facultad de Ciencias, Universidad Autónoma de Barcelona, Bellaterra, Barcelona, Spain

Abstract: Traditionally gravity and thermodynamics have been considered as two independent subjects with practically no relation between each other. Textbooks on gravity essentially ignore thermodynamics implications and vice-versa. In this talk I shall argue by means of several simple examples that both subjects are more closely related than one may think at first sight.

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Thursday, June 22 | 11:00 AM → 12:30 PM

Seven hints that early-time new physics alone is not sufficient to solve the Hubble tension

Speaker: Dr. Sunny Vagnozzi, Department of Physics, University of Trento, Povo (TN), Italy

Abstract: It appears by now established that solving the Hubble tension requires new physics operating at early times, i.e. prior to recombination. But is that really the end of the story? Based on Miller’s law (which states that the number of objects the average person can hold in working memory is about seven), I will present seven independent hints pointing towards the fact that the Hubble tension requires more than just early-time new physics, and will discuss my personal thoughts about what the most promising scenarios might be moving forward (also keeping in mind the potential tension in S8).

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Thursday, June 29 | 11:00 AM → 12:30 PM

Quantum integrable black holes

Speaker: Prof. Roberto Casadio, Dipartimento di Fisica e Astronomia, Universita di Bologna, Bologna, Italy

Abstract: It is common to assume that quantum gravity belongs at the Planck scale, but a possibly much larger width for the ground state emerges in the (non-perturbative) quantisation of the Oppenheimer-Snyder model of dust collapse that naturally recovers Bekenstein’s area law. The effective geometry for such quantum black holes can then be obtained from coherent states which describe integrable singularities without inner horizons. The extension to quantum (differentially) rotating black holes with similar properties is also described.

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Thursday, July 6 | 11:00 AM → 12:30 PM

Tracing beyond GR physics with gravitational waves – qualitative vs quantitative

Speaker: Dr. Daniela Doneva, Theoretical Astrophysics Group, Eberhard Karls University of Tübingen, Tübingen, Germany

Abstract: Gravitational waves are among the ultimate tools to test fundamental physics and promise to answer the long-waiting question about the nature of gravity in the regime of strong fields. The degeneracies between different effects are a serious obstacle, though, to fulfilling this goal since modified gravity often leads to smaller cumulative changes. In the present talk we will focus on a few examples of interesting new effects we can observe in the gravitational wave spectrum that differ qualitatively from the standard picture in general relativity. This includes gravitational phase transition of neutron stars, jumps in the gravitational wave emission from merging black holes, and inverse chirp signal of extreme mass-ratio inspirals. Such effects are valuable because they are a smoking gun of beyond-GR physics that can be easily traced in observations.

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

Is H0 a constant in LCDM?

Speaker: Dr. Eoin O. Colgain, Atlantic Technological University, Ash Lane, Sligo, Ireland

Abstract: Starting from simple mathematics, I will argue that LCDM tensions, if physical, require redshift evolution of integration constants in the LCDM model. I will present observations supporting this (expected) evolution.

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

Axion-Like-Particles, Direct Detection & Solar constraints

Speaker: Prof. Mar Bastero-Gil, Departamento de Fısica Teorica y del Cosmos, Universidad de Granada, Granada, Spain

Abstract: The Sun could produce Axion-Like Particles (ALPs) with masses in the keV-range. A fraction of them would be trapped in the solar gravitational field and accumulate over cosmic times. Their decay into photons would contribute to the observed solar X-ray flux, whose measurements can be used to constrain ALP models. This has been studied considering the Primakoff process the main production mechanism in the absence of a couplings to electrons. However, ALP production in the solar core is dominated by the photon coalescence, and this will improve the bound on their coupling to photons by an order of magnitude for ALP masses between 3 keV and 40 keV. For non-vanishing coupling to electrons, Compton process soon become the dominant one. At the same time, trapped ALPs can be Compton-absorbed while crossing the Sun during their orbits, and this counter-balances the production, partially or entirely depending on parameter values. This results in two well-defined regime in the exclusion limits, with a transition triggered by the coupling to electrons. Out of the transitional region, the solar X-ray constraints on ALPs are exclusively governed by the coupling to photons.

