Proca seminars series

11 Nov 2021, 09:00 → 1 Jul 2024, 10:30 Europe/Zurich

The goal of this online seminar is to bring Theoretical Physics closer to experiment and to discuss current topics in high energy theory, cosmology & astrophysics. This intends to renew & continue the tradition of "Proca seminars" (1946-1955) initiated by Alexandru Proca.  The seminar lasts 1h, followed by a questions session of no exact time limit (usually 1/2h). Some of the past talks  are also found here.

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Next online seminar:

Title:           UV-IR connections in scattering amplitudes: power of unitarity and causality

Speaker:      Dr Anna Tokareva (Hangzhou Institute for Advanced Study, UCAS, China)

Date:            Thu 28 Feb 2024, 10:00h Paris time (GMT+1h), 18:00h Seoul time (GMT+9h).

Zoom link:  click here

Abstract:    Very basic assumptions about the structure of the fundamental theory describing the nature at high energies can allow for a set of valuable constraints on the low energy EFT. The constraints are independent of the concrete underlying fundamental theory at high energies. The low energy physics is bounded from the set of quite natural assumptions about the structure of the full theory, such as unitarity, causality and Lorentz invariance. In this talk I will consider the EFT of photons (or other U(1) gauge field) and compare different approaches to obtain bounds on its Wilson coefficients. I will present an analytic derivation of the implications of unitarity (linear and non-linear positivity bounds) and compare these constraints with the requirement of causal propagation of the photon modes around non-trivial backgrounds generated by external sources. I will show that the low energy causality condition can give complementary constraints to the positivity bounds. Applying both constraints together can significantly reduce the allowed region of the photon EFT parameters. Besides constraints on the IR theory, the analyticity and unitarity assumptions can set bounds on the UV theory. I will show this connection using the example of the graviton mediated scattering amplitudes. Surprisingly, the form of the forward limit singularities at low energies can give an insight into the of high energy limit of graviton scattering described by the unknown physics.

 

 

Thursday 11 November

1 TBA

TBA

2 Detecting dark matter signals by radio observational data

Abstract: In the past few years, some studies claimed that annihilating dark matter with mass 30-50 GeV can explain the GeV gamma-ray excess in our Galaxy and the gamma-ray spectrum of some other sources. In this talk, we present our latest analysis using the radio continuum spectral data of the central halo of the Ophiuchus cluster and the radio halo of the A4038 cluster. We have found that 40-50 GeV dark matter annihilating via the b quark channel can best explain the radio continuum spectral data of the data. The statistical significance can be larger than 6 sigma, which indicates a very strong signal of dark matter annihilation.

Speaker: Dr Man Ho Chan (Education University of Hong Kong)

Slides

Thursday 18 November

3 The gravitational spin Hall effect of light

Abstract: The propagation of electromagnetic waves in vacuum is often described within the geometrical optics approximation, which predicts that wave rays follow null geodesics. However, this model is valid only in the limit of infinitely high frequencies. At large but finite frequencies, diffraction can still be negligible, but the ray dynamics becomes affected by the evolution of the wave polarization. Hence, rays can deviate from null geodesics, which is known as the gravitational spin Hall effect of light. By considering the WKB approximation for Maxwell's equations, I will briefly present the main steps of a covariant derivation of the polarization-dependent ray equations describing the gravitational spin Hall effect of light. I will also discuss the relation of these equations with the well-known MPD equations, as well as the observer dependence of the position of massless spinning particles.

Speaker: Dr Marius Oancea (Max Planck Inst for Gravitational Physics, Potsdam)

Slides

Thursday 25 November

4 Symmetric teleparallel gravity - the limit of a Proca theory?

Abstract: Alternative geometrical formulations of General Relativity, and the corresponding alternative interpretations of the gravitational interaction, are reviewed. It is shown that General Relativity can be understood as an integrable gauge theory of translations in the "symmetric teleparallel" geometry. This framework allows the canonical resolution of the long-standing foundational problem of the localisation of gravitational energy. The resolution is clarified from the perspectives of the Noether theorems and the pre-metric formalism. The integrability (i.e. flatness, or "teleparallelism") is the property of a massive connection at large distances. This suggests the natural interpretation of teleparallel gravity as the limit of a (non-Abelian generalisation of) Proca theory, where the Planck mass is the mass of the connection. Spacetime has a resolution limit, since the integrability breaks down at microscopic distances near the Planck scale.

Speaker: Prof. Tomi Koivisto (University of Tartu)

Slides

Thursday 2 December

5 Carroll Symmetry & Memory Effect for gravitational waves

Abstract: The motion of test particles after the passage of a Gravitational Wave could (in principle) provide a way to detect the Wave. The isometries of a plane gravitational wave, identified as Levy-Leblond's „Carroll” group, act as symmetries for the particle subject to the wave ; the associated conserved quantities determine the trajectories.