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Thursday, October 05 | 11:00 AM → 12:30 PM

From the Flamm-Einstein-Rosen bridge to the modern renaissance of traversable wormholes

Speaker: Prof. Francisco S. N. Lobo, Departamento de Fısica, Faculdade de Ciencias da Universidade de Lisboa, Edifıcio C8, Campo Grande, Lisbon, Portugal

Abstract: We consider the possibility of multiply-connected spacetimes, ranging from the Flamm-Einstein-Rosen bridge, geons, and the modern renaissance of traversable wormholes. A fundamental property in wormhole physics is the flaring-out condition of the throat, which through the Einstein field equation entails the violation of the null energy condition. In the context of modified theories of gravity, it has also been shown that the normal matter can be imposed to satisfy the energy conditions, and it is the higher order curvature terms, interpreted as a gravitational fluid, that sustain these non-standard wormhole geometries, fundamentally different from their counterparts in general relativity. We explore interesting features of these geometries, in particular, the physical properties and characteristics of these `exotic spacetimes'.

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

Indirect Evidence for Dark Matter Density Spikes around Stellar-Mass Black Holes

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

Abstract: It has been suggested for a long time that dark matter would form a density spike around a black hole. However, no promising evidence has been observed so far to verify this theoretical suggestion. In this talk, I will report the existence of a dark matter density spike around each of the two nearby stellar-mass black holes (A0620-00 and XTE J1118+480). The dynamical friction between dark matter and the companion stars can satisfactorily explain the abnormally fast orbital decays in the two binaries. The calculated spike index for A0620-00 and XTE J1118+480 are 1.71 and 1.85 respectively, which are close to the lower regime predicted by the stellar heating model. It may provide a possible indirect evidence for the existence of dark matter density spikes around stellar-mass black holes. We anticipate that analyzing observational data of nearby black hole X-ray binaries would be a new way to reveal the nature of dark matter.

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

Electromagnetically K-mouflaged dark matter and the secret supersymmetry of haloes

Speaker: Dr. Jose Beltran-Jimenez, Departamento de Fısica Fundamental and IUFFyM, Universidad de Salamanca, Salamanca, Spain

Abstract: I will discuss a cosmological scenario where dark matter is provided with a dark electric charge. This dark electromagnetic sector features a screening mechanism so that all the effects only appear at low redshift when dark matter is sufficiently clustered. Within these models, it is natural to have a universe described by a Lemaitre model instead of a FLRW. Instead of solving the full relativistic equations, I will consider a Newtonian approach that is sufficient for a matter dominated universe with, possibly, a cosmological constant. In this scenario, it is possible to explain the Hubble tension in terms of the dark electric repulsion between dark matter halos. After reviewing some phenomenological consequences, I will proceed to analysing the deformability of dark matter haloes due to the dark electromagnetic interaction and show an intriguing resemblance with the vanishing of the Love numbers of black holes. Finally, a remarkable relation with super-symmetric quantum mechanical systems will be explained.

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

Promising findings for simultaneous alleviation of major Cosmological tensions through the $\Lambda_{\rm s}$CDM cosmology

Speaker: Prof. Ozgur Akarsu, Department of Physics, Istanbul Technical University, Maslak, Istanbul, Turkey

Abstract: In this talk, we will begin with a brief introduction to the $\Lambda_{\rm s}$CDM model. This model explores a recent conjecture suggesting a rapid transition of the universe from anti-de Sitter vacua to de Sitter vacua. Specifically, the cosmological constant switches sign from negative to positive at redshift $z_\dagger\sim 1.7$, inspired by the graduated dark energy (gDE). Following the introduction, we will present the results of a comprehensive observational analysis. This analysis demonstrates that $\Lambda_{\rm s}$CDM, which predicts $z_\dagger\approx 1.7$, simultaneously addresses the major cosmological tensions present in the standard $\Lambda$CDM model. These tensions include the $H_0$, $M_B$, and $S_8$ tensions, along with other less significant tensions such as the BAO Ly-$\alpha$ discrepancy. We will conclude with a brief discussion of the possible observational and theoretical implications of these findings for our current understanding of the universe.


Thursday, November 02 | 11:00 AM → 12:30 PM

Cosmology with wide-area extragalactic radio surveys

Speaker: Dr. Jacobo Asorey Barreiro, Departamento de Física Teorica, Universidad Complutense de Madrid, Madrid, Spain

Abstract: We are currently living in a blooming era of wide field optical cosmological surveys. In particular, extragalactic radio surveys will map the matter distribution at high redshifts crucial to discriminate between current cosmological theoretical models. I will address two types of radio observations. One regarding the use of intensity mapping as a low-resolution spectroscopic technique. The other one uses radio-continuum information. Using the full sky HIR4 simulations, I will show the prospects of measuring the growth rate of around redshift z=1 and measuring the expansion rate of the Universe by cross-correlating intensity mapping information with DESI. I will also address some of the cosmological prospects of using the Australian Square Kilometre Array Pathfinder (ASKAP) with the Evolutionary Map of the Universe survey (EMU), especially considering the EMU pilot survey and the first cosmological results using data from the Rapid ASKAP Continuum Survey.


Thursday, November 09 | 11:00 AM → 12:30 PM

Connecting Cosmic Inflation to Particle Physics with LiteBIRD, CMB S4, EUCLID and SKA

Speaker: Dr. Lei Ming, School of Physics, Sun Yat-Sen University, Guangzhou, China

Abstract: We show that next generation CMB experiments will be capable of the first ever measurement of the inflaton coupling to other particles, opening a new window to probe the connection between cosmic inflation and particle physics. For our analysis we introduce a simple analytic method to estimate the sensitivity of future CMB observations to the reheating temperature and the inflaton coupling. This sensitivity is based on the impact that the reheating phase after cosmic inflation has on the redshifting of cosmic perturbations. Applying our method to LiteBIRD and CMB-S4 we find that, within a given model of inflation, these missions have the potential to impose both an upper and a lower bound on the inflaton coupling. Further improvement can be achieved if CMB data is combined with optical and 21cm surveys. Our results demonstrate the potential of future observations to constrain microphysical parameters that can provide an important clue to understand how a given model of inflation may be embedded in a more fundamental theory of nature.

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

Status and future of 21-cm cosmology

Speaker: Dr. Andrei Mesinger, Scuola Normale Superiore, Pisa, Italy

Abstract: The 21-cm hyperfine line of neutral hydrogen is set to revolutionize studies of the first billion years, spanning the cosmic dawn of the first stars and eventual reionization of our Universe. I will discuss the potential of this probe in learning about the unknown astrophysics of the first galaxies as well as physical cosmology. Current upper limits on the cosmic 21-cm power spectrum already provide new insights into the heating of the intergalactic medium, and the X-ray sources in the first galaxies. I will discuss the upcoming steps, including the main challenges, that will eventually lead to the Nobel prize-worthy 3D map of half of our observable Universe with the Square Kilometer Array telescope.


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

Scalar field emulator via anisotropically deformed vacuum energy

Speaker: Prof. Nihan Katirci, Department of Electrical and Electronics Engineering, Faculty of Engineering, Dogus University, Umraniye/İstanbul, Turkey

Abstract: In this talk, we introduce a generalization of the usual vacuum energy via preserving zero inertial mass density. In return for zero inertial mass density, vacuum energy yields a particular form of anisotropic pressure. This ‘ansiotropicallly deformed vacuum energy' couples to the shear scalar in a unique way, such that they together emulate the canonical scalar field with an arbitrary potential. This opens up a new avenue by reconsidering cosmologies based on canonical scalar fields, along with a bonus that the kinetic term of the scalar field is replaced by an observable, the shear scalar. There has been recent suggestions to address Hubble tension by reanalyzing the cosmological data by breaking down of the RW framework, e.g., allowing anisotropic expansion in the late universe and at the end of this talk, we will show that via deformed vacuum energy, it is possible to generate anisotropies at cosmological scales in the late universe, which in turn can pave the way for considering such approaches to address some of the cosmological discrepancies.

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

Hearts of Darkness: Theory and Phenomenology of quantum gravity regularised black holes

Speaker: Prof. Stefano Liberati, SISSA, and INFN Sezione di Trieste, Trieste, Italy

Abstract: Black holes are the purest expression of gravity and at the same time the places where our best theory of gravitation, Einstein’s General Relativity, meets its demise in the form of singularities. We expect that any successful theory of quantum gravity should however be able to regularise these uncharted regions leaving an interior that can be modelled via a smooth spacetime. The question is then if this can be done without any modification showing outside the trapping horizon of these objects. In this talk I shall describe how a quantum gravity agnostic approach can go a big length in predicting possible regular outcomes of a regularised gravitational collapse. Remarkably, all of the viable spacetime we shall encounter sport an interesting phenomenology suggesting a quite limited set of possible long living objects. We shall see that connecting these insights with current observations hold promise for distinguishing these regular spacetimes from the singular General Relativity black holes, and so heralding the advent of a new branch of quantum gravity phenomenology.