Speaker: Peter Horvathy (Institut Denis Poisson, Tours/Orleans Univ)

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Slides

Thursday 9 December

6 Conserved quantities for Non-relativistic systems via the Eisenhart-Duval lift

Abstract: In this talk I will discuss conserved quantities of some non-relativistic systems originating from symmetries of a particular class of pp-waves called exact plane gravitational waves. In particular, I will consider periodic gravitational waves and corresponding non-relativistic systems called ion traps. I will show how the symmetries of the former yield conserved quantities for the latter.

Speaker: Dr M. Elbistan (Bogazici Univ Istanbul & Institut Denis Poisson, Tours/Orleans Univ)

Slides

Monday 13 December

7 Cosmic inflation from broken conformal symmetry

Abstract: A period of rapidly accelerating expansion is expected in the early Universe implemented by a scalar field slowly rolling down along an asymptotically flat potential preferred by the current data. In this paper, we point out that this picture of the cosmic inflation with an asymptotically flat potential could emerge from the Palatini quadratic gravity by adding the matter field in such a way to break the local gauged conformal symmetry in both kinetic and potential terms. The metric Einstein gravity with a positive cosmological constant could be recovered either in the absence of the matter field or by adding the matter field in a way that preserves the local gauged conformal symmetry (refs. 2110.14718, 2003.08516, 1810.05536).

Speaker: Dr Shao-Jiang Wang (Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing)

Slides

Thursday 16 December

8 Connections between Weyl geometry, quantum potential, and quantum entanglement

Abstract: We study the relationships between the Weyl geometry, quantum entropy and quantum entanglement based on the Weyl geometry.

Speaker: Prof. Shi-Dong Liang (Sun Yat-Sen University SYSU, CN)

Slides

Thursday 6 January

9 Higher-spin BMS symmetries

Abstract: We study higher-spin extensions of the Carrollian definition of BMS symmetry and their relation to the asymptotic kinematics of a massless scalar field near null infinity. In particular, we revisit Sachs's unitary representation of the BMS group, generalise it to any dimension, and use it to define a higher-spin algebra in Minkowski spacetime.

Speaker: Dr Xavier Bekaert (Institut Denis Poisson, Tours Univ & Universite d’Orleans)

Slides

Thursday 13 January

10 Curvature-Matter Couplings in Modified Gravity

Abstract: We review a plethora of modified theories of gravity with generalized curvature-matter couplings. The explicit nonminimal couplings, for instance, between an arbitrary function of the scalar curvature and the Lagrangian density of matter, induces a non-vanishing covariant derivative of the energy-momentum tensor, implying non-geodesic motion and consequently leads to the appearance of an extra force. Applied to the cosmological context, these curvature-matter couplings lead to interesting phenomenology, where one can obtain a unified description of the cosmological epochs. In addition to this, these models are extremely useful for describing dark energy-dark matter interactions, and for explaining the late-time cosmic acceleration. We explore the physical and cosmological implications of the nonconservation of the energy-momentum tensor by using the formalism of irreversible thermodynamics of open systems in the presence of matter creation/annihilation.

Speaker: Prof. Francesco Lobo (University of Lisbon)

Slides

Thursday 20 January

11 Black holes, bosonic stars and ultralight dark matter

Abstract: We discuss, with concrete examples and real data, how strong gravity systems and observables could assess the nature of dark matter, or at least part of it. In the theoretical modelling, I will focus on fuzzy dark matter, bosonic stars and black holes with synchronised hair. The real data examples will concern the GW190521 gravitational wave event and the EHT measurement of the M87* shadow.

Speaker: Prof. Carlos A. Herdeiro (University of Aveiro)

Slides

Thursday 27 January

12 Testing General Relativity with black hole X-ray data

Abstract: The theory of General Relativity has successfully passed a large number of observational tests. The theory has been extensively tested in the weak-field regime with experiments in the Solar System and observations of binary pulsars. The past 5-6 years have seen significant advancements in the study of the strong-field regime, which can now be tested with gravitational waves, X-ray data, and mm Very Long Baseline Interferometry observations. In my talk, I will summarize the state-of-the-art of the tests of General Relativity with black hole X-ray data, discussing its recent progress and future developments.

Speaker: Prof. Bambi Cosimo (Fudan U, Shanghai and Tubingen Univ)

Slides

Thursday 3 February

13 Modified theories of gravity - foundations and models

Abstract: In the first part of the talk I am discussing General Relativity, in particular its basic ingredients and 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. 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. Boundary terms in the action will play a crucial role in establishing the equivalence between different theories. This setup will also allow us to discuss models where local Lorentz invariance can be broken and also models where local diffeomorphisms can be broken. In general this is done by introducing appropriate length scales.

Speaker: Prof. Christian Boehmer (University College London)

Slides

Thursday 10 February

14 Pair Productions in Charged Black Holes

Abstract: We study the spontaneous pair production, including the Schwinger mechanism and the Hawking radiation, of charged scalar particles from the near horizon region of (near) extremal charged black holes in the probe field limit. Analytical expressions for pair production, vacuum persistence and absorption cross section are found, and the spectral distribution is given a thermal interpretation. The pair production in near-extremal black holes has a factorization into the Schwinger effect in AdS and in Rindler space, measuring the deviation from extremality.