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Thursday, December 07 | 11:00 AM → 12:30 PM

Highly relativistic solutions of the Einstein-Vlasov system and a comparison to solutions of the Einstein-Dirac system

Speaker: Prof. Hakan Andreasson, Department of Mathematical Sciences, Chalmers University of Technology, University of Gothenburg, Gothenburg, Sweden

Abstract: In the first part of this talk I will review some results on highly compact stationary solutions of the Einstein-Vlasov system. A comparison to Wheeler's idealized spherically symmetric geons will be made which indicates that solutions of the massless EV system have very similar properties. In the second part I will focus on a comparison between solutions of the Einstein-Dirac system and the EV system. In 1999 Finster et al. found for the first-time static solutions to the Einstein-Dirac (ED) system in the case of two fermions with opposite spins. Recently this study has been extended to a larger number of particles by Leith et al. In particular, they construct highly relativistic solutions. The structure of the solutions is strikingly similar to the structure of highly relativistic solutions of the EV system. Since solutions of the EV system are classical whereas solutions of the ED system have a quantum signature this study provides insights about the transition from quantum to classical behaviour for a small number of particles. A particular feature in this context is that the pressure may be negative in the case of a few particles for solutions of the ED system, but as the number of particles increase (above 16) it becomes non-negative, i.e. it behaves classically.

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Thursday, December 14 | 11:00 AM → 12:30 PM

Black holes and boson stars with new scalar hair

Speaker: Dr. Betti Hartmann, Department of Mathematics, University College London, Gower Street, London, UK

Abstract: Static, spherically symmetric black holes can carry scalar hair when coupling standard Einstein gravity minimally to a self-interacting complex scalar field and a U(1) gauge field. For this scalar hair to exist, the frequency of the scalar field needs to be fine-tuned. In this talk, I will discuss these solutions and point out that for sufficiently large gravitational coupling, the space-time splits into two distinct parts: (a) an inflating interior and (b) an exterior which is described by the extremal Reissner-Nordström solution. Moreover, for a specific range of parameters, the scalar hair develops spatial oscillations, i.e. black holes can carry so-called wavy scalar hair. I will also comment on the globally regular counterparts of these solutions: charged boson stars.

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

Effective field theory of black hole perturbations with timelike scalar profile

Speaker: Prof. Shinji Mukohyama, Center for Gravitational Physics and Quantum Information, Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto, Japan

Abstract: Many dark energy (DE) models are based on a scalar field with timelike gradient. In this talk we begin with a review of the systematic construction of the effective field theory (EFT) describing perturbations around the Minkowski background with a timelike scalar profile and its extension to cosmological backgrounds, i.e. the ghost condensation and the EFT of inflation/DE. If one hopes to learn something about the EFT of DE from black holes (BHs) then one needs to consider BH solutions with timelike scalar profiles. We thus extend the EFT to arbitrary backgrounds. Finally, we discuss applications of the general EFT to study perturbations of BH with a timelike scalar field responsible for DE.

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Thursday, January 18 | 11:00 AM → 12:30 PM

Modified theories of gravity - a unified approach to metric and metric-affine models

Speaker: Prof. Christian G. Boehmer, Department of Mathematics, University College London, Gower Street, London WC1E 6BT, United Kingdom

Abstract: The talk begins with a general overview of General Relativity focussing on the basic ingredients of its mathematical structure. This will naturally lead the way to consider various modifications or extensions of General Relativity, many of which have been studied recently. I will briefly review metric-affine models and how these fit into GR. Next I will discuss modified gravity models based on generalised geometries and on actions no longer linear in curvature. The main part of the talk will discuss how these many different theories can be studied using a single unified approach which also shows the equivalence of some of these models. We will finish off by showing a cosmological application where torsion drives a period of early-time inflation without affecting the late-time matter dominated behaviour. No matter sources are required to drive inflation and it becomes a purely geometrical effect.

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Thursday, January 25 | 11:00 AM → 12:30 PM

Cosmological models with arbitrary spatial curvature in the theory of gravity with non-minimal derivative coupling

Speaker: Prof. Sergey Sushkov

Abstract: We investigate isotropic and homogeneous cosmological scenarios in the scalar-tensor theory of gravity with non-minimal derivative coupling of a scalar field to the curvature given by the term $(\zeta/H_0^2) G^{\mu\nu}\nabla_\mu\phi \nabla_\nu\phi$ in the Lagrangian. In general, a cosmological model is determined by six dimensionless parameters: the coupling parameter $\zeta$, and density parameters $\Omega_0$ (cosmological constant), $\Omega_2$ (spatial curvature term), $\Omega_3$ (non-relativistic matter), $\Omega_4$ (radiation), $\Omega_6$ (scalar field term), and the universe evolution is described by the modified Friedmann equation. In the case $\zeta=0$ (no non-minimal derivative coupling) and $\Omega_6=0$ (no scalar field) one has the standard $\Lambda$CDM-model, while if $\Omega_6\not=0$ -- the $\Lambda$CDM-model with an ordinary scalar field. As is well-known, this model has an initial singularity, the same for all $k$ ($k=0,\pm1$), while its global behavior depends on $k$. The universe expands eternally if $k=0$ (zero spatial curvature) or $k=-1$ (negative spatial curvature), while in case $k=+1$ (positive spatial curvature) the universe expansion is changed to contraction, which is ended by a final singularity. The situation is crucially changed when the scalar field possesses non-minimal derivative coupling to the curvature, i.e. when $\zeta\not=0$. Now, depending on model parameters, (i) There are three qualitatively different initial state of the universe: an {\em eternal kinetic inflation}, an {\em initial singularity}, and a {\em bounce}. The bounce is possible for {\em all} types of spatial geometry of the homogeneous universe; (ii) For {\em all} types of spatial geometry, the universe goes inevitably through the {\em primary quasi-de Sitter} (inflationary) epoch when $a(t)\propto e^{h_{dS}(H_0t)} $ with the de Sitter parameter $h_{dS}^2={1}/{9\zeta}-{8\zeta\Omega_2^3}/{27\Omega_6}$. The mechanism of primary or {\em kinetic} inflation is provided by non-minimal derivative coupling and needs no fine-tuned potential; (iii) There are {\em cyclic} scenarios of the universe evolution with the non-singular bounce at a minimal value of the scale factor, and a turning point at the maximal one; (iv) There is a natural mechanism providing a {\em change} of cosmological epochs.

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Thursday, February 01 | 11:00 AM → 12:30 PM

Lévy-Leblond Equation and Eisenhart-Duval lift in Koopman-von Neumann Mechanics

Speaker: Prof. Zurab Silagadze

Abstract: The report is based on the paper B.K. Parida, A. Sen, S. Dhasmana, Z.K. Silagadze, Lévy-Leblond Equation and Eisenhart-Duval lift in Koopman-von Neumann Mechanics (Mod. Phys. Lett. A. I will talk in more detail about the topics that appear in the title of the paper: the Lévy-Leblond equation, Koopmann-von Neumann mechanics, and the Eisenhart-Duval lift. These branches of physics are not as well known as they deserve, in my opinion. Finally, I will briefly discuss our work that served as the starting point for the report.

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Thursday, February 08 | 11:00 AM → 12:30 PM

Probing the very small scale dark matter distribution with gravitational waves

Speaker: Prof. Masamune Oguri, Center for Frontier Science, Chiba University, Chiba, Japan

Abstract: The spatial distribution of dark matter at small scale is a key for understanding the nature of dark matter, yet it is difficult to probe. I argue that it would be possible to probe the very small scale dark matter distribution with gravitational waves, as wave optics effects caused by intervening dark matter distributions modify their amplitudes and phases. I will discuss the expected signals, including how those amplitude and phase fluctuations could be enhanced due to gravitational lensing magnifications.

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

Quantum Improved Regular Kerr Black Holes

Speaker: Prof. Chiang-Mei Chen, Department of Physics, National Central University, Chungli, Taiwan

Abstract: In this talk, I will discuss the quantum improvement of Kerr black holes with mass-dependent scale identifications in asymptotically safe gravity. We find that a physically sensible identification can only be a function of $M r$ and the area $A = 4 \pi (r^2 + a^2)$ where $M$ is the mass of the black hole and $a$ is an angular momentum parameter. We then discuss various properties of the rotating quantum black holes for a simple choice of the identification. We show that the resulting regular rotating black holes have the following nice properties: (i) admitting a consistent black hole thermodynamics at the horizon, (ii) resolving the ring singularity, (iii) partially eliminating closed time-like curves present in the classical Kerr black holes.

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Thursday, February 22 | 11:00 AM → 12:30 PM

Primordial black holes, cosmological dark matter, and galactic antimatter

Speaker: Prof. Alexander D. Dolgov

Abstract: It is argued that the dense population of the early universe with well developed galaxies and supermassive black holes (quasars), observed by HST and JWST, nicely fits the conjecture that the galaxies and quasars are seeded by primordial black holes (PBHs), proposed in our work more than 30 years ago. This idea of galaxy seeding by massive black holes is rediscovered in recent publications by several groups. It is shown that the predicted log-normal mass spectrum of PBHs very well agrees with astronomical observations. Our other prediction of noticeable amount of antimatter in the Galaxy (positrons, antinuclei, antistars) is also confirmed by the data. It is argued that the cosmological dark matter may totally consist of PBHs.