Speaker: Prof. Chiang-Mei Chen (Taiwan National Central University)

Slides

Thursday 17 February

15 The Neutron Star Zoo

Abstract: A massive star ends its life as a violent supernova explosion. The compact core left over could form an ultra-dense object called a neutron star. Neutron stars are more massive than the Sun but as small as a city, and they possess the strongest magnetic field in the Universe. These remarkable properties make them ideal laboratories for studying physics under extreme conditions. Since the first pulsar was discovered in the 1960s, all neutron stars were thought to spin rapidly and emit broadband radiation. However, the emergence of different populations, including magnetars, dim isolated neutron stars, central compact objects, and millisecond pulsars, over the past decades has revolutionized this simple picture. In this talk, I will describe the diverse properties of the neutron star populations and highlight some recent results from multi-wavelength studies. I will also discuss attempts to unify different classes of objects.

Speaker: Dr Stephen Chi-Yung Ng (Hong-Kong University)

Slides

Thursday 24 February

16 Entangled relativity

Abstract: Entangled relativity is a new theory of relativity that does not allow gravity to be treated separately from matter fields. Unlike general relativity, the theory therefore satisfies the last of Einstein's three original demands for an acceptable theory of relativity, which he named Mach's Principle. Indeed, unlike in general relativity, inertia cannot be defined ex nihilo in entangled relativity, because the definition of the theory demands the existence of matter in the first place. Apart from that, the theory does not assume any new ingredient with respect to general relativity: for instance, spacetime is described by a four-dimensional manifold, the metric tensor encodes the mechanical properties of space as well as the inertia of bodies and gravitation, and its curvature is generated by matter fields. In fact, the theory even possesses one less free parameter w.r.t. general relativity, as Newton's constant no longer is a fundamental constant in entangled relativity, but results from the primordial evolution of the universe. The theory also has the nice property to possess general relativity as one of its limits (for fairly generic situations when the density is not way higher than the central density of neutron stars).

Speaker: Dr Olivier Minazzoli (Cote d'Azur Observ., Nice)

Slides

Thursday 3 March

17 The universe evolution and modified gravity: an introduction.

Abstract: A basic introduction to modified gravity is presented. Special attention is paid to F(R) gravity. The realistic, "exponential F(R)" gravity is also discussed. It is shown how it may describe the whole universe evolution from inflation to dark epoch. Ghost-free extended F(R) and F(G) gravities are briefly introduced. The reconstruction of F(R) gravity from inflationary indices is discussed. This seminar is aimed at young people and is based on the review: "Modified Gravity Theories on a Nutshell: Inflation, Bounce and Late-time Evolution" by S. Nojiri, V. K. Oikonomou and S. Odintsov, e-Print: 1705.11098 [gr-qc] Phys Rept 692 (2017) 1.

Speaker: Prof. Sergey Odintsov (Barcelona, ICREA/IEEC)

Slides

Thursday 17 March

18 Metric-Affine Chern-Simons Modified Gravity (a first look)

Abstract: Modifying GR by means of a Chern-Simons term coupled to a scalar field offers an interesting opportunity to explore parity violations in the gravitational sector. As usual, the original representation of this theory appeared in the metric formalism, though there are good reasons to reconsider it within a metric-affine approach. By doing so, one may improve the original action in order to make it projectively invariant. Though the connection equation is hard to tackle, we manage to obtain an approximate solution around a Schwarzschild background, which allows us to explore small perturbations and quasi normal modes. We compare our findings with those obtained in the metric formulation and in GR, finding that the metric-affine version presents certain features that could facilitate their detectability via gravitational waves observations.

Speaker: Prof. Gonzalo Olmo (Valencia U.)

Slides

Thursday 24 March

19 Searching gravitational wave signals with autoregressive approach and deep learning

Abstract: We are developing a novel framework of searching gravitational wave signal by coupling the stochastic autoregressive modeling and deep learning. This proposed framework holds the potential for an automatic pipeline for noise reduction, event candidate detections and template-free parameter estimation. We are also working on a deep learning algorithm by incorporating generative adversarial network. By increasing the training set and resolving the imbalance class problem, we can better separate the environmental/instrumental noise from the astrophysical signals. Currently, we are testing the method with LIGO O1/O2 data.

Speaker: Dr David Hui (Chungnam Natl. Univ. - South Korea)

Slides

Thursday 31 March

20 Beyond the Reissner-Nordstrom paradigm: static black holes with non-spherical horizons

Abstract: Static (single) black holes (BHs) in electrovacuum can only have an electric monopole, and they are necessarily spherically symmetric. We discuss two different mechanisms to circumvent this result. In the first case, we show that static BHs in AdS-electrovacuum can have an arbitrary electric multipole structure, and present explicit examples of static BHs with no continuous (spatial) symmetries. The second example consists in asymptotically flat scalarized BHs in a class of Einstein-Maxwell-scalar (EMS) models. The corresponding BHs bifurcate from the Reissner-Nordstrom BH trunk, forming an infinite (countable) number of branches, and possess a large freedom in their multipole structure. Unlike the case of electrovacuum, the EMS model admits static, asymptotically flat, regular on and outside the horizon BHs without spherical symmetry and even without any spatial isometries, which are thermodynamically preferred over the electrovacuum state.