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Thursday, February 29 | 11:00 AM → 12:30 PM

Ricci inverse gravity wormholes

Speaker: Dr. Ghulam Mustafa, Department of Physics, Zhejiang Normal University, Jinhua, P. R. China

Abstract: In the current talk, we shall discuss two new wormhole solutions in the background of fourth-order, newly modified Ricci inverse gravity. Two new classes of wormhole solutions are analyzed by showing the valid region for the main part of wormhole geometry under the effect of involved parameters. The embedded diagrams for both generic shape functions should be presented, connecting upper and lower Universes. Energy conditions are included in the current analysis to check the existence of these wormhole solutions. In the maximum regions, all energy conditions are violated, which confirms the presence of exotic matter in the background of Ricci inverse gravity. Stability analysis for both wormhole solutions is explored within the scope of the speed of sound parameters. In conclusion, some highlights from this research confirm the realistic nature and viability of these wormhole solutions in Ricci inverse gravity.

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Thursday, March 07 | 11:00 AM → 12:30 PM

Tensions in the Concordance Model of Cosmology

Speaker: Prof. Arman Shafieloo, Korea Astronomy and Space Science Institute, Daejeon, Republic of Korea

Abstract: I review the status of the standard model of cosmology in light of recent observations discussing the current tensions and some different alternatives including some early and late universe solutions to the problem. I will also briefly discuss the near-future of the field at the era of the next generation of astronomical/cosmological surveys.

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Thursday, March 14 | 11:00 AM → 12:30 PM

Hidden conformal symmetry for dyonic Kerr-Sen black hole and its gauged family

Speaker: Dr. Muhammad F. A. R. Sakti, High Energy Physics Theory Group, Department of Physics, Faculty of Science, Chulalongkorn University, Bangkok, Thailand

Abstract: In this talk, we explore the conformal invariance of the dyonic Kerr-Sen black hole and its gauged family. We consider a neutral massless scalar probe on the black holes’ background within the low-frequency limit and exhibit that the solution space possesses SL(2,R)×SL(2, R) isometry. We then successfully derive the Bekenstein-Hawking entropy using Cardy entropy formula from 2D CFT. Furthermore, we also calculate the absorption cross-section from gravity side for generic non-extremal dyonic Kerr-Sen black hole and near-extremal gauged dyonic Kerr-Sen black hole. Our calculations show that those quantities are in a perfect match with the calculation from CFT. Our findings further support the duality between rotating black holes and CFTs.

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Thursday, March 21 | 11:00 AM → 12:30 PM

Cosmological observational constraints on the power law f(Q) type modified gravity theory

Speaker: Dr. Sanjay Mandal, Faculty of Mathematics and Computer Science, Transilvania University, Brasov, Romania

Abstract: In this talk, I will discuss an alternative framework for advancing our understanding of dark energy beyond the confines of standard General Relativity (GR), within the realm of f(Q) modified gravity theory. The discussion commences with a succinct overview of the dark energy enigma, followed by an examination of the fundamental elements that provide opportunities for modifying or extending GR to address current challenges. To comprehensively grasp the accelerated expansion phenomenon of the universe, we will delve into a power-law type f(Q) cosmological model. After that, this model will confront observational datasets, contrasting it with the $\Lambda$CDM model. Additionally, a detailed comparison between the predictions of the power-law model and the $\Lambda$CDM model through various cosmological applications will be presented. Later on, I will conclude the findings by demonstrating how the f(Q) model remains capable of elucidating current observational results related to cosmological parameters, providing a compelling and coherent explanation for the accelerating expansion of the Universe, albeit with distinctions from the $\Lambda$CDM paradigm. We will finish off by discussing some future perspectives.

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Thursday, March 28 | 11:00 AM → 12:30 PM

Dipole $\Lambda$CDM model: Towards a realistic model building in dipole cosmology

Speaker: Prof. Shahin Sheik-Jabbari, School of Physics, Institute for Research in Fundamental Sciences (IPM), Tehran, Iran

Abstract: Dipole cosmology is the maximally Copernican generalization of the FLRW paradigm that can incorporate bulk flows in the cosmic fluid. In this paper, we first discuss how multiple fluid components with independent flows can be realized in this set up. This is the necessary step to promote “tilted” Bianchi cosmologies to a viable framework for cosmological model building involving fluid mixtures (as in FLRW). We present a dipole $\Lambda$CDM model which has radiation and matter with independent flows, with (or without) a positive cosmological constant. A remarkable feature of models containing radiation (including dipole $\Lambda$CDM) is that the relative flow between radiation and matter can increase at late times, which can contribute to e.g., the CMB dipole. This can happen generically in the space of initial conditions. We discuss the significance of this observation for late time cosmic tensions.

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Thursday, April 04 | 11:00 AM → 12:30 PM

Ultracompact stars in the light of minimal geometric deformation

Speaker: Dr. Muhammad Zubair, Department of Mathematics, COMSATS University Islamabad, Lahore, Pakistan

Abstract: In this talk, we present an analytical version of the charged gravastar model with non-uniform anisotropic features in the framework of $f (R, T )$ theory. In order to introduce anisotropy in the system, we consider the gravitational decoupling method by means of the Minimal Geometric Deformation (MGD). In the interior region, we obtain a family of interior solutions, which smoothly join a conformally deformed Reissner–Nordström exterior solution, as well as the standard Reissner–Nordstr\"{o}m solution at the stellar boundary $R = a(\τau)$, where $\tau$ is the proper time. In both cases, we have different ranges of $\lambda$ for the smooth joint at the stellar boundary. The ultracompact interior solution representing a family of charged stellar models satisfies some of the fundamental properties of a stable configuration, which includes regularity at the center, positive energy density with monotonically decreasing behavior from the center outwards, and non-uniform pressure with monotonic profile. All the energy bounds except the strong one are satisfied inside the ultracompact interior for all the values of $\lambda$, compatible with regularity condition.

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

Discretized Finsler-Hamilton Geometry: An Approach to Quantize General Relativity

Speaker: Prof. Abdel Nasser Tawfik, Research Center, Faculty of Engineering, Future University in Egypt, New Cairo

Abstract: We suggest an approach to generalize Finsler-Hamilton geometry and gravitize the fundamental theory of quantum mechanics, the Heisenberg uncertainty principle. The Riemann geometry is extended to incorporate the distance-momentum duality symmetry, and the application of the Relativistic Generalized Uncertainty Principle (RGUP). At the point x in the Finsler-Hamilton space, we consider a test particle with mass m, velocity y and momentum p, whose Finsler structure function F(x,y) is discretized as F(x,p), and on which RGUP can be applied. A generalized metric tensor on this geometric structure can be derived. On the generalized Riemann manifold, the resulting metric tensor looks very similar to the Weyl tensor. We determine some properties of the additional curvatures that emerge at relativistic scales, and conclude that an attenuation, or even a regularization of space, and of the initial singularity, can be obtained.

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Thursday, April 18 | 11:00 AM → 12:30 PM

Effective speed of gravitational waves and the mirage of modified luminal gravitational wave propagation

Speaker: Dr. Antonio Enea Romano, Theoretical Physics Department, CERN, CH-1211 Geneva, Switzerland

Abstract: We derive an effective equation and action for the propagation of gravitational waves (GW), encoding the effects of interaction and self-interaction in a time, frequency and polarization dependent effective speed. In terms of an appropriately defined effective metric, the effective action takes the form a massless Klein-Gordon action. This effective approach predicts that for theories with matter coupled to the Einstein frame metric the ratio between gravitational and electromagnetic (EM) luminosity distance depends on the effective speed, while for Jordan frame matter coupling it depends on the effective Planck mass. We discuss how the frequency and polarization dependence of the GW-EM distance ratio and time delay provide a new test of general relativity and its modifications, and more in general of the interaction of GWs with other fields. As an application, consistent with the effective theory of dark energy, we show that for a luminal modified gravity theory, the gravitational-wave propagation and luminosity distance are the same as in general relativity, but depending on the matter gravity coupling, the electromagnetic luminosity distance can be modified w.r.t general relativity. Partially based on Phys.Lett.B 851 (2024) 138572, Phys.Rev.Lett. 130 (2023) 23


Thursday, April 25 | 11:00 AM → 12:30 PM

Ghosts without runaway instabilities

Speaker: Prof. Cedric Deffayet, Laboratoire de Physique de l’École Normale Supérieure, ENS, Université PSL, CNRS, and Sorbonne Université, Université Paris Cité, Paris, France

Abstract: I discuss simple mechanical models with a ghost interacting with a positive energy degree of freedom, and yet, the phase-space motion can be shown analytically and numerically to be fully stable.


Thursday, May 09 | 11:00 AM → 12:30 PM

Probing the primordial Universe with 21cm and galaxy surveys

Speaker: Prof. Roy Maartens, Department of Physics and Astronomy, University of the Western Cape, Cape Town, South Africa

Abstract: The Square Kilometre Array Mid-Frequency radio telescope array is being built in South Africa, with early observations by the precursor MeerKAT array already underway. At the same time various new optical / infrared telescopes have started, or will soon start, observing the cosmos. Next-generation surveys of the matter distribution will not only provide a major advance in precision on the standard cosmological parameters - they will also take the first steps beyond the Planck CMB telescope in probing the primordial Universe. In particular, 21cm and galaxy surveys will be able to constrain the level of primordial non-Gaussianity at similar precision to Planck, but in a completely different way. By combining the results from different surveys, the aim will be to improve on Planck constraints and thus advance our understanding of the very early Universe.