Speaker: Prof. Eugen Radu (University of Aveiro)

Slides

Thursday 7 April

21 TBA

TBA

Speaker: Dr Matthew Lake (Frankfurt Institute for Advanced Studies)

Thursday 14 April

22 Cosmology with gravitational waves: current results and future prospects

Abstract: Gravitational-wave (GW) cosmology has recently been established as a new observationally-driven research field, and is expected to expand rapidly in the future thanks to the increasing number of ever more accurate GW observations. In this talk I will review the current status of GW cosmological results obtained with the LIGO and Virgo interferometers, and will survey the results expected with future detectors. I will first introduce the most common methodologies employed to extract cosmological information from GW data, including new tests of LCDM which cannot be performed with more common electromagnetic observations. I will then show how these methods are employed to produce cosmological constraints with current GW data collected by LIGO and Virgo, and how they will usher an era of precise GW cosmology with next generation detectors, such as the Einstein Telescope on the Earth and LISA in space.

Speaker: Dr Nicola Tamanini (Univ Toulouse CNRS & MPI, Albert Einstein Inst Postdam)

Slides

Thursday 21 April

23 Planar Carrollian dynamics

Abstract: It is known that the effective dynamics of a system restricted to a plane are in a sense richer than those in the "bulk" thanks to the observation of effective quasi particles in the plane, anyons. While this is a condensed matter result, one may wonder if similar effects arise in General Relativity on some hypersurfaces. A good candidate for the study of this phenomenon is Carroll geometry, as it turns out that important physical structures in Lorentzian spacetimes carry such geometry, such as black hole horizons, conformal null infinity, or even some hypersurfaces inside gravitational waves. Hence, we will look at the effective dynamics on such structures by considering intrinsic motions on 2+1 dimensional Carroll geometries. The distinct behavior of the planar case is due to the presence of two non trivial central extensions of the Carroll algebra. Elementary particles thus feature two additional (effective) physical parameters. Without these parameters (for instance in higher dimensions), it is known that Carroll particles have trivial motions, they do not move. However, we will show that these parameters lead to non trivial motions when particles are coupled to an external electromagnetic field.

Speaker: Dr Loic Marsot (Univ de Marseille U & Univ de Toulon CNRS)

Slides

Thursday 28 April

24 Aspects of Gravitational Field and Cosmological Consequences Based on Finsler and Finsler-like Geometries.

Abstract: We present the gravitational dynamics with generalized field equations in Finsler and Finsler-like geometries. In this approach, the gravitational field is considered in the framework of extra dimensions with more degrees of freedom. The Einstein‐Finsler‐like gravity theories can be considered as natural candidates for the investigation of Lorentz violation, local anisotropies and provide a geometrical background for the emergence of dark matter and dark energy. In addition, modified gravity theories are extended in the framework of a geometry of an enriched spacetime with extra degrees of freedom. Friedmann equations are derived in a locally-anisotropic form of spacetime. In such theories, the concept of nonlinear connection, which connect the external and internal structures of spacetime, plays a crucial role. Some cosmological consequences are shown for the Finsler-Randers, Schwarzschild-Finsler-Randers models and generalized scalar-tensor theories.

Speaker: Dr Panayotis Stavrinos (National and Kapodistrian University of Athens)

Slides

Thursday 5 May

25 Can Modified Gravity challenge LCDM?

Abstract: The late time cosmic acceleration is one of the most puzzling phenomena in modern cosmology. Its modeling within General Relativity (GR) through the cosmological constant (L) results in the LCDM scenario. Although the latter gives a precise description of the Universe, it is known that it still contains a number of unresolved problems. These lead researchers to look for modified gravity models, for example by including additional degrees of freedom. In this talk I will present the phenomenology and the cosmological bounds of theories consistent with the gravitational-wave event GW170817. In particular I will discuss models which solve the Hubble tension between Planck and local measurements and for which data show a statistically significant preference over LCDM.

Speaker: Dr Noemi Frusciante (Univ of Lisbon)

Slides

Thursday 12 May

26 An invariant approach to Weyl's unified field theory

In this talk we revisit the Weyl's unified field theory, published in 1918, shortly after general relativity was discovered. As it is well known, in order to extend the program of geometrization of physics started by Einstein to include the electromagnetic field, H. Weyl developed a new geometry which constitutes a kind of generalization of Riemannian geometry. However, despite its mathematical elegance and beauty, a serious objection was made by Einstein, who considered Weyl's theory not suitable as a physical theory since it seemed to lead to the prediction of a not yet observed effect, the so-called "second clock effect". In this talk, our aim is to discuss Weyl's proposal anew and examine its consistency and completeness as a physical theory. Finally, we propose new directions and possible conceptual changes in the original work. As a simple application, we solve the field equations assuming a Friedmann-Robertson-Walker universe and a perfect fluid as its source. Although we have entirely abandoned Weyl's attempt to identify the vector field with the 4-dimensional electromagnetic potential, which here must be simply viewed as part of the space-time geometry, we believe that in this way we could perhaps be led to a rich and interesting new modified gravity theory.

Speaker: Dr Iarley Lobo (Paraiba University)

Slides

Thursday 19 May

27 Multi-messenger astronomy of cosmic particle accelerators

As a synergy of coordinated ground- and space-based observations targeting the four extragalactic messengers (electromagnetic radiation, cosmic rays, neutrinos, and gravitational waves (GWs)), multimessenger astronomy is a very useful tool for studying the most energetic phenomena of the universe. In my talk I will review recent findings and questions about the high-energy particle emission by cosmic particle accelerators, focusing on radio, X-ray, gamma and neutrino data.

Speaker: Dr Emma Kun (Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Budapest)

Slides

Thursday 26 May

28 Cosmological dynamical systems in modified gravity

Dynamical systems theory is a useful tool for assessing cosmological models. In this talk, I will look at applying it to second-order modified gravity theories in an approach that leads to several model-independent results. Moreover, as well as being model independent, the formulation is equally applicable to three second-order geometric modifications of gravity, teleparallel f(T) gravity, symmetric teleparallel f(Q) gravity, and the metric theory f(G) gravity. This is due to the equivalence of all three theories when working with the FLRW metric (in appropriate coordinates/tetrads). This approach reveals generic information about all possible models in any of the three theories, irrespective of the choice of function f. The existence and stability criteria for the fixed points are then analysed and discussed.

Speaker: Dr Erik Jensko (University College London (Department of Mathematics))

Slides

Thursday 2 June

29 Quantum Conformal Gravity

Conformal symmetry is a natural symmetry in physics since it is the full symmetry of the light cone. If all particles are to get their masses by symmetry breaking then conformal symmetry is the symmetry of the unbroken Lagrangian. Like Yang-Mills theories conformal symmetry has a local extension, namely conformal gravity, a pure metric-based candidate alternative to the non-conformal invariant standard Newton-Einstein theory of gravity. With its dimensionless coupling constant quantum conformal gravity is power counting renormalizable. Since its equations of motion are fourth-order derivative equations conformal gravity has long been thought to possess unacceptable ghost states of negative norm that would violate unitarity. However on constructing the quantum Hilbert space Bender and Mannheim found that this not to be the case. Conformal gravity is thus offered as a completely consistent and unitary quantum theory of gravity, one that requires neither the extra dimensions nor the supersymmetry of string theory. As formulated via local conformal invariance there is no intrinsic classical gravity, with gravity instead being intrinsically quantum-mechanical, with the observed classical gravity being output rather than input. The contribution of the graviton loops of conformal gravity enables conformal gravity to solve the cosmological constant problem. Like Yang-Mills the potential of conformal gravity contains both a Newtonian term and a linear potential. Together with a quadratic potential that the theory also contains conformal gravity is able to explain the systematics of galactic rotation curves without any need for galactic dark matter. Since all mass is to be dynamical there cannot be a fundamental double-well Higgs potential in the theory. Instead, the Higgs boson is generated dynamically, with the hierarchy problem then being solved.

Speaker: Prof. Philip Mannheim (University of Connecticut (US))

Slides

Thursday 9 June

30 Seeking the first radio-powerful AGN within the Epoch of Reionisation

The existence of powerful Active Galactic Nuclei (AGN) has now been established well within the first Gyr of the Universe, through the observations of tens of optically or near-infrared selected Quasi-Stellar Objects (QSOs) up to the currently highest redshift of z~8. Theoretical work has been developed showing how super-massive (M∼10^9 M_sun) black holes can exist at even earlier epochs, depending on a unknown but necessarily quick assembly and growth of a suitable “seed”. Some of these sources should be bright at radio wavelengths, and detectable well within the Epoch of Reionisation (EoR). These detections will be paramount to the understanding of the earliest stages of galaxy evolution but even more - they will allow the direct study of neutral hydrogen in the Epoch of Reionization, through the Square Kilometer Array project (SKA) observations of the HI 21cm forest against such background radio sources. However, recent radio surveys have shown that, even if very high redshift radio AGN may have been already detected, their identification has proven all but impossible. Attempts to "break this wall" are on-going, but at a frustratingly slow pace. In order to focus the development of future SKA and SKA-pathfinder surveys we have initiated a systematic exploitation of state-of-the-art galaxy formation and evolution models in order to understand how the identification of EoR radio AGN can finally be achieved. Finally, I will also discuss how radio and X-ray exploration of the EoR appear to be set to become increasingly connected in the near future.

Speaker: Prof. Jose Afonso (University of Lisbon)

Slides

Thursday 16 June

31 Scale and Weyl invariance in metric-affine theories of gravity

Scale invariance is a desirable feature of any quantum field theory that wants to be complete in the ultraviolet and, as a consequence, more predictive. With this motivation in mind, we discuss a geometrical construction, based on the gauging of Weyl symmetry, of theories of gravity that are scale invariant. We pay particular attention to the behavior of affine degrees of freedom, such as spacetime's torsion, under Weyl transformations.

Speaker: Dr Omar Zanusso (Univeristy of Pisa (Italy))

Slides

Thursday 23 June

32 TBA -- seminar cancelled

Thursday 30 June

33 Enigmas of the Sp(h)in(x)

Beyond the well-known epistemological and foundational problems of quantum theory (measurement theory, entanglement, classical limit, etc.), there are others conceptual questions which have been somewhat neglected in the past decades. I will discuss six of these open questions, all connected with the notion of spin: the riddle of the position operator (or Dirac vs Wigner); the puzzle of the intrinsic magnetic moment; the spin-forces connection; the spin-statistics connection; the spin-charges connection; the non-renormalization of spin.

Speaker: Prof. Jean-Marc Levy-Leblond (University of Nice)

Slides

Thursday 7 July

34 Metric-Affine Gravity as an effective field theory

Abstract: I will present several aspects of general Metric-Affine gravity theories: different gauge choices, different choices of variables, the relation to metric theories of gravity, important special subcases, the choice of Lagrangian from the point of view of effective field theory, the occurrence of ghosts/tachyons and ways to avoid them. I will also mention Weyl theory as a special case.

Speaker: Prof. Roberto Percacci (SISSA Trieste, Italy)

Slides

Thursday 14 July

35 Scale and Weyl Invariance in Einstein-Cartan gravity

I will show how Einstein-Cartan gravity can accommodate both global scale and local scale (Weyl) invariance. To this end, I will present a wide class of models with non-propagaing torsion and a nonminimally coupled scalar field. In phenomenological applications the scalar field may be associated with the Higgs boson. For global scale invariance, an additional dynamical field — the dilaton — is needed to make the theory phenomenologically viable. In the case of Weyl symmetry, the dilaton is spurious and the theory reduces to a sub-class of one-field models. Eliminating non-propagating degrees of freedom, I will derive an equivalent theory in the metric formulation of gravity and briefly discuss possible implications for phenomenology.

Speaker: Dr Georgios Karananas (Munich University, ASC)

Slides

Thursday 21 July

36 On de Sitter moduli stabilisation and inflation from string compactifications

An overview will be presented on the efforts to obtain an accelerating universe in string compactifications including some recent work on brane inflation and moduli stabilisation.

Speaker: Prof. Fernando Quevedo (DAMTP, Cambridge University)

Thursday 1 September

37 Minimal theories and modified gravity models

I will describe the minimal theories of modified gravity that I have been working on recently. In particular, I will describe why/how minimal theories might help resolve today's tensions in cosmology.

Speaker: Prof. Antonio De Felice (Yukawa Institute for Theoretical Physics, Kyoto Univ, Japan)

Slides

Thursday 8 September

38 Black holes solutions in metric-affine gravity with dynamical torsion and nonmetricity

In this talk, I will introduce and explain the geometrical role of torsion and nonmetricity tensor by considering post-Riemannian manifolds to construct theories of gravity. Then the trinity of gravity will be presented. After that, I will discuss a gravitational model which allows the independent dynamical behaviour of the torsion and nonmetricity fields to be displayed in the framework of Metric-Affine gauge theory of gravity. It will be shown that it is possible to construct exact black hole solutions within this theory. Particularly, I will show the first known isolated gravitational spherically symmetric system characterized by a metric tensor with independent spin and dilation charges. Finally, I will show a new axially symmetric solution in our theory which describes a Plebansky-Damiansky type D black hole solution valid in the decoupling limit between the orbital and the spin angular momentum.

Speaker: Dr Sebastian Bahamonde (Tokyo Institute of Technology, Japan)

Slides

Thursday 15 September

39 Shadowgraphy: the art of producing images of compact objects

The optical appearance of a compact object illuminated by its accretion disk is determined by a delicate - and hard to simulate - interaction between the background geometry and the geometrical, optical, and emission properties of the disk. In this talk I will discuss the images generated in optically and geometrically thin disks with several choices for the emission profiles, acting over (spherically symmetric) compact objects of different types: (modified) black holes, wormholes, and boson stars. Our discussion places a fundamental role to the notion of critical curve(s), namely, the locus of unstable bound null geodesics, and how (when present) they affect the balance between the main emission of the disk and the higher-order structure created by those light rays turning more than one (half-)orbit around the compact object. We finally reflect upon the simplifications and caveats of our analysis, and on the change of testing these objects using very-long baseline interferometry.

Speaker: Dr Diego Rubiera-Garcia (Departamento de Fısica Teorica, Universidad Complutense de Madrid)

Slides

Thursday 22 September

40 Hidden symmetries from distortions of the conformal structure

In the study of geodesic motion, it is well known that the mass of the test particle breaks the symmetries generated by proper conformal Killing vectors; the latter generating integrals of motion only for null geodesics. In this work we study the fate of these broken symmetries. We show that, for a generic spacetime, they are substituted by nonlocal symmetry generators, while, under special geometric conditions, they result in hidden symmetries of the geodesic action. A prominent example of this situation is given by considering a general pp-wave spacetime. We then proceed by introducing an additional symmetry breaking effect and study its further consequences on the conformal structure. This is realized by departing from Lorentzian geometry and by adopting a Bogoslovsky-Finsler type of line element.

Speaker: Dr Nikolas Dimakis (Center for Theoretical Physics, Sichuan University, China)

Slides

Thursday 29 September

41 Observational signatures of hot spots orbiting horizonless objects

Pushed by a number of advances, electromagnetic observatories have now reached the horizon scale of supermassive black holes. The existence and properties of horizons in our universe is one of the outstanding fundamental issues that can now be addressed. Here we investigate the ability to discriminate between black holes and compact, horizonless objects, focusing on the lensing of hot spots around compact objects. We work in particular with boson and Proca stars as central objects, and show that the absence of a horizon gives rise to a characteristic feature -- photons that plough through the central object and produce an extra image. This feature should be universal for central objects made of matter weakly coupled to the standard model.

Speaker: Dr Joao Luis Rosa (University of Tartu, Estonia)

Thursday 6 October

42 Geometrical formulations of gravity and its coupling to matter ***

I will first introduce 3 alternative but completely equivalent formulations of GR, where gravity is assigned to either curvature, or torsion or to non-metricity (mentioning extensions of GR on the way). I will then discuss how matter can couple to these different geometries and whether the minimal coupling principle can be preserved.

*** note the change of time to 14:00h

Speaker: Prof. Lavinia Heisenberg (ETH Zurich and Heidelberg ITP)

Wednesday 12 October

43 Challenges of an accelerating universe

Abstract: I will discuss theoretical constraints on de Sitter vacua in effective field theories, supergravity and string theory, as well as implications in models of inflation.

Speaker: Prof. Ignatios Antoniadis (LPTHE Univ Pierre et Marie Curie Paris & CNRS)

Slides

Thursday 13 October

44 Dark Light Boson Emission from Supernovae

Supernovae are good laboratories to investigate new physics beyond the Standard Model which contains light dark particles. This is because the additional energy leakage sources by the dark particle emission can change the star evolution significantly. We re-examined the dark light boson (axion, dark gauge boson) emission from supernovae especially focusing on the effects of local contact interactions, pion-mediated Thomson-like process, decay of dark particle outside/inside the star, and medium effects on the effective charge of the nucleon. We obtained new constraints. In this talk, I will explain the physics behind the constraints.

Speaker: Dr Chang Sub Shin (IBS Daejeon & Chungnam National University, South Korea)

Slides

Thursday 27 October

45 Distinguishing different axion models with precision low energy axion couplings

I discuss the low energy couplings of the three most common types of axions: KSVZ-type, DFSZ-type and string-theoretic axions, while focusing on the possibility to discriminate these three types of axions from each other by the measurable couplings to the photons, nucleons, and electrons.

Speaker: Prof. Kiwoon Choi (IBS, Center for Theoretical Physics of the Universe, Daejeon - South Korea)

Slides

Thursday 10 November

46 Quantum Gravity and the Swampland

The Swampland is defined as the set of consistent Quantum Field Theories that cannot be coupled to Quantum Gravity. The goal of the Swampland is to find or to conjecture, based on the intuition gained from String Theory, general principles that a field theory coupled to quantum gravity should respect. One example is the "gravity as the weakest force" (or "weak-gravity conjecture"). I will summarize some of the recent conjectures and their potential implications for particle physics and cosmology.

Speaker: Prof. Emilian Dudas (Ecole Polytechnique Paris)

Slides

Thursday 24 November

47 The old and the new muon g-2 puzzle

The recent muon g-2 measurement at Fermilab confirms the previous BNL result, hence emphasizing the longstanding discrepancy between the experimental value and the Standard Model (SM) prediction based on the use of e^+ e^- → hadrons data to evaluate the leading hadronic vacuum polarization (HVP). This is what I call the “old” muon g-2 puzzle. On the other hand, a recent lattice result by the BMW collaboration shows a tension with the above mentioned SM prediction (this constitutes the “new” g-2 puzzle). The BMWc result has recently been confirmed by other lattice computations, although only concerning a window of the whole energy range integration. In this talk I plan to summarize the overall situation concerning the two muon g-2 puzzles and, in particular, I discuss the possibility that new physics may contribute to the e^+ e^- → hadrons cross-section to solve or at least alleviate the new g-2 puzzle.

Speaker: Prof. Antonio Masiero (University of Padova and INFN - Italy)

Thursday 8 December

48 TBA

seminar cancelled by the speaker.

Thursday 15 December

49 Seeking de Sitter vacua in the string landscape

The implications of perturbative string loop corrections on the moduli stabilisation problem and de Sitter vacua are investigated within the framework of type IIB string theory. In particular, the role of one-loop logarithmic corrections arising from a configuration of intersecting D7-brane stacks and a localised Einstein-Hilbert term descending from the ten-dimensional string action are examined. It turns out that such contributions are capable of generating a minimum of the potential along the volume modulus. It is further shown how de Sitter vacua are ensured when D-terms associated with the universal abelian factors of D7 brane stacks are taken into account.

Speaker: Prof. George Leontaris (University of Ioannina - Greece)

Slides

Thursday 22 December

50 Axion quality problem and its solution

No global symmetry is exact in the presence of gravity. The global U(1) PQ symmetry for the QCD axion is explicitly broken by non-perturbative effects of gravity such as axionic wormholes. The gravitational violation of the PQ symmetry due to wormholes is large enough to invalidate the PQ mechanism, which is entitled as the axion quality problem. We suggest a novel solution to this quality problem based on non-minimal coupling in metric and Palatini formulation of gravity.

Speaker: Prof. Seong Chan Park (Yonsei University Seoul, South Korea)

Slides

Friday 20 January

51 On the Weyl gravity extension of Higgs inflation

We consider the embedding of Higgs inflation with a non-minimal coupling into the Weyl gravity. In this model, the effective current-current interactions from the heavy Weyl gauge field cancel the non-canonical Higgs kinetic term in Einstein frame, so the unitarity problem of the original Higgs inflation becomes less severe. For a simple case where the couplings of the heavy Weyl gauge field appears from the non-minimal couplings to the Ricci curvature scalar in Weyl gravity, we find that the resultant model for Higgs inflation is the same as in the Palatini formulation for Higgs inflation. The crucial difference of our model from the Palatini formulation for Higgs inflation is that there is a light Weyl gauge field coupled to the Higgs fields. We also generalize the unitarization of Higgs inflation with general covariant kinetic terms for the dilaton and the Higgs fields, and realize a successful Higgs inflation, interpolating between the Palatini formulation for Higgs inflation and a Higgs-like inflation. We also discuss the Higgs mechanism for the light Weyl gauge field with an extra singlet scalar and show some interesting signatures for Higgs physics.

Speaker: Prof. Hyun Min Lee (Chung-Ang University Seoul - South Korea)

Slides

Friday 17 February

52 Inflation and Dark Matter with Weyl Symmetry

The cosmological observations of cosmic microwave background and large-scale structure indicate that our universe has a nearly scaling invariant power spectrum of the primordial perturbation. However, the exact origin for this primordial spectrum is still unclear. We present a theoretical framework and several viable models to explain cosmic inflation and dark matter simultaneously, both are important ingredients for the formation of cosmic structures. We also discuss the connection with usual Starobinsky inflation and difference in the prediction on the magnitude of primordial gravitational waves, which may be probed by future experiments on cosmic microwave background.

Speaker: Prof. Yong Tang (Beijing, Chinese Academy of Sciences)

Slides

Friday 3 March

53 Aspects of Palatini Inflation and Quintessence

In this seminar we give an overview of various modified theories of gravity in the Palatini formalism and discuss their inflationary predictions. We then show how the Palatini formalism can be used to construct models of quintessential inflation where a single scalar field can play the role of the inflaton at early times and of dynamical dark energy at late times.

Speaker: Dr Alexandros Karam (NICPB National Institute Tallinn)

Slides

Thursday 8 February

54 Constraining torsion theories with Weyl (gauge) invariance

Torsionful theories of gravity with local SO(3,1)xD(1) invariance generally propagate two vector degrees of freedom, i.e., the torsion vector and the Weyl vector potential. We show that these are tightly intertwined by imposing a projective symmetry on the theory: this enables us to get rid of the Weyl potential and to interpret the torsion vector as the connection 1-form for Weyl gauge invariance. By further imposing a second type of projective invariance we can get rid of the torsion vector too, obtaining a conformal theory. Thence, we focus on the tensor irrep of the torsion, which is unaffected by Weyl transformations, and we derive its conformal quadratic action. We conclude by making remarks about the general features of Weyl gauge invariant theories with torsion.

Speaker: Dr Dario Sauro (University of Pisa (Italy))

Slides

Thursday 15 February

55 TBA

TBA

Wednesday 28 February

56 UV-IR connections in scattering amplitudes: power of unitarity and causality

Very basic assumptions about the structure of the fundamental theory describing the nature at high energies can allow for a set of valuable constraints on the low energy EFT. The constraints are independent of the concrete underlying fundamental theory at high energies. The low energy physics is bounded from the set of quite natural assumptions about the structure of the full theory, such as unitarity, causality and Lorentz invariance. In this talk I will consider the EFT of photons (or other U(1) gauge field) and compare different approaches to obtain bounds on its Wilson coefficients. I will present an analytic derivation of the implications of unitarity (linear and non-linear positivity bounds) and compare these constraints with the requirement of causal propagation of the photon modes around non-trivial backgrounds generated by external sources. I will show that the low energy causality condition can give complementary constraints to the positivity bounds. Applying both constraints together can significantly reduce the allowed region of the photon EFT parameters. Besides constraints on the IR theory, the analyticity and unitarity assumptions can set bounds on the UV theory. I will show this connection using the example of the graviton mediated scattering amplitudes. Surprisingly, the form of the forward limit singularities at low energies can give an insight into the of high energy limit of graviton scattering described by the unknown physics.

Speaker: Dr Anna Tokareva (Hangzhou Institute for Advanced Study, UCAS, China)

Slides

Thursday 14 March

57 TBA

TBA

Thursday 28 March

58 TBA

TBA

Thursday 11 April

59 TBA

TBA