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

Matter-Geometry couplings and the variation of the energy-momentum tensor

Speaker: Dr. Shahab Shahidi, School of Physics, Damghan University, Damghan, Iran

Abstract: In this talk, I will provide a comprehensive discussion on the variation of the matter-energy momentum tensor. A key issue with previous approaches is that they yield different results depending on the specific matter Lagrangian, such as $L_m = -\rho$ and $L_m = p$. We will show that the new method presented here addresses this inconsistency. We will show that previous approaches incorrectly took the second variation of the matter Lagrangian. Following a review of the thermodynamic background for a perfect fluid, we will derive the new, correct variation. Finally, we will compare the two methods using a simple $f(R,T)$ model with non-minimal matter-geometry couplings.

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

Bootstrapped Newtonian compact objects

Speaker: Dr. Octavian Micu, Institute for Space Sciences, Bucharest-Magurele, Romania

Abstract: Bootstrapped Newtonian gravity is a bottom-up approach constructed by adding several gravitational self-interaction terms to the Poisson equation for Newtonian gravity, and it is used to analyze static spherically symmetric compact sources. A noteworthy feature of bootstrapped Newtonian gravity is the absence of a Buchdahl limit, which means that a matter source can be held in equilibrium by a large enough (and finite) pressure for any (finite) compactness value. The solutions for the gravitational potential can effectively describe the interior of macroscopic black holes in the quantum theory and predict deviations from general relativity in the strong field regime of very compact objects. An overview of the results obtained within this model will be presented.

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

Parametric resonance of Alfven waves driven by ionisation-recombination waves in the weakly ionised solar atmosphere

Speaker: Prof. Istvan Ballai, Plasma Dynamics Group, School of Mathematics and Statistics, University of Sheffield, Sheffield, UK

Abstract: Parametric coupling of waves is one of the most efficient mechanisms of energy transfer that can lead to the growth or decay of waves. This transfer occurs at frequencies close to their natural frequencies. In partially ionised solar plasma there are a multitude of waves that can undergo this process. In my presentation I will present results of a recent study on the parametric coupling of Alfven waves propagating in a partially ionised solar plasma with ionisation-recombination waves that appear in a plasma in ionisation non-equilibrium. Depending on the parameters that describe the plasma (density, temperature) the coupling can lead to a parametric resonance. Our study determines the occurrence conditions of parametric resonance, by finding the boundaries between stable and unstable regions in the parameter space. Our results show that collisions and non-equilibrium recombination can both contribute to the onset of unstable behaviour of parametrically resonant Alfven waves.


Thursday, June 06 | 11:00 AM → 12:30 PM

Semi-Symmetric Metric Gravity: from the Friedmann-Schouten geometry with torsion to dynamical dark energy models

Speaker: Lehel Csillag, Department of Physics, Babeș-Bolyai University, 1 Kogalniceanu Street, Cluj, Romania

Abstract: In this talk, I will provide a detailed discussion on the dark energy models within semi-symmetric metric gravity. Essentially, this theory is based on a connection with a special type of torsion, first introduced by Friedmann and Schouten. The Einstein equations in this geometry contain additional terms, which arise from torsion. Interpreting these as geometric-type dark energy effects, I will present three models based on three different equations of state. In each case, the predictions of the considered model are compared with the observational data of the Hubble function, and the standard $\Lambda$CDM paradigm. The findings indicate that semi-symmetric metric cosmological models give a good description of the observational data. Consequently, Friedmann's initially proposed geometry emerges as a credible alternative to standard general relativity, in which dark energy has a purely geometric origin.


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

An Einstein-Legendre modified f(R) gravity theory

Speaker: Prof. José Pedro Mimoso, Departamento de Física, Faculdade de Ciencias, Universidade de Lisboa, Campo Grande, Lisboa, Portugal

Abstract: I'll report on the investigation of a new formulation of f(R) gravity, dubbed Einstein-Legendre f(R) gravity, in which a Legendre transform is included as a dynamical term. This leads to a theory with second-order field equations that describes general relativity with a self-interacting scalar field, without requiring the introduction of conformal frames. We find that the quadratic version of scalarized f(R) gravity reduces to general relativity with a massive scalar field, and explore its implications for Friedmann cosmology. We also explore alternative cases that illustrate the features of this modified f(R) gravity, and classify the possible asymptotic regimes. Our findings suggest this scalarized f(R) gravity may lead to simplified description of cosmological applications. The proposed formulation could offer a new perspective on the relationship between f(R) gravity and scalar–tensor theories. [Based on the ref: Eur.Phys.J.C 83 (2023) 433 • e-Print: 2305.04011 [gr-qc]